February 11, 2021: A Jaunt Through Our Insanely Active Pattern

There have already been almost too many wintry systems across the central and eastern U.S. since late January to keep track of, including a 20-30”+ whopper of a storm where I’m living in northern New Jersey. And for better or worse, there are three more impactful systems clearly signaled by operational and ensemble guidance between this weekend and the end of next week.

The pattern giving us this activity (representative EPS snapshot above) is not all that favorable for I-95 snowstorms. Before you throw beer bottles at me, I’m not saying it will rain on I-95, but except for maybe Boston don’t expect any of these storms to be predominantly snow along the I-95 corridor. However, it is still an impressive pattern and favors a lot of impactful wintry weather across the CONUS.

The trough over the western and central U.S. has a tap of truly Arctic air, with prolonged, strong high-latitude blocking and a cross-polar flow filling Canada with very cold air as a large piece of the tropospheric polar vortex gets displaced into North America. The lobe of the tropospheric polar vortex grazing the US / Canadian boarder is bringing this Arctic air with it into the CONUS.

However, the problem for I-95 getting all snow is the EPO is rising, the PNA is rather neutral, the NAO is somewhat negative but is east-based, and the coldest air dumped in over the western U.S. This all allows a Southeast Ridge to crop up. That’s good for the Ohio Valley and Great Lakes, but not so much the Mid-Atlantic. However, there is still strong confluence over southeastern Canada and New England in this pattern, favoring surface high pressure there and cold air damming east of the Appalachians. With the air over Canada Arctic in nature, it will be much colder at the surface on the East Coast than the 500mb maps suggest. The pattern strongly favors primary lows tracking into the Ohio Valley, with the confluence encouraging re-development off the East Coast. The primary lows will drive mid-level warmth north ahead of them, and any re-development may help keep places like Upstate NY and New England snowier, but farther south this is an icy look.

Note how there are multiple shots of Arctic air into southeastern Canada as pieces of the TPV move through next week. This will give us a quality airmass to tap. Also note the TPV generally consolidating and the jet becoming more zonal towards the end of next week as the AO briefly spikes, giving us at least a brief break in this very active wintry pattern after next week.

Now that the broad stage has been set, let’s look a little closer at the 3 systems on the radar from this weekend through next week…

This Weekend:

There are a few pieces here that will produce precip, some of it wintry. A vort max rounding the base of the polar vortex will produce a rather light swath of high-ratio snow from the central Plains into the Upper Midwest and Great Lakes Friday – Saturday. The shortwave over the western Gulf will move northeast and bring primarily warm air advection-driven precipitation to much of the southern and eastern U.S Friday night – Sunday. The next shortwave behind it over NM and northern Mexico will largely get sheared out as the next, more potent shortwave dives in from the northwest later this weekend (though if it survives, it is what is bringing the shot of lighter precip later Sunday into early Monday to the SE / Mid Atlantic that some models have). What really hurts over the eastern U.S. is the fact that the wave itself is rather weak, the wave spacing is poor, and there’s broad confluence over much of the eastern U.S. which will tend to shear this feature out as it lifts northeast.

The result will be a weak attempt at a primary low into the Ohio Valley, and a weak attempt at a coastal. Neither will be strong and there won’t be a ton of QPF to play with, with the most QPF over the southeast and lower Mid Atlantic where the shortwave will still be stronger and where PWATs are higher. There may also be a relative uptick over PA, NY and southwestern New England as the vort max rounding the PV adds a bit of lift and improves jet support while the weak coastal makes its closest pass:

QPF will then begin diminishing again heading east across New England as this feature passes and we get into subsidence behind it.

This just isn’t a great set-up for snow due to a lack of robust coastal re-development and warm mid-levels. The bulk of the precip will be driven by low-mid level WAA. This lift briefly improves (with some suggestion of fgen ramping up too) Saturday evening / night as the best upper jet support glances the region, but the strongest fgen is located well below the DGZ across the board and is occurring near the retreating 850mb 0C line, just not a great set-up for a “thump” of snow before flipping to ice or rain. The relatively best shot of snow may be from northern PA and Upstate NY into interior New England where it will be colder and where all of the lift will briefly line up in an area cold enough for snow, but even here more than a localized few inches seems like a stretch.

The problematic aspect to this system is that there will be a tap of very cold and dry air to dam east of the terrain, with the weak coastal also giving a little isallobaric tug to the flow Saturday afternoon – night and re-enforcing this cold air damming. This will allow temperatures to hold below freezing west of the coastal front through much of VA and potentially into NC during this precip – note that cold air damming often trends colder as we get closer. Along much of the I-95 corridor temperatures will be well-below freezing during any wintry precip, so it may accrete or accumulate rather well and impact travel. There isn’t a ton of QPF to go around so in general I don’t expect enough freezing rain for significant power outage issues, though the one exception may be the lower Mid Atlantic (or interior NC if it’s cold enough). The low-levels are quite cold as you head northwest of the I-95 corridor which may favor sleet vs freezing rain for many areas for part of the storm, though a narrow corridor of predominantly freezing rain may play out somewhere near or just NW of I-95.

Monday – Tuesday:

Similar to the weekend wave, but also different. The lobe of the polar vortex is pulling east. While there is strong confluence / surface high pressure behind this feature (and a shot of cold air into the Northeast with it), it is moving to the east ahead of our ejecting shortwave. The shortwave ejecting is much larger as well. With a southeast ridge in place, this will send a robust primary low into the Ohio Valley early next week, followed by secondary development off of the Northeast coast.

There is some uncertainty with regards to the exact details, as exactly how consolidated the wave is when it ejects out will influence the strength of the primary low into the Ohio Valley and can influence the coastal re-development. Some guidance ejects a little piece on Monday, resulting in a wave of overrunning precip well ahead of the main storm, and potentially resulting in a slightly weaker primary low. The next shortwave will also need to be monitored for wave spacing issues if it trends faster, which could also weaken the primary low. How quickly the PV moving across SE Canada ahead of the storm can also influence things; if it moves through slower it may favor a somewhat colder solution, but if it lifts out faster could favor a more amped primary low into the Ohio Valley and warmer solution for the East Coast.

The tl;dr here is to expect a primary low of sorts into the Ohio Valley, followed by eventual re-development off the Northeast coast…this isn’t going to go anywhere as it’s too obvious of a set-up that’s been shown strongly by most operational and ensemble guidance since it came into range, but it may trend more or less amped.

In terms of where this may produce wintry precip…given the record cold surging to the TX Gulf Coast this weekend and early next week, wintry precip is likely all the way to the coast in TX. As the primary lifts into the Ohio Valley a swath of moderate to locally heavy snow is likely across the Mississippi Valley into the Ohio Valley and southern / eastern Great Lakes, though note that there are ways this trends more or less amped. I generally feel these types of lows into the Ohio Valley trend more amped as we get closer, but the very cold air and dominant polar jet may limit that if the lobe of the TPV doesn’t lift out as quickly.

In the east, there will probably be some overrunning snow ahead of the primary low from the northern Mid Atlantic into New England, followed by some enhancement from the coastal from NY into New England. Much of the I-95 corridor (especially Philly and north) may see some snow from overrunning, but should flip to ice before getting a ton of snow.

It’s possible that Boston and Hartford are mainly snow if the primary low is on the weaker side and the coastal takes over quicker…NYC is close to the line but I don’t think they can avoid going over to ice or rain at some point. I think the farthest south this can be primarily snow, even with the colder solutions, is from central PA to maybe northern NJ (but outside of NYC itself) into southern New England (though as of now, I’d favor some ice even in this corridor unless we see the primary low trend weaker…that’s the farthest south I could see mainly snow if this trends less amped and colder).

Despite the primary low likely flooding the mid-levels with warmth, the TPV will bring a shot of very cold and dry air to southeast Canada and New England that will funnel east of the Appalachians into the Mid Atlantic. West of the coastal front will be cold with this storm, certainly through DC and probably through most of VA (the GFS usually underdoes low-level cold in CAD). It is less clear-cut into the Carolinas, though I feel at least interior NC is in play for being cold enough for another round of freezing rain with this storm.

Basically, a lot of areas should get snow from this system, even if the exact placement can wobble. Even in the Northeast many should see snow before any ice, but the very cold / dry air funneling east of the Appalachians combined with a robust primary low into the Ohio Valley screams an icy set-up for a large chunk of the region, likely including NYC, Philly, Baltimore, DC, and possibly even Richmond again. Hartford and Boston are a little more on the fence for mainly snow vs a fair amount of ice, but very well could end up in the latter camp too. My gut here is that this is more likely to trend a little more amped vs flatter as we get closer.  

Thursday – Friday:

So this is almost the same thing and I’m tired of labeling and typing out the same thing over and over again (since it’s like midnight). The key difference here for this storm is that there isn’t a lobe of the TPV sitting over SE Canada ahead of our storm, allowing for the SE ridge to extend all the way into eastern Canada. There is enough confluence for some surface high pressure and cold air damming ahead of the storm, but the quality of the air is not as good, the high pressure isn’t as strong, and the Miller B / coastal will be much more apt to track close to the coast given a weaker high pressure and less confluence north of the storm:

There is enough high pressure ahead of the storm for more cold air damming, and probably enough high pressure and confluence to coax a Miller B again, but overall it will be warmer than the Monday – Tuesday storm.

Using the GFS for demonstration purposes, note how along with weaker high pressure ahead of the storm overall (per the EPS images), the cold / dry air over New England and SE Canada is a couple of notches less impressive than what the Monday – Tuesday storm is working with. It’s still cold and dry enough that there may be overrunning snow ahead of the storm and again cold air damming, quite possibly into the Mid Atlantic, but it will be easier for the mid-levels to warm, and the cold air damming may eventually erode a bit more than it will with the early week system.

The result is still another potential swath of snow, both with the primary low from the southern Plains into the Ohio Valley and southeast / eastern Great Lakes and likely again over the interior Northeast, and still more ice, but overall it will probably be a notch or two warmer in the Northeast than the early-week storm.

Note how the ensemble mean snow swath is actually fairly similar on the southern edge to Monday – Tuesday in the Northeast, but definitely has the heaviest swath located even farther inland.

Overall, 3 systems are coming up. In the interior Northeast they may be mainly snow, but closer to I-95 they all likely feature a fair amount of sleet and freezing rain (and maybe some plain rain farther south). Large swaths of impactful wintry weather can be expected with all 3, but especially the Monday – Tuesday and Thursday – Friday storms. Finer details are still a long way from being worked out, but there’s plenty to keep an eye on, including the risk for substantial icing through the week and power outage potential.

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Winter Chill Will Stick Around Through February

The pattern coming up is going to feature plenty of cold / Arctic air over North America to tap, along with a continued tendency for blocking over the Atlantic. 

A Rossby wave making machine will be in place over the western Pacific, encouraging persistent north Pacific blocking and a cross polar flow into Canada. This is occurring while a tendency for high latitude blocking continues across the board for multiple reasons.

This loop of the GFS helps show the continued cross polar flow into Canada, and how the retrograding -NAO helps shove the tropospheric polar vortex towards the CONUS this weekend and next week, bringing a cold snap to much of the Lower 48. With the Pacific blocking likely continuing for the forseeable future, the model shows Arctic air reloading towards the end of the run in mid-late February. 

One of the reasons we will remain “blocky” for the forseeable future is the continued downwelling of the weakened stratospheric polar vortex into the troposphere, which encourages a -AO and blocking:


On top of that, there’s been a recent uptick in tropical forcing across the western hemisphere, and western Pacific forcing will continue until further notice:

This deposits momentum in the tropics and subtropics (enhances the sub-tropical jet). Because momentum is a conserved quantity, the increase in momentum in the tropics and subtropics results in a decrease in momentum in the higher-latitudes, which causes a tendency for blocking. The 12z GFS is shown as an example of this, note how the stronger sub-tropical jets over the eastern Pacific and eastern Atlantic occur beneath weaker higher-latitude flow, and how this encourages high latitude blocking:

The persistent western Pacific forcing will keep our Pacific blocking in place for most of February, and the Arctic air dropping into North America will encourage continued cyclogenesis near the east coast, which will help encourage Atlantic blocking as well. 

Basically, the US will lean cold for much of February. There will be a baroclinic zone across the southern / eastern US, though the cold will seep south and east at times given the -NAO and the quality of the cold that will be available. The Arctic air may lead to near to below average precipitation for a lot of the CONUS (save for perhaps the southeast/east coast), but the pattern will stay at least somewhat active given the baroclinic zone and hints of a subtropical jet. The GEFS and EPS weekly forecasts for the next 30 days speak for themselves: 

While suppression and lack of moisture may be a problem at times, we are already in a rather wintry stretch that will probably last through all of February, and perhaps into early March. 

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Sizing up the January 31 – February 2 Storm Potential

I think it’s safe to say “here we go” for Sunday – Tuesday in the Midwest and Northeast!

The pattern features a west-based -NAO, 50/50 low, large shortwave, confluence ahead of it over New England and SE Canada, and ridging out west:

In addition, this storm will occur while the NAO is transitioning, the PNA is spiking, and the AO is persistently, deeply negative:

This all leads to a robust low pressure tracking across the Ohio Valley and then re-developing off the Mid Atlantic coast, with a strong high pressure over southeast Canada:

The TPV that gets shoved through the Northeast Friday – Saturday (and probably bringing snow squalls, gusty wind, and the coldest airmass of the winter so far) will ensure that there is an actual cold airmass in place in front of the storm, with reasonable cold behind it as well:

At first blush, there appear to be some similarities to what was modeled for this Thursday and will end up really suppressed on the East Coast. Here is this Thursday’s set-up:

Note how there’s little wave spacing in front of the storm, with a PV also depressing heights over the Great Lakes, and little ridging behind the storm. This resulted in a solution that was a bit too far south and too late of a bloomer for a more notable East Coast storm. Compare to what’s modeled with the Sunday – Tuesday system:

More wave spacing both in front of and behind our storm, a more amped shortwave itself, and more room to breath over the Great Lakes. This storm should be more amped across the board. 

The trends at 500mb over the last few days on the EPS have been for more blocking, a more optimally placed 50/50, a more amped shortwave, and more ridging and wave spacing to the west. All generally argue for a more amped storm, but also force the track farther south especially over the Midwest:

Surface trends match, along with a much better high over southeast Canada:

After these trends, that are still ongoing, the EPS mean 500mb evolution is classic for a slow-moving, sprawling winter storm from the Midwest to the East Coast:

Most of the 12z GFS, Canadian, and Euro ensemble members have a notable swath of snow somewhere across the Midwest or East Coast. Location and amounts vary considerably, especially on the East Coast (note that some snow in the Great Lakes, New England, and southern Mid Atl / SE are from prior events, but any larger swaths are from this storm):

We definitely have a storm to track for a lot of areas! In the Midwest, I feel that eventually the strength of the wave should limit southern shifts, though a couple more are possible if the western ridging or blocking trend any stronger. On the East Coast, I feel this is more likely to trend more amped than extremely suppressed. However, quality cold in front of the storm and good blocking should increase the odds of overrunning snow in front of the storm anyways, even into the Mid Atlantic, and should eventually limit how far north this can trend. 

It’s too early to lock in snow in a given spot, but a bunch of areas are in the game for this one. 

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12/10/2020: But Actually: Four Systems to Watch on the East Coast Next Week

The weather models are running just as quickly and as often as they can. Outputting nothing. Outputting snow. Outputting rain. Back and forth. Life and death. Snip snap! The fingers are typing and posting on Twitter even faster, if that’s even possible. But yes, actually, there are four “systems” that may produce snow on the East Coast next week, and one of them looks rather delectable, especially from the interior / northern Mid Atlantic into New England.

 A quick note on how this pattern is coming to be…

TropicalTidbits.com

The combination of a negative East Asian Mountain Torque event (lower pressure east of the mountains) and piece of the tropospheric polar vortex dropping into Asia is encouraging a somewhat retracted Pacific jet stream (after it was quite strong / extended to end November and start December) and wave breaks over the northern Pacific. This is allowing heights to rise in the northern Pacific, dropping the EPO and WPO. This is greatly increasing polar air in North America after it lacked to start December and prevented last weekend’s storm from being snowier. And by snowier, I mean producing more than 0 snowflakes in New Jersey.

Tropical forcing is centered near 120E right now, registering as a phase 5 MJO, projected to move into phase 6 next week:

https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/clivar_wh.shtml

The RMM plots have had a hard time tracking this current MJO as it’s constructively interfering with lower frequency forcing near 120-150E. However, it seems there’s still a coherent MJO in there that’s currently in phase 5 and may get into phase 6 or 7 next week. You can see how the lift really intensified over the Indian Ocean last month as the MJO approached the low frequency forcing near Indonesia…and how it seems the MJO is now emerging on the other side towards the western Pacific:

https://ncics.org/portfolio/monitor/mjo/

The CFS (above), EPS and GEFS all have some uptick in 200mb upward motion as far east as the Dateline over the next 5-7 days. A phase 5-7 MJO in mid-December strongly suggests blocking in both he EPO and NAO domain:

http://www.atmos.albany.edu/facstaff/roundy/waves/rmmcyc/index200reg.html

This period of potential blocking, aided strongly by tropical forcing along with a recent +East Asian Mountain Torque that really beefed up the sub-tropical jet, has been talked about many (including myself) since November. I personally was not as sold on legitimate NAO blocking, but that does seem likely to occur this weekend through next week.

weathermodels.com

It’s quite possible that a disruption to the stratospheric polar vortex, causing it to get split in the lower stratosphere for a time, with a lobe dropping into Asia and another into North America, with ridging / weaker flow in between over the north Pacific and north Atlantic, is encouraging this tropospheric blocking. 

stratobserve.com

One can definitely see on the 3D vortex plot from stratobserve.com how the tropospheric and stratospheric blocking across the north Atlantic may be somewhat coupled, and also how the lobe of the tropospheric polar vortex dropping into North America is tied to the stratospheric vortex as well. This activity within the polar vortex is complimenting what the tropical forcing suggests quite nicely…and this disruption is likely tied to a recent strong MJO progression across the Indian Ocean and strong East Asian Mountain Torque event.

I guess this is a long way of saying high latitude blocking, including in the NAO domain (2020 baby!) is quite well-supported next week. Though, do note that phase 5-6 tropical forcing in late December supports troughing over western North America and a Southeast Ridge, and as the MJO eventually weakens and the low frequency forcing takes over later in the month troughing very well may try to slide west. But let’s not skip to that, let’s break down next week’s potentials…

Monday:

Dig polar jet, dig! Ah that’s been the rub against the Monday – Tuesday threat, the lack of ridging out west would keep the polar jet too bottled up to get enough stream interaction for a storm to deepen and come up the East Coast. Is there hope on that front?

There is quite a bit of blocking over the Arctic and north Atlantic. The polar vortex is over Hudson Bay! There isn’t a 50/50 low yet, but the Atlantic side is not inhospitable. The Pacific side is a little more…eh? If ridging off the West Coast is a bit stouter, or if there is just a bit more “traffic” over the Atlantic, perhaps the northern piece can dig a little more.

The trend to raise heights a bit over the Davis Strait and nudge the PV a bit west…and trend on the West Coast to have a little more ridging between shortwaves…may just force the northern stream to dig a bit more. That would result in a better chance at a phase and a more robust coastal. The rub here is there’s no 50/50 low yet (or “traffic”), so a stronger storm would try to cut a bit more. So, I-95 would require a perfectly timed phase for legitimate snow, but inland is in a better position if we trend towards a better phase.

Both the EPS and GEFS runs at 18z like this slightly more phased idea for whatever reason…seems it’s the northern Mid-Atlantic most likely to get snow, especially NW of I-95, with some potential this gets into NY or New England if we see more wholesale changes to slow down / dig the polar jet shortwave more.

weatherbell.com

There are a number of EPS members that are stronger than the mean, though most are northwest of the mean, and the strongest ones are well northwest of the mean. This is evident on both the spread (all to the north/northwest of the mean) and map of the member low locations:

My verdict on this one is to watch for the polar / northern jet shortwave to dig more. If it can, it opens the door for more snow, but may also shift that snow NW of I-95 pretty quickly if we aren’t careful.

Wednesday – Thursday:

This is currently the big-ticket item generating most of the images getting posted on weather social media. A primary low moves towards the Great Lakes, as it runs into confluence over southeast Canada the energy gets shunted east and sparks a coastal (Miller B) off the northern Mid Atlantic or New England coast. With reasonably cold air in place to the north, this may produce a good amount of snow for someone…

The pattern is pretty delectable on the Atlantic side. A strong -AO, -NAO, and 50/50 low. The -NAO and 50/50 low coax that strong confluence over southeast Canada, supporting a surface high pressure that forces the Miller B coastal to form. The tropospheric PV and 50/50 low, which was a lobe that broke off from the PV, mean there’s actual cold air available to tap, unlike last weekend when a robust Nor’easter produced more rain than snow. The Pacific pattern isn’t ideal…the ridge axis is a little southwest of preferable and the polar vortex is a little west/northwest than an ideal location. So, this would likely act to limit any threat in the Southeast and lower Mid-Atlantic. But, the Atlantic blocking and cold air over Canada being so robust would pretty much guarantee snow from much of PA and northern NJ into NY and New England, assuming there’s a storm.

The trends on the EPS over the last few days for this time period are remarkable. The NAO has trending much more negative, with that -NAO also trending more west based, with the strongest anomalies over the Davis Strait! There has also been a trend for a much stronger and more optimally placed 50/50 low. These trends are a result of trends with the Monday system for a more amped northern stream and potentially stronger coastal. The AO blocking has also trended stronger, and perhaps also as important, ridging over the Aleutians has been trending towards mainland Alaska. One more trend in that direction and all of a sudden the PV is nudged a little to the east, which would be a better spot.

Right now, it’s quite a good set-up north of the Mason-Dixon line, especially inland, but if the Pacific side trends a bit better could become something interesting in the lower Mid Atlantic or Southeast.

These are impressive ensemble mean snow swaths for over a week out for the EPS and GEFS this afternoon / evening. The greatest signal is from eastern PA through NYC and Boston, initially favored NW of I-95 but then right along it. There is considerable spread on both sides through. Also note how there is some potential for snow with the primary low over the Ohio Valley too on the other side of the Appalachians.

While there’s great agreement on a general low track off the East Coast next Wednesday – Thursday, there is still a good bit of spread in both track and speed, and intensity at all points. This likely has a lot to do with how well this shortwave interacts with the polar stream, and how strong the Monday system becomes, since it turns into the 50/50 low.

My thoughts on this one are that the Atlantic side is stellar, and the shortwave ejected east across the country is strong. There should be a decent Nor’easter with snow for at least part of the Northeast or Mid-Atlantic. If the Pacific side isn’t as ideal, with the PV centered a bit to the west and the ridge axis southwest of what’s preferable, though some improvements may still occur there based on current trends. These improvements could really up the ante for snow somewhere if they occur.

Otherwise, how the Monday system pans out will also impact this second threat. A stronger Monday system may force this system farther south, turning it into more of a Mid Atlantic problem and perhaps limiting how far north any snow can get. A weaker Monday system could weaken the confluence over Canada enough to allow the primary to cut a bit more, which would limit the threat on the southern extent by pumping in more warm air before the coastal tries to form.

A storm seems likely for someone, but a lot can change where and exactly how much. Wow, I know, I’m such a prophet.

Next Friday and Sunday:

The Atlantic pattern remains decent through next weekend and the AO stays negative…however, the EPO rises and the PNA tries dipping, which will try to flood the CONUS with Pacific air and make it harder to see enough northern stream interaction for a phase or good winter storm. How the Wednesday-Thursday storm plays out will also impact the pattern for Friday’s storm, and Friday’s storm will have an impact on the Sunday storm. See a pattern? This is fun.

If the Pacific side holds onto a more neutral pattern just a bit longer that could really make one of these a more plausible storm. Is that possible? My gosh these questions are why I should be sober when I write. I ask fewer weenie questions while sober.

After the ongoing negative East Asian Mountain Torque, we’ll have a positive torque develop much of next week into the following weekend. This will act to extend the Pacific jet again. There will be a brief window next week where this may encourage a stronger low near the Okhotsk Sea (NW Pacific). As some of this momentum gets dispersed into the sub-tropical jet over the East Pacific, this may combine with the Okhotsk low to try to wedge a ridge between that low and the PV edging into the EPO domain from western Canada. That’s a long way of saying it’s possible it takes the EPO a bit longer to rise than the current ensembles, though it seems inevitable to eventually happen.

My impression is that unless the Wednesday – Thursday storm falls apart that it will make the Friday system unlikely to happen. Or, it will shove it so far south it’s mainly a Mid Atlantic or even Southeast event. The hostile Pacific pattern may limit the potential on next Sunday, though if there is enough polar influence it may again threaten the Mid Atlantic or southern New England.

The takeaways to all of this are that a lot supports high-latitude blocking next week, and we have a few systems that can take advantage. The Wednesday – Thursday is the most likely to be a major storm for the greatest number of people as Monday, Friday, and next Sunday’s threats all need a bit of work. But all are close enough not to rule any out. That’s to say, let the model watching continue…

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Jim Sullivan’s 2020-21 Winter Outlook & Write-Up

It’s past November’s midpoint, so I’m clearly late to the party with this more technical write-up on what I think will happen this winter. My thoughts to date have been posted by me at times on weather forums and hinted at on Twitter, and my employer has also posted a couple of YouTube videos…but, this is the first time I’ve been able to fully sit down and write everything down. Given how late it is, my thoughts have evolved some from October.

This write-up will have 3 sections of discussion and then finish with the actual forecast. I’ll go through the drivers heading into the winter and what they may mean, some analogs based on said drivers, and also the recent / current pattern, how that may move forward and how that compares to some of the analogs.

Since I understand some people just want the maps, I will post them first, but the fun obviously comes from the write-up 😉

December temperatures:

Please note that I expect a week or two of normal to below normal temperatures in the Great Lakes, Northeast, and perhaps Ohio Valley and Mid Atlantic, along with windows where snow is possible, despite the overall mild temperatures in December.

January temperatures:

February temperatures:

March temperatures:

Total seasonal snowfall (including what’s already fallen, where applicable):

Potential Drivers This Winter:

Plot from: tropicaltidbits.com

We’ll start with the moderately strong La Nina event across the Pacific. This developed over the summer and has strengthened through the fall, and the atmosphere has clearly responded. The La Nina is currently moderately strong and will likely be at moderate to borderline strong strength for the winter.

Plot from: http://mikeventrice.weebly.com/

A recent weakening of the trade winds has allowed for both the surface and sub-surface to warm slightly, however, most  guidance suggests another period of strong trade winds developing the rest of this month and into early December, which likely will result in further cooling.

Plot from: https://www.cpc.ncep.noaa.gov/

Despite a little bit of recent warming, it is still quite cold below the surface across much of the Pacific, particularly east of the Dateline. Another period of enhanced trades would result in upwelling and notable surface cooling into December. Various models depict more strengthening into early-winter and suggest that a strong La Nina tri-monthly peak may still be in play, though the modest warming this month will probably make that difficult to pull off. Most guidance shows the La Nina peaking in intensity in December and then beginning to weaken:

Plot from: https://www.ecmwf.int/en/forecasts

Plot from: https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

So, let’s start with the La Nina…here’s what the last 10 have gotten us when averaged together:

Gee thanks for nothing La Nina. Except for 10-11 and 17-18. Those are kind of OK. The last 10 Ninas, on average, fit the stereotype of starting colder than they end for the eastern U.S.:

The last 10 moderate-strong La Ninas only are similar for the winter as a whole, though are a little uglier in the NAO and EPO domains:

There were a couple of fairly good winters in there, and even some of the duds had brief but intense cold shots…though on the whole they’re not great.

In general, La Nina winters feature a stronger stratospheric polar vortex and limited high-latitude blocking, with the signal stronger in moderate to strong events with a +AO, NAO, and EPO on the above composite. There are a few exceptions, even with moderate to strong events, but with all else being equal they are not more supportive of high-latitude blocking than a neutral or warm ENSO.  

With that said, where the coolest water with the La Nina is associated can make a significant difference. This paper examines different response to East Pacific La Nina events and Central Pacific events:

https://link.springer.com/article/10.1007/s00382-014-2155-z

The tl;dr here is that “East Pacific” La Nina events tend to have a slightly more poleward north Pacific ridge, and more notably, a modest -NAO…as compared to “Central Pacific” events that have a flatter north Pacific ridge, and a +NAO.

The question is, is this event central, eastern, or more basin-wide?

From the same paper, above is the typical SST departures with time in East Pacific, Central Pacific, and hybrid La Nina events. The key difference between eastern and central events is that in eastern events, the cooling originates near South America and propagates west, while in central events the cooling is centered near 150W. Central events also tend to be stronger, though there are exceptions both ways.

Here is this year’s evolution. It’s clearly NOT central Pacific based…and the drift west in SST anomalies is typical of east based events. You can argue that for a time in late-summer, it appeared more centrally based, though it’s been hybrid or east-based this fall. Per the referenced paper, La Ninas don’t switch from EP to CP based (or vice versa) from fall to winter, so I don’t think this event is “becoming central Pacific based”…though you can argue there are some hybrid characteristics, which per the paper can lead to some tendencies of both with regards to the pattern response. This does suggest, at first blush, that we don’t have the “worse” flavor of La Nina and may maintain some hope for the event weakening during the second half of winter.

The mechanism for East Based La Ninas producing a more -NAO is not really explored in the paper. A couple of thoughts are that the low-frequency response is slightly closer to “neutral” conditions in the EP events, as the cooling isn’t as far west. Other research (that I’ll reference later) suggests that La Ninas in general don’t have as robust of a blocking response to the MJO as El Nino / neutral ENSOs do. Another thought is that the MJO may be more active into the western Pacific during EP events than during CP events. The paper did present rainfall rate anomalies in CP and EP events:

The CP events (B) have a stronger negative precipitation anomaly that is centered farther west, focused at or just west of the Dateline. The EP events have a weaker anomaly centered slightly farther east, closer to neutral years, perhaps muting the effect La Nina has on dampening the MJO into the western Pacific somewhat and allowing for more blocking to result. In a somewhat concerning trend, most of the seasonal models, that overwhelmingly have a +NAO for December-February and in some cases have a flatter north Pacific ridge, all have precip anomalies for the winter that look more like a CP event:

The JMA:

Euro:

Jamstec:

And the CFS:

This has been consistent on the majority of seasonal model runs for the last few months. This could be a red flag that despite the SST profile clearly not behaving like a CP event, we may still have a heightened risk for a +NAO (and EPO) for December-February as a whole, due in large part to the response to this La Nina. Or, it could mean the seasonal models are a little too +NAO and warm…hmm.

Other SST Areas of Interest:

1 is La Nina

2 is the Indian Ocean. Last year saw a strong positive Indian Ocean Dipole event, with cool waters near Australia and warm waters closer to Africa. This contributed to convection/forcing frequently sitting over the western Indian Ocean during the fall of 2019 and into the first half of winter, with a dearth of forcing over the Pacific, especially in the critical period when the polar vortex began wrapping up in December and early January. This fall, the IOD is neutral to perhaps slightly negative. This argues for the low frequency forcing centering farther east than last winter, but in tandem with the much stronger La Nina doesn’t necessarily promise that the MJO frequently gets far into the Pacific.

3 is the Western Pacific warm pool…as has often been the case in recent years, it’s quite warm and expansive. This, along with the neutral to weakly negative IOD, does argue for convection getting into the western Pacific and perhaps helping to amp the north Pacific ridge due to enhancing Rossby Wave Trains across the Pacific. As referenced above, if the convection doesn’t wander close to the Dateline, this still may not dump the cold air in where the East Coast wants it on a consistent basis.

4 is the North Pacific…it’s quite warm across the board, but this is a negative Pacific Decadal Oscillation. The warm waters near Japan and cooler waters near the southern Alaskan and western Canadian coast are characteristic of a -PDO, and the short-medium range pattern argues for these anomalies worsening. A –PDO tends to be associated with a weakened Aleutian low and a more negative PNA, which causes troughing to be biased towards the Pacific Northwest.

A note that the off-equator SSTs are quite warm over the central and eastern Pacific (a positive Pacific Meridional Mode, or PMM). This is one of the stronger +PMM events observed during autumn during a La Nina. This may add a bit of kick to the sub-tropical jet over the next couple months.

5 is the northern Atlantic. The SST pattern here has been used, especially more so in the past, to predict the winter NAO. The configuration this spring – now has been slightly more conducive to a -NAO than recent years, with cold waters south of Greenland/Iceland and somewhat “warmer” waters closer to Greenland and Iceland. While not as unfavorable to a -NAO as most recent years, it still is displaced north of where it is prior to -NAO winters, and doesn’t really move the needle in the face of other factors if they support a +NAO winter.

The Quasi-Biennial Oscillation:

Plot from: https://acd-ext.gsfc.nasa.gov/Data_services/met/qbo/qbo.html

The QBO is a typically nice downwelling of westerlies and easterlies in the stratosphere. This is shown on the image above. However, the QBO basically “skipped” the easterly (negative) phase in 2016. After one and a half normal oscillations from 2017-2019, we had an extremely anemic easterly QBO late 2019-early 2020, and will be in a westerly (positive) QBO for the winter.

An easterly QBO tends to favor a weaker stratospheric polar vortex, which can lead to more high latitude blocking in the troposphere, while a westerly QBO tends to favor the opposite. Yes, there are easterly/negative QBO years with limited high-latitude blocking, and westerly QBO years with blocking…and yes, sudden stratospheric warming events (SSWs) can still occur in a +QBO and lead to a weaker polar vortex and increased blocking potential. In fact, per this graphic posted by Dr. Amy Butler on Twitter (@DrAHButler), half of westerly QBO La Nina winters featured a sudden stratospheric warming event (filled circles had a SSW):

This may be a significant wild card this winter, as otherwise, the combination of a La Nina and positive QBO (along with our current, absolutely putrid pattern) favor a strong stratospheric polar vortex, which is less favorable for tropospheric high-latitude blocking. This was a major culprit in the extremely mild winter of 2019-20. I will circle back more to this topic when tying all the drivers into our current / short-term pattern later.

Aside from the typical PV and blocking implications discussed each year, a phenomenon (that I frankly did not notice on my own when I started dabbling in seasonal forecasts) that has gotten increased attention in recent years is the QBO’s apparent influence on the north Pacific ridge in La Nina winters. In general, +QBO winters have a more poleward ridge, while -QBO winters have a flatter ridge (interestingly, EP La Ninas have a more poleward ridge, while CP La Ninas have a flatter ridge).

Here is a look at La Nina winters since 1980, that per a combination of the CPC’s 50mb monthly QBO data the QBO diagram above had a +QBO at or below 50mb through the winter. The most debatable inclusions are 17-18 and 11-12, but both had a positive anomaly at 50mb through at least December per the CPC data and held onto it just below that level through at least February.

Here are the -QBO La Ninas:

And the difference:

The +QBO La Ninas DO have a more positive AO and NAO than the -QBO La Ninas, but the differences in the north Pacific are beneficial for the Northeast U.S. Keep in mind, La Nina strength, location, and other factors are ignored for this exercise and many of these anomalies may not be statistically significant given the somewhat small sample size, but it’s food for thought, at the very least, and has worked more times than not in La Ninas over the last 15 years.

Solar Cycle:

We are coming off of another deep minimum in the 11-year sunspot cycle. Many papers (here’s one I quickly pulled up, though there are more https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005JD006283 ) have found a relationship between the solar cycle and the stratospheric polar vortex strength and tropospheric high-latitude blocking in winter. In general, solar minimums correlate with a weaker SPV and increased blocking, while maximums correlate with the opposite. I’m not going to spend a ton of time on this as I think other factors are already in control, but it was a factor weighed in the analogs.

Low Frequency Forcing:

We’ve clearly had a La Nina response to the low frequency forcing this summer and fall, with rising motion over the Indian Ocean and sinking motion over the Pacific. It can be argued the forcing drifted east some over the course of the summer and fall. Another way to visualize this is on a map showing velocity potential from September – November 15:

This shows a similar idea, though do note that the low-frequency forcing is strongest near Indonesia (near 120E). This signal was focused closer to 40E (much farther west) in fall of 2019.

This does suggest that convection may be most amplified this winter in MJO phases (3-6) that are not cold over the eastern U.S. (phases 5-6 are OK in early-mid December, which I will get to later). However, keep in mind that the MJO getting into the western Pacific can set off a wave train that eventually leads to blocking, especially if it continues propagating. Here are lagged composites for phase 5 of the MJO in DJF, which are quite warm with a strong SE ridge when the phase 5 occurs and in the short-mid range, but turn much blockier a few weeks later:

This low frequency forcing IS a warm signal/risk for the bulk of winter, but likely isn’t as bad as last winter’s in terms of potentially encouraging MJO propagations that can lead to temporary breaks in the milder weather down the line.

The location of this forcing was a factor weighed when determining analogs.

Atmospheric Angular Momentum:

Atmospheric angular momentum is, extremely basically, how much westerly flow there is across the globe. Typically, El Ninos feature a stronger / extended Pacific jet due to increased forcing over the Pacific, and higher AAM. La Ninas typically feature the opposite.

Above is the seasonal correlation to AAM in DJF. The correlation is weak in much of the U.S., but in general higher AAM features a +PNA pattern with higher heights over Canada and lower heights over the southern U.S…basically a typical El Nino pattern. If you inverse the colors (use imagination) and emulate the correlation for a -AAM, it looks much more like a La Nina with higher heights off the West Coast and across the southern U.S. and lower heights over much of Canada.

The AAM over the last year or so has been interesting. It was quite low in the fall of 2019, likely due to increased Indian Ocean forcing and limited Pacific forcing, which contributed to a very ridgey pattern in the mid-latitudes (in particular, across the Pacific) and limited disruption to the stratospheric polar vortex, allowing it to get quite strong by early 2020. It rose later in winter, though was distributed poorly and didn’t shake-up the +AO pattern. The AAM fell again over the summer as La Nina developed and strengthened. It has risen this fall and isn’t dropping back off yet. This atypical AAM for a La Ninia may explain why we’ve had cold weather over western North America and warmth nearly everywhere else in November (and quite possibly December, more later), which is not how La Ninas typically play out (usually, La Ninas start colder than they finish in the eastern US).

The fact that the AAM is kind of high for a La Nina isn’t necessary bad, but how it’s currently distributed is poor for winter weather fans in the eastern U.S. and Europe to say the least:

Plot from: https://atlas.niu.edu/gwo/

Momentum in the tropics and especially sub-tropics is quite weak. The mid-latitudes are rather unremarkable, but momentum (westerly flow) is high north of 60N, indicative of a +AO and lack of blocking. This distribution was seen at times in January-March of 2020 when all attempts to develop blocking failed.

Plot from: stratobserve.com

Part of the reason we couldn’t shake the anomalous westerly momentum in the high latitudes in early 2020 was because the stratospheric polar vortex was very strong and coupled to the troposphere. Nothing was moving that, apparently…despite the increase in AAM and more active MJO during the second half of last winter, and despite a lot of model hints that we would eventually move that. Fortunately, the stratospheric polar vortex, while becoming strong again, is not yet coupled to the troposphere (note how zonal wind anomalies below the white line are still comparatively weaker than higher up). If this doesn’t change in the short-medium range, it may provide for some hope for an eventually blockier pattern if other factors can align.

Analogs:

Now that we’ve more than thoroughly discussed some of the potential drivers that are evident this summer-fall, let’s start diving into the analogs. Factors I weighed while searching for possible analogs included:

  1. La Nina, with preference for first year events and moderate to strong hybrid or east-based events
  2. The Indian Ocean, with preference given to years with a neutral or weakly negative IOD in the fall
  3. The fall PMM, with preference given to years with a higher PMM in the fall
  4. The PDO, with preference given to years with a somewhat negative PDO
  5. The QBO, with preference given to years with a westerly QBO, and some tolerance for years with descending easterlies, but westerlies below 50mb through the winter
  6. The October teleconnections (AO, NOA, PNA, and EPO)
  7. The September-October low frequency forcing
  8. The general hemispheric pattern and AAM in September and October
  9. The speculative November pattern
  10. The speculative November MJO when that data is available
  11. Years with a large / strong West Pac warm pool
  12. The solar cycle (minimum or coming off a min)

There weren’t many analogs that check all of those boxes to an acceptable degree, but if I had to give a few tiers:

Top tier: 1988-89

“Good” tier: 64-65, 99-00, 08-09, 10-11, 17-18

Potentially useful tier: 73-74, 75-76, 95-96, 96-97, 07-08, 11-12, 16-17

Other years evaluated: 54-55, 55-56, 67-68, 70-71, 71-72, 74-75, 83-84, 84-85, 85-86, 98-99, 00-01, 05-06

Here are some composites when using the top 3 tiers with weights:

It fits the general La Nina theme, and looks more like the +QBO La Ninas with more poleward ridging in the north-central Pacific, but also a more +AO overall. December is generally the coldest month, with -EPO and some -NAO potential, with January likely quite up and down but with the AO, EPO, and NAO all trending more positive and the cold backing west. The February look has a robust SE ridge, though the deep vortex near Hudson Bay would probably be deceptively chilly /  snowy from the Great Lakes into New England.

Unfortunately, most of these years were horrible matches to the pattern we’re seeing this November. The few years that made an analog tier higher than “also ran” that had a November pattern that was even somewhat palatable compared to this November were: 64-65, 73-74, 88-89, 99-00, and 11-12. Here was their composite November pattern:

This is not by any stretch perfectly like what this November will look like after the second half of the month, but is somewhat tolerable overall, especially compared to the whole analog package’s November pattern (not shown). Arguably, 1988 and 2011 are the best matches to this year. For reference, here is what this November has done through mid-month:

For some idea of how the second half of the month may change this, the next 10 days on the most recent EPS run as of this writing on Wednesday:

Not perfect by any stretch, but still somewhat OK overall. Here is what those years did in December, and then the rest of the winter:

Overall, this is the shittier end of the larger analog pool. However, without giving the rest away yet (dramatic effect!), I do not think this December’s pattern will end up like the composite pattern in the above Decembers.

With that said, a select couple of those years did end up turning much colder in December and / or January:

64-65 is actually one of the higher scoring analogs…the only thing that really held it back from being a tier higher is a lack of MJO data, distance back in time, and a marginal November pattern match.

88-89 was one of my highest scoring analogs.

Not the greatest analog, but was similar in enough areas to be rated somewhat high.

Most of the winters with similar November patterns to this year were not good winters overall, though more than half featured a legitimately cold period at some point in December or January, and January of 2000 did pull off a pretty good East Coast snowstorm. It is worth noting that 64-65, 88-89, and 99-00 all had at least some similarities to this year WRT the September-October hemispheric pattern/AAM, low frequency forcing, QBO, and in 88-89 and 99-00, some similarities to the November MJO (MJO data not available back to 1964). Worth noting that these analogs have several similarities to this year, are a decent match to this November’s pattern, and have notable cold and/or snow over the eastern U.S. in either December or January. It is possible…

Out of all of the analogs, the only ones that had some similarity to this month’s MJO, the QBO, the low frequency forcing in fall, and the hemispheric pattern / AAM in September-October are 88-89, 95-96, 99-00, 08-09, and 17-18. I will throw 64-65 in there, as despite a lack of MJO data it is a good match in the other areas. However, the November pattern in 1995, 2008, and 2017 was not like this year. WRT the 95 and 08 MJO, they’re not perfect matches, but may have a similar move towards phases 4-6 as some models have over the next two weeks, but a couple of weeks earlier in those years. Given everything, a final blend of analogs includes those 6 years with weighting…an interesting note that 4/6 of these winters featured a SSW.

My final maps don’t look exactly like these composites, mainly because we’re far enough into this fall to speculate about the December pattern and its possible ramifications without analogs, but some takeaways are that December into January may offer the best shot at blocking and colder weather into the eastern U.S. February is the greatest risk to see a strong SE ridge. While March is mild on the mean, it’s somewhat muted compared to February, and the pattern February into March would keep Canada cold, so if the SE ridge isn’t dominant it could allow for an opportunity to turn chilly and try snowing in March in the Midwest and Northeast.

Ongoing – Medium Range Pattern:

As has been the case much of November, the pattern is not conducive for cold / snow in much of the central and eastern U.S. The poor distribution of AAM is evident, with ridging in the mid-latitudes pushing anomalous westerly flow to 50-60N, resulting in a +AO. There is a small block near Alaska, that will dump enough cold into Canada that a couple of systems may produce some thread-the-needle snow from the Midwest/Great Lakes into parts of the Northeast through the weekend after Thanksgiving, though otherwise for now a southeast ridge will keep mild air dominant over the rest of the central and eastern U.S.

Convection is very active across the Indian Ocean right now; a combination of the low-frequency base state, a Kelvin Wave, and the MJO appear to be causing this. The MJO is progressing through phase 2 at a somewhat respectable amplitude, contrary to some prior model predictions. This Twitter thread by @griteater features a nice loop showing the increase in convection and upper-level divergence as the MJO arrived in the Indian Ocean (definitely a good Twitter follow for those interested in mid-long range forecasting, he also posts on some of the common forums such as AmericanWx and 33andrain).

This analysis on Dr. Mike Ventrice’s website shows similar, with the MJO and Kelvin wave constructively interfering right now. There may be brief subsidence behind the Kelvin wave, though I think the robust nature of the MJO and low-frequency forcing will offset that and allow the convection to continue propagating east. Yet another plot, this from Carl Schreck, shows the current constructive interference, and the potential for the low-frequency forcing centered near 120E (as discussed earlier) and warm SSTs to keep convection going to at least 120-150E into early-mid December:

Plot from: https://ncics.org/portfolio/monitor/mjo/

In general, the GEFS has consistently been more bullish on getting the MJO into phases 6-7 than the Euro and CFS. Again, given the low-frequency forcing centered near where the MJO enhances convection in phases 5-6 and the warm waters in those areas, I do think convection becomes active between 120-150E (and perhaps a bit farther east) later November and into early December. It is possible that the MJO itself doesn’t make it out into the Pacific, as the low frequency forcing will dampen the MJO’s signal in the short term, and that same LF forcing suggests suppressed convection once farther east into the Pacific. However, convection between 120-150E is not bad through early December, given wavelengths still shorter than mid-winter, and a compelling argument can be made that the MJO is a significant driver at the moment. A few various tools illustrate this I think:

The lagged October-December phase 2 composites on the CPC’s website (each lag is 5 days) shows a similar pattern in the first few pentads to what most ensembles suggest occurring over the next 1-2 weeks, with troughing over the northern Pacific, Alaska and near the West Coast, along with ridging over the central and eastern CONUS. That then gives way to a more +PNA pattern. The anomalies in the later pentads are generally not statistically significantly, so although that’s still not a great pattern it’s not particularly concerning if other factors suggest differently. An interesting note is that the strong low across the northern Pacific that floods much of North America with warmth in the shorter term does not necessarily persist in these composites.

Another tool, from Paul Roundy (http://www.atmos.albany.edu/facstaff/roundy/waves/), shows the typical MJO response at much finer scales than three-month periods. In transition seasons, when the wavelengths typically change quicker than a 3-month mean will capture, these regressions add quite a bit of additional value.

The phase 2 regression for the week of November 17th shows a number of similarities to the pattern we’re actually seeing…and, moving the regressions forward through phases 3-6 with time shows a fair amount of similarity to the pattern showing up on many of the medium range ensembles in late November, followed by an increase in blocking near the West Coast and in the NAO domain, should we get increased convection to near 120-150E by early December.

The CPC October-December Phase 5 lagged composites show similar as well:

Both Paul Roundy’s tool, and the CPC composites, show a much warmer response to Phase 5-6 with a Southeast ridge during winter, so as the wavelengths get longer, this teleconnects to what people traditionally think it does in winter…warmth for the eastern U.S. But since wavelengths are still shorter, that’s not yet the case, and it can actually be helpful for winter weather fans in the central and eastern U.S.

There should be an increase in AAM in the tropics and sub-tropics in late-November as the MJO propagates east towards the far western Pacific, and as a strong East Asian Mountain Torque event occurs…

There has been a fairly persistent signal among the GEFS and EPS for a block/ridge near and east of the Ural Mountains to dump cold air and high pressure into eastern Europe for a prolonged period of time in late-November into the beginning of December, leading to a prolonged, significant +EAMT event:

This will lead to an acceleration of the east Asian-Pacific jet (the tropical forcing moving east towards 120-150E will also contribute to that). There’s also some minor signal on the EPS for an increase in West Pac tropical cyclone activity around the beginning of December (the GFS and GEM have had a number of false alarms while the EPS has remained much more bearish thus far).

This will add momentum in the sub-tropics, which may eventually improve the distribution of momentum across the mid-high latitudes and lead to a regime more favorable for high-latitude blocking, and also will encourage a change in the Pacific pattern in late-November and early December. With convection most active well west of the Dateline, this strong jet will want to break somewhere over the central or eastern Pacific (the +PMM and active STJ may initially cause this to occur farther east than it otherwise would in a La Nina). This will cause a significant attempt at raising heights over the western U.S. and into Alaska, which may increase the PNA and drop the EPO.

You can see this on Thursday’s 12z EPS, which is more bearish than the GEFS in getting convection into the western Pacific (also note the sub-tropical jet enhancement from the positive PMM east of Hawaii):

The wave breaks want to happen somewhere over the central or eastern Pacific, though initially the TPV over Alaska may be stubborn. The stratospheric PV will also briefly swing towards Alaska around Thanksgiving, which doesn’t help this cause:

So, we likely won’t see an immediate rise rise in heights over Alaska, but if the SPV moves away from Alaska and the TPV backs west…which the EPS and GEFS consistently show both at the moment…the wave breaks would eventually have a much easier time trying to raise heights on the West Coast and into Alaska. Initially in late November and early December, the lack of height rises over Alaska will result in the PNA rising, but the EPO won’t drop immediately. The op models consistently have a very progressive, wavy pattern with sub-tropical support to end November and start December with a +PNA look. The ensembles don’t look very stormy yet in this regard, but their mean pattern by the end of November / early December is too smooth:

(A note that this is a couple runs old now as of this posting, and it has trended a bit sharper with its ridge and trough axis)

The wavelengths in early December aren’t long enough to support a trough from 160E to 100W, and a ridge from the West Coast of the U.S. to eastern Canada. While it’s likely it’s generally troughy and generally ridgey in those areas, the pattern will probably be quite a bit wavier than an extended ensemble mean implies, which will allow for a bit more cold air to be available than modeled, and along with the active sub-tropical jet may bring snow potential when / if there’s enough polar jet influence. Given the STJ involvement, if marginally cold air can get displaced well into the CONUS any snow threats, in a more Nino-like fashion, may not favor the northern tier.

December will start quite mild across most of the CONUS with a progressive, but active pattern with a somewhat enhanced sub-tropical jet. That’s pretty baked in based on with the very +EPO to end November. But it very well may be stormier than the smoothed ensemble mean implies, with at least some transient cooler shots too. And, once the TPV starts sliding west of Alaska, it’s quite possible heights rise along the West Coast and towards Alaska quicker than the smoothed means suggests too. A day 12-13 51-member ensemble mean will not resolve something like a wave break at all.

While I think there’s legitimate potential for at least a temporary -EPO developing in early-mid December, I’m not as confident in Atlantic side blocking following suite. It is possible, as both the Roundy and CPC plots show a -NAO response to phase 5-6 forcing in the late-fall, and there have been hints of it on longer range guidance (the ECM monthlies have consistently had a weak -NAO for December. Let’s point out what, other than the Euro monthly, may point to a negative NAO:

An MJO propagation into the western Pacific (in particular, phases 5-7) in late November or early December, as mentioned, can favor the NAO dropping into early-mid December. In addition, Rocky Mountain Torque will remain weak if not somewhat negative into the mid-range (it shouldn’t increase until we start seeing EPO-induced cold shots, early December at the earliest). This favors a weaker jet from eastern North America into the western Atlantic.

With an active STJ late November / early December and a +PNA, the potential is there for strong cyclogenesis in the central or eastern CONUS, which may help raise heights / perhaps cause a wave break over the northwestern Atlantic. This, along with some (but not consistent) hints on guidance of a dropping NAO into early December, and the tropical forcing, does give multiple ways to get the NAO to drop. A couple of sources for pause:

MJO Phase 7 response with a warm ENSO
MJO phase 7 response with a cold ENSO

From this paper: https://journals.ametsoc.org/jcli/article/31/13/5293/92625/The-Impact-of-the-Madden-Julian-Oscillation-on

In general, the MJO does not produce as robust of an increase in blocking during a cold ENSO as it does during a warm ENSO in the winter. Note how phase 7 with a warm ENSO has a strong, statistically significant correlation to a -NAO. There are hints of a lowering in the NAO following a phase 7 with a cold ENSO, but it’s weaker and for the most part not statistically significant. Note than there is a temporary increase in blocking along the West Coast and into Alaska shortly following a phase 7 during a cold ENSO.

This suggests that while a -NAO isn’t impossible if we get the MJO to propagate into the western Pacific over the coming couple of weeks, the cold ENSO may otherwise lower the chances of it delivering the goods compared to otherwise. This does, however, further point to height rises along the West Coast being a possibility.

My other concern:

After the PV gets shunted away from AK and allows for a better opportunity for the wave breaks to raise heights there, it moves towards the eastern North America – northeastern Europe sector. While it’s not strongly coupled to the troposphere right now, this may discourage prolonged blocking in the North Atlantic, or at least make it so everything needs to go just right for it to occur.

My overall thought on the NAO over the next few weeks is that while I think we will see attempts for it to go negative…and at times, the op models will do it in a big way…I’m not confident we pull it off. A legitimate -NAO ups the ante along the East Coast in December, though one may not be needed for something if the Pacific side improves as I think it may. Basically, I lean narrowly towards a +NAO for December, with perhaps a couple of short dips, with a lower potential for a wave break in just the right spot to pull it off. Not impossible, but not really predictable at this stage…perhaps I’m just bitter from watching most -NAO attempts fail the last 7 or 8 winters, with the notable exception of March of 2018.

Thoughts on tropical forcing deeper into December:

There is a divergence among guidance, with the GFS in particular keeping forcing active between 120-150E for much of December, while the EPS and CFS, along with the JMA, generally appear to focus it farther west in the Indian Ocean by mid-December, causing the attempt to raise heights on the West Coast into Alaska to be brief and be quickly replaced by a pattern similar to what we have now with the TPV frequently settling near AK. Even if convection does remain active near Indonesia and spark a change to a pattern we’d more typically expect in a La Nina December, as the wavelengths look more like winter later in the month that does teleconnect to a Southeast Ridge. Although I wish other guidance showed it, the low frequency forcing has frequently flared up around 120E this fall, which may point to the GFS solution having some validity. This would somewhat prolong cold risks, probably focused around the second week of the month, before trending milder late in the month.

The CFS weeklies have consistently appeared to develop a robust, eastward propagating MJO for mid-late December that may get into the western Pacific in early January. While the EPS mean VP anomalies focus mostly over the Indian Ocean on the most recent weeklies, a majority of members have a coherent MJO signal at some point beginning mid-late December. The low frequency forcing amplifies this signal over the Indian Ocean and there’s quite a bit of disagreement on timing at this point, so the ensemble mean looks like a standing wave over the Indian Ocean with no MJO activity, but that likely isn’t realistic. An amplified MJO moving across the Indian Ocean would bring a warmer signal to the eastern U.S. as it occurred, but could lead to a cool-down at some point (likely towards mid January) for the eastern U.S. as it attempts to emerge over the Pacific. This would have ramifications on the January forecast.

Thoughts on the polar vortex into December / beyond:

As referenced above, the polar vortex is quite strong, but not well-coupled with the troposphere yet, and is occasionally getting knocked around and stretched a bit. So, while it is currently a signal for mild weather, it’s not an entirely hopeless proposition for it to weaken at some point as it was last January – March. The pattern showing up somewhat consistently on guidance the rest of November into early December is likely a more favorable pattern for disrupting the polar vortex than what we saw early this month.

The CFS is shown, though the EPS and GEFS have occasionally had the look of an Aleutian low and Scandinavian-Ural high as well. These are both features that stand out on composites of the precursor tropospheric pattern to SSW events:

A is the proceeding pattern in the 45 days prior to a vortex split, C is the pattern in the 45 days prior to a displacement. Both similar, though the Aleutian low and Scandinavian ridge are more amplified proceeding the splits. Another set of maps from the same paper shows similar

Here is a link to the paper: http://web.mit.edu/jlcohen/www/papers/CohenandJones_JC12.pdf

The polar vortex will again be quite strong in the short-medium term, but runs of some longer range guidance over the last week or so has slowly opened the door to some weakening mid-late December…shown are the CFS, GEFS and EPS:

CFS and GEFS PV plots from https://simonleewx.com/

My thought is the PV will weaken at least some in mid-late December, though I don’t think a SSW is likely before the New Year unless we set off more tropospheric blocking in the shorter term. I do think that the ridge across the Urals supporting continued opportunities for +EAMT in December, and the possibility of another more robust MJO between mid-December and mid-January, may give a window for a SSW to occur in January. I think after that, typical Nina forcing would favor the PV strengthening into February if it isn’t significantly weakened by mid-late January. A 2018 outcome where the La Nina weakens late in the winter and the MJO becomes active would then be the only hope for a PV disruption later in the season.

So, I see two clear opportunities to weaken the PV in December into the first half of January. If this successfully occurs, an MJO propagation into the Pacific in January could provide for a temporary colder and blocky pattern to start 2021, with the +PMM leading to some continued STJ activity. After that, the PV may very well get strong and coincide with a return to Indian Ocean forcing to provide for significantly milder weather heading into February. This is followed by an outside shot at a prayer to end the season if we can get one more coherent MJO as the La Nina begins to fade.

Putting it all together…the forecast:

December:

A +PNA to end November and start December leads to a very mild / warm start to the month across the CONUS (slightly cooler over the Southeast / East Coast)…however, an active STJ and a wavier pattern than the ensemble means will show at this distance may allow for enough polar influence for an opportunity or two at a winter storm across the central or eastern U.S. Given the +PNA and active STJ, this may occur in areas such as the Ohio / Tennessee Valley, Appalachians, Southeast or Mid-Atlantic, farther south than what you may expect in a La Nina and more typical of an El Nino. A +PNA, split flow, and active STJ are features that are frequently seen in an El Nino!

This window is brief, the first week or so of the month and with a questionable amount of polar air to work with, so it may not work, but there should be a bit to track on the models as this comes into range. Thereafter, the SPV moving away from Alaska, persistent +EAMT, and convection getting into the 120-150E area (and likely persisting a bit more as the GEFS has due to the lower frequency forcing in that region this fall) likely allows for a window of -EPO, and a pattern more typical of a December La Nina. I think there’s still enough momentum in the Pacific jet that this dumps into the Midwest and Northeast as opposed to the Rockies and Plains as typically occurs with EPO shots.

This trends towards cold edging towards the Northwest / northern Rockies into late December, with a Southeast Ridge cropping back up due to forcing becoming more focused on the Indian Ocean, and due to Indonesian convection beginning to teleconnect to a Southeast Ridge in later December.

With the start of end of December quite possibly warm for a large portion of the country, the month as a whole will be mild across the CONUS.

While I think the NAO likely ends up positive for the month, if we take advantage of the potential to develop a -NAO in early or mid-December, it would up the ante for snow in the Northeast and Mid Atlantic in that window, and may slow the warm-up later in the month. Even without a -NAO, I think there are opportunities for some snow in December in the central and eastern U.S. (even the southwest may have a window with the STJ, the NW and Rockies will be active and trend colder later in the month as well).

January:

January may start cold in the Northwest U.S. and warm across the southern and eastern U.S. However, I think the MJO has another opportunity to propagate east during the first half of the month. I believe the PV will be weaker and more receptive to the MJO producing a blocking response than it is in early December. This may cause another period of -EPO, -NAO, and a somewhat active sub-tropical jet for a couple of weeks starting the first or second week of January. This will bring another window of winter farther southeast towards the Ohio Valley, Mid-Atlantic, and perhaps even the Southeast.

I think we start trending towards a Southeast ridge later in January as the MJO fades and Indian Ocean forcing crops back up. How much blocking develops in the first half of January will of course influence how quickly this warm-up occurs.

February:

This is generally the warmest month in the analogs, with an amped central Pacific ridge dumping cold into Alaska, western Canada, and the northwest / north-central CONUS. If we have persistent Indian Ocean forcing and a strengthening PV to open the month, that will probably be the case again this year. If there’s cold available in Canada, the +NAO may lead to confluence east of New England that leads to high pressure over eastern Canada that can cause cold air to seep down into New England and perhaps the Great Lakes in February, causing some wintry threats in these areas…however, the pattern I envision for February is not snow-friendly in the Ohio Valley, and especially the Southeast and Mid-Atlantic.  

March:

Cold air likely remains available in Canada, so if the MJO becomes active again in late-February or March it may shake-up the pattern enough to bring it south and bring one last shot of winter in March. If this does not occur early enough, the Southeast Ridge dominates much of March and keeps the cold over the Northwest, Plains, and Midwest / northern Great Lakes / northern New England.

Teleconnection Guesses:

AO: Generally positive, but may go neutral or negative briefly in early-mid December, and perhaps a bit more legitimately in January

NAO: Generally positive, may briefly go negative in early-mid December, with greater potential for a couple-few weeks of a variable / negative at times NAO in January

PNA: Negative

EPO: Generally somewhat positive, but will dip at times. Mid December and perhaps mid-late January may offer windows.

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Spitballing Some Analogs / Considerations for Winter 2020-2021

It’s early, but some scattered thoughts based on some of the looking I’ve done into this winter…

Given a La Nina, westerly QBO, climate trends, and majority of long range guidance at this point, a warm winter has to be favored across the southern and eastern U.S. at this point, especially compared to the 1981-2010 normals which are almost obsolete at this point given how much warming we’ve continued to see this decade. I don’t think that can be ignored and is the logical starting point. Warm in the southern U.S. overall, mild up the east coast (but not as constantly warm farther north), and cold across the northwest-north-central U.S. 

With that said, La Ninas with a westerly QBO tend to feature more ridging near Alaska and generally more cold farther southeast into the CONUS than easterly QBO La Ninas. This has been pointed out by many people (who are generally smarter than me) over the last decade or so. Also, this La Nina is currently centered over the eastern Pacific, which tends to open the door for a colder outcome compared to La Ninas that are more basin-wide. An extraordinarily warm and large western Pacific warm pool and negative IOD may encourage an active MJO this winter over the western Pacific, and the persistent western Indian Ocean standing wave that severely limited Pacific forcing last winter through this summer may finally be weakening (not 100% sold on that yet, but there appears to at least be some more variability in the near future). Warm water near the Maritimes can be dangerous, as sub-seasonal forcing in that area is warm in the winter for us (and remember, I think the baseline here is a mild to warm winter across the southern and eastern U.S.), but the warm West Pac may encourage an active enough MJO to give us windows of opportunity. While the PDO is rather close to neutral, waters are currently quite warm over the NE Pacific…we’ll see if that changes in October though with more storminess in that area. The warm waters over the NE Pacific and extensive western U.S. drought are a result of a persistently ridgy pattern there this summer…does that continue into the cold season, or will outside forces coerce a change? 

So, off the cuff, there’s an urge (especially after last winter) to forecast an all-out torch-fest. There are several reasons for optimism…and the pattern in the short-term, with an amped Pacific jet resulting in an amped west coast ridge and hints of Scandinavian blocking, would be beneficial both short and long-term if it ends up being a semi-regular occurrence over the next few months, as it is not warm as it occurs and also isn’t favorable for the stratospheric polar vortex strengthening uncontrollably. Is this just a brief mid-fall blip, or is it a sign of things to come? Remember, we had a hell of a blip / head fake last November and early December…I bit on it myself. 

Looking at some analogs, there are some very warm winters that show up in here, actually a couple of block-busters, and a few OK years. The first set focuses on west QBO La Ninas that reasonably match in at least 2 of the following 4 areas: Indian Ocean (negative IOD), east based La Nina, warm West Pac, and Warm NE Pacific. As a note, 1973-74 would have made this set, but it was an exceptionally strong La Nina so I chose to exclude it. 

The DJF 500mb composite and temperature/precip anomalies:

Temps with 1981-2010 climo:

Went back and forth on which climo period is best to use given 55-56 was in a noticably cooler climo, so for completeness here is the same map with a 1951-2010 climo:

Precip:

The bulk of the cold in these winters is frontloaded, with November and December both colder than normal for much of the northern and eastern U.S:

January is a more mixed month, with February featuring an incredible warm signal over most of the southern and eastern U.S. March is more mixed and leans cooler over the NE. 

A second set of years I played with focuses less on SST distribution and more on QBO and early-mid fall patterns and tropical forcing. There were few that matched all areas (though, 95-96, 10-11 and 17-18 were at least reasonable in all fields analyzed, even if not perfect). They show a similar (but slightly muted) signal overall for the whole winter, and still show a preference for any cold being more front-loaded with a strong warm signal for February:

(because all but 1 analog are after 1980 and this is a larger set, will not include the longer climo base period map…but it would make it appear warmer compared to the climo as it did to the other set)

Some overall takeaways…yes, a mild winter is favored (temperature wise) over the southern and eastern U.S., but there are ways to get cold air at times. Analogs are not as warm as you’d think (a disclaimer about ENSO-based analogs generally being too cold the last few winters is needed here, though last winter was exceptional due to the record wind speeds and cold observed with the stratospheric PV between January and March), and there is a strong signal for early cold and snow potential over the Great Lakes, Ohio Valley, Northeast and Mid-Atlantic among the analogs. Given the global pattern setting up heading into early October, I think the idea of an early blocky / cold pattern is there if we can get sub-seasonal forcing into the western Pacific between late-October and some point in November. However, for as surprisingly chilly as the analogs may be early, they’re more mixed in January and are exceptionally warm overall for February (and mixed / leaning cool for March in the Northeast), underscoring this type of winter does have “warm risks” that may be substantial. We’ll probably have a better idea on how the start of the winter will go in a few weeks when we see what actually happens heading through October.

So, I’m not overly pessimistic or optimistic at this point. I am curious though. 2017-18 is a decent analog (that winter held onto a +QBO below 40-50 mb all the way through), and I can see how an overall similar evolution to that winter plays out (though quite possibly without the SSW and hence, a more typical March).

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Late Night Scribbles on June 3rd Mid Atlantic Severe Threat

ML L

COD-GOES-East-regional-northeast.radar.20200603.053000.gif-over=map-bars=

Some nice nocturnal storms at present as a shortwave glances the edge of an EML advecting into the northeast.

There’s higher end potential with Wednesday’s action, though uncertainty still exists.

It seems like there are two mechanisms to watch for thunderstorm development and severe weather:

  1. Remnant MCS or MCV from tonight’s activity over the Great Lakes over PA/NJ during the morning and into the afternoon
  2. The southward advancing cold front during the afternoon and evening

A shortwave tracking from the Great Lakes towards northern New England will cause modest 500mb PVA and height falls during the afternoon hours, along with some right-entrance support in the upper-levels.  This, along with the front, provides “okay” forcing in central/northern PA and NJ, though forcing is weaker the farther south you go.

What happens with number one is uncertain, and will affect number two…

Let’s discuss the environment briefly:

We have an unusually untapped EML for the Northeast advecting in tonight into Wednesday…as dew points increase into the low-mid 60s on Wednesday, moderate MLCAPE somewhere on the order of 1500-2500 J/KG will develop by early-mid afternoon where sufficient heating occurs.  The NAM dew points seem overdone.  The mid-upper level speed max glancing the region will contribute to moderate to strong deep-layer shear of 30-50 knots (increasing to the north) across the northern mid-Atlantic.  Some mid-level dry air will contribute to over 1000 J/KG of DCAPE in the warm sector, with mixing to near or above 850mb where heating occurs yielding decent low-level lapse rates and inverted-V soundings.

These ingredients will support large hail and microbursts with any cellular activity, along with swaths of any damaging wind with any lines or clusters, with locally significant wind damage with any bows.

Let’s try to take a stab at the convective evolution now:

The MCS over the Midwest/Great Lakes as of this writing is the big wild card.  It’s running a bit faster than guidance, but is gradually becoming less organized and isn’t diving south of guidance yet.  There are a few ways this MCS can go into Wednesday:

  1. It runs slightly faster than guidance and moves into eastern PA late-morning and into NJ by the noon hour.  It holds together and brings showers/storms, with perhaps some severe threat developing by the time it gets into NJ…likely not a high-end solution though.  If this occurs, it likely nudges the frontal convection south, from parts of central/northern IN/OH into central/southern PA, perhaps northern MD, DE, and southern NJ.  This would to an extent limit the threat compared to what the environment suggests is possible.
  2. It doesn’t run faster than guidance and weakens considerably by morning.  However, it leaves an MCV and likely contributes to differential heating, which encourages new storm development by late-morning across central or northern PA that tracks into NJ early-mid afternoon.  Given the environment, MCV-aided fresh development a little later in the day could yield significant severe in eastern PA or NJ…with frontal convection from central and northern IN/OH into southern PA, MD, and DE.
  3. It does whatever the 18z NAM did and completely dissipates in the morning, with unabated heating along the front into northern PA and NJ, with frontal convection firing along I-80 early to mid-afternoon.

Option 1 seems most likely, though option 2 isn’t off the table if the MCS falls apart/slows down some by morning…option 2 has a fair amount of model support, but since it’s running on the fast side of guidance now I’d slightly favor option 1.  Option 3, aka the 18z NAM, will likely not happen.

The frontal convection from the Ohio Valley into the Mason-Dixon area will be in an environment of weaker forcing and more moderate, mostly uni-directional shear but with strong instability beneath the EML.  The weaker forcing may somewhat temper the coverage of severe weather, but splitting cells with a large hail / microburst threat are initially favored given the moderate but uni-directional shear.  Despite the weak forcing, the sloppy storm mode and flow parallel to the front does argue for fairly quick upscale growth into small lines and clusters…any lines or clusters that bow could produce corridors of severe wind given the high DCAPE, low-level inverted-V and sufficient shear.  Where exactly the northeastern extent of the frontal convection is over eastern PA/NJ will depend on how the MCS plays out.

The tornado threat generally isn’t much across the board, but if we see the MCV play out the right way over eastern PA/NJ (ie, later timing so it destabilizes in front of it), there could be a non-zero QLCS-style tornado risk.  Otherwise, I’d be rather surprised if there was a tornado with the cold frontal convection.

I can see two areas where the SPC considers enhanced risk probabilities or higher…one is E PA into NJ IF the leftover MCS/MCV is slow enough.  This could warrant 30 or 45% wind probabilities along with “hatching”, but, is highly conditional and I’m not sure we get it out of the gate at 6z.  The other is perhaps eastern OH or western PA into southern PA and perhaps northern MD or DE if there’s high enough confidence in scattered strong convection along the cold front here.  Despite weaker forcing and somewhat less shear, the thermodynamics are very favorable for this region, and scattered strong convection would yield enough severe coverage to justify a 30% wind risk in that region.  Given a fairly messy storm mode it’s hard to expect more than a 15% hail risk being justified, but if there are any more persistent discrete supercells  they could produce locally significant hail given the moderate to strong CAPE and steep mid-level lapse rates.

Fingers crossed that when I roll over and peak at the radar at 8 AM that the MCS isn’t already messing everything up!

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Some Refined Thoughts for NE Ohio

I drew this map around 5 PM and this incorporates snow after that, so a bit of this has already fallen.

2-26 NE OH

Some reasoning:

My thoughts on tonight haven’t changed much…the synoptic snow this evening is not that heavy or efficient, but may drop up to an inch or two in the Cleveland area and east side. Lake enhancement ramps up starting 9 or 10 PM and is most intense about 11 PM – 6 AM east of Cleveland, tapering slightly faster in the Cleveland area and secondary Snowbelt (and lingering past sunrise in NW PA). I still think the duration of the lake enhanced snow and improving ratios will allow for 4-6″ in the hills of the secondary Snowbelt by morning (when including the ongoing synoptic snow) and 4-7″ for most of the higher terrain east of Cleveland, with 6″+ likely in northern Geauga County and most of interior NW PA. Hi-res models continue to have 0.4-0.5″ of QPF as snow in the higher terrain by morning which supports the potential for 6″+ in the most favored hills with improving ratios overnight. Winds ramp up overnight as the ratios improve so it could be a pretty wintry scene by morning. It will likely only be 2 or 3″ tonight closer to the lake and in Cleveland proper, dropping off towards Sandusky where most of their snow was from the synoptic snow today with lighter snow from here on out. With a strong NW flow 2″+ totals tonight likely make it down to Mansfield, Akron and near Youngstown.

There’s likely a bit of a lull Thursday morning, though lake effect conditions become passable in the afternoon. Bands are likely disorganized Thursday afternoon but may drop a Coating to 2″ as they go, and may start southwest of Cleveland and in the secondary Snowbelt before shifting into the primary Snowbelt towards evening. Thursday evening through Friday morning is interesting as a surface trough and vort max move through slowly, adding lift. The edge of the better synoptic moisture just grazes NE Ohio, though with a long fetch and some Lake Michigan moisture should be sufficient. Although instability and moisture both are marginal to moderate, they are well focused in the DGZ Thursday night into Friday morning, so ratios will be good. In terms of band placement, NAM guidance has a band well southwest of Cleveland Thursday evening but the RGEM lifts it into the primary Snowbelt quicker…given the westerly flow, I heavily favor the RGEM, with a few W-E oriented bands likely from northeast Cuyahoga, Geauga and Lake through NW PA and SW NY. Given the good ratios, long fetch, and synoptic lift for a pretty long period of time I expect these bands to drop 3-6 or 4-8″ type amounts Thursday evening through Friday morning on parts of the primary NE Ohio Snowbelt into NW PA and SW NY. There will be little south of Cleveland, though some snow showers and the synoptic lift could produce a local coating to 1″ in the rest of NE Ohio in this window (though nothing organized outside of the primary Snowbelt).

There’s likely a relative lull later Friday morning into the afternoon, before a fairly deep lobe of synoptic moisture and embedded shortwave drop across Lake Erie Friday night into Saturday morning. As winds slowly shift from W to NW over the lake convergence will increase over the Snowbelt and there will be some upslope, with snow spreading south into the Cleveland area and eventually perhaps the secondary Snowbelt south of Cleveland again. Ratios again look high with a decent amount of moisture and some synoptic lift, so I think another few to locally several inches can occur Friday night into Saturday, most in the higher terrain east of Cleveland into NW PA and SW NY, but also with some snow for the Cleveland area and secondary Snowbelt.

Given the multiple windows for snow and ratios, I think a widespread 10″+ is likely for the hills in the primary NE Ohio Snowbelt into NW PA, with 15-20″ where bands are especially persistent. The Cleveland area and lakeshore will see less, but occasional periods of snow will gradually add up some. The secondary Snowbelt I’m debating if I went a bit too high, but if they get a few inches tonight, a little bit Thursday afternoon, and a couple inches or so Friday night into Saturday can squeak out the low end. Most of the snow west of Lorain and south of Medina, Akron and Youngstown will fall tonight, though they may see some snow showers at times Thursday through Saturday that can add additional light amounts.

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Some Ohio Snow Scribbles

With a NE Ohio focus…

Suppose I should post this before it starts snowing…

With the 850mb low track still near or just west of Sandusky into western Lake Erie, expect the heaviest synoptic snows to fall in NW Ohio. There, a long duration light snow from tonight (Tuesday night) through Wednesday evening, along with a period of moderate snow Wednesday afternoon when some modest fgen and deform ramp up will likely lead to a widespread 4 to locally 7″. The rain/snow line will slowly progress southeast during the day, with the west side changing over by early-mid afternoon, and Cleveland and the Snowbelt late-afternoon or early evening.

In north-central and NE Ohio there will be less synoptic snow as the weak deform band quickly swings through, thinking the synoptic snow is 1-2 or 3″ in Cleveland/Akron and 2-4″ on the west side…generally, the synoptic snow will be heavier closer to the lake as the best lift passes to the north. I do like the lake enhanced setup…temperatures get cold enough for lake enhancement from west to east between 8-10 PM and are quite optimal for several hours overnight with a NW flow backing towards WNW, with 700mb moisture not stripping away Cleveland points east until close to sunrise Thursday. Usually when the lake induced instability gets above the -10C level (which occurs late-evening) there will be moderate to heavy orographic lift snow in these setups in the terrain downwind of the lake until the 700mb moisture strips away. This gives a prolonged window from late evening through around sunrise Thursday of good lake enhanced snow east of Cleveland (tapers off slightly quicker south of Cleveland).

My general rules of thumb in these lake enhanced scenarios (when it’s cold enough, which it will be by late evening) are to take the highest QPF and assume at least a 15:1 ratio. Some hi-res models have up to half an inch of QPF as lake enhanced snow through Thursday morning, which yields potentially over half a foot. Another rule is that once it gets cold enough that rates in the higher terrain will be generally 0.5-1″ per hour until the 700mb moisture strips away…given the duration (on top of the light synoptic snow in the early-mid evening) this also suggests over half a foot of lake enhancement in the prone spots.

I’d like to see the hi-res stuff just a little juicier for widespread 6″+ totals, but think anyone in the hills south or east of Cleveland gets 4-7″, with 6″+ being likely in the highest terrain of Geauga (and also NW PA) through Thursday morning. With wind gusts of 25-35 mph and an increasingly fluffy snow overnight, there will be blowing and drifting snow and it will be quite wintry by Thursday morning. There will be much less lake enhanced snow near the lakeshore and I’d expect the immediate lakeshore to come in on the low ends of the ranges across the board…worried about the western lakeshore missing the better synoptic snow to the west and better lake enhanced snow to to the east, so we’ll see if those 4″ totals can be met in northern Erie County. Otherwise, I think the terrain does fine.

2-25 OH final

2-25 NE OH

In terms of the additional lake effect, conditions quickly become favorable for traditional lake effect Thursday morning and afternoon, though winds go westerly (maybe even slightly S of W at times) by late Thursday through the day Friday. The lake effect will likely be moderate and accumulate efficiently Thursday afternoon into Friday morning with a long fetch and little shear, good lift and moisture in the DGZ, good synoptic support from a vort max rotating around the closed low to our northeast, and the edge of the synoptic moisture grazing Lake Erie…but, I think the heaviest snows in this period are focused near or just north of the 322 corridor in the northern Snowbelt into NW PA with a more westerly flow. Given the duration of this setup and likely high ratios, where bands persist could see another 6-10″ of snow Thursday afternoon through Friday morning.

There may be a lull Friday afternoon as we briefly get into some synoptic-scale sinking motion, but conditions become favorable again Friday night into Saturday morning. A lobe of deep moisture and a good vort max move through, with winds slowly shifting from W to NW. I think this is more of a lake enhanced setup with weaker instability but abundant moisture and some lift, but good snow growth given the temperature profiles. Winds will favor broad convergence over the Cleveland area and Snowbelt which will help focus snow, and orographic lift will help too. Given the decent duration and likely good ratios, this likely supports another 4-8″ type accumulation from eastern Cuyahoga County into southern Lake, most of Geauga, inland Ashtabula and the higher terrain in NW PA Friday night into Saturday morning. Some accumulations likely get into the secondary Snowbelt south of Cleveland by Saturday morning as winds go more NW.

The lake effect likely winds down on Saturday as ridging builds in…time permitting I’ll post a map for just the lake effect Thursday afternoon-Saturday on Wednesday.

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Random Scribbles as Winter 2019-20 Continues to Flail Away

The winter of continued failure continues as any promising signals constantly get muted as we try to pull them inside of day 10.

In terms of snow from here on out, I think the Plains, Midwest, Great Lakes and perhaps northern NY into central/northern New England do just fine.  I think the Ohio Valley and most of the Mid Atlantic struggle, with places like southern New England more on the bubble but probably not great from here on out.  There are a couple of colder windows between now and mid-March, but also a couple of very warm windows in the eastern U.S…

February opens very mild. I am not opposed to 7-10 days where the cold actually presses far enough south to drop Upstate NY and New England to somewhat colder than average, and the northern Mid-Atlantic and Ohio Valley to near average somewhere around the second week of February, but am worried about a prolonged and significant warm-up after the middle of the month along the East Coast, with what may be winter’s last attempt at much of anything then possibly arriving late-February or early March. I agree with many on February likely coming in warmer than normal overall across the Ohio Valley and Mid-Atlantic, closer to normal in New England (but leaning warm in southern New England), and perhaps on the cold side over the central U.S. into the Upper Midwest and Great Lakes.

The second week of February interests me a bit as there will be a prolonged +East Asian Mountain Torque through the first week or so of the month, extending the Pacific jet through about mid-month. This will coincide with when the stratospheric PV is temporarily disrupted, which may increase the odds of a +EAMT actually dropping the EPO substantially. The EPS has trended towards a more -EPO over the last couple of days. The issues is tropical forcing; does it cooperate, or does it support a continued -PNA? Where the Kelvin Wave interacts with the low frequency forcing and amplifies in the Pacific may be key. Amplifying near the Dateline as the CFS has may support a brief rise in the PNA near or just before mid-February; amplifying much farther west as the EPS has would likely maintain and even intensify the -PNA. At the moment I’d lean towards the EPS, but not with 100% certainty. The prospect of a fairly significant -EPO does increase the risk of cold into the U.S. with a legitimate tap of Arctic air, though it will likely dive into the western and eventually central U.S. If the PV gets displaced towards Hudson Bay it may depress heights enough over New England to allow cold to seep south, but we’d need a rise in the PNA to really dislodge bigger cold into the Mid-Atlantic, which seems less than likely.

I am worried that a lot of warm signals occur at the same time just after mid-February after some confluence of colder signals (but not unanimous with tropical forcing question-marks) around the second week of the month. How quickly warmth returns may be influenced by the PV’s location, if the PNA does rise and push the PV to eastern Canada, it may be sneakily slow to warm across the northern tier (though the southern Mid-Atlantic and Southeast are different). It’s currently well-agreed-upon that the stratospheric PV quickly rebounds after the first week of February and is again quite strong around mid-month. This while the EAMT is projected to go negative by the EPS and CFS, while any Pacific tropical forcing from the Kelvin Wave likely dies down. This all suggests a retracted Pacific jet and very negative PNA developing after mid-February along with a +AO and +NAO. Yuck.

How the MJO plays out may modulate the duration and intensity of this warm-up. As Mike Ventrice pointed out on Twitter, there is signal for a strong MJO starting in mid-February. While the mean VP anomaly plots quickly wash out to the lower-frequency forcing in the Indian Ocean and central Pacific on the EPS weeklies from Monday, many individual members have a very strong MJO developing over the Indian Ocean somewhere between February 10th and the end of the month and propagating east. This is initially a very warm signal if it passes through the Maritimes at a high amplitude, which many members show. If this occurs around or just past the middle of February when other signals argue for warmth, it could get very ugly. Could we get 80 degrees into PA or NJ in February for the second time in three years? Maybe that’s too aggressive, but man a lot is pointing to that warm-up.

As much as I hate to seem like I’m wishcasting, a strong MJO would eventually significantly weaken the strat PV in later February or early March and as it worked east into the Pacific would eventually teleconnect to cold. That exact evolution is uncertain but would bring what would likely be the last chance of legitimate wintry weather either at the end of February or beginning of March, perhaps lasting up to a few weeks if stuff breaks right. Maybe the end of the season as we start shifting towards spring brings the best chance of a shake-up to actually happen.

At the end of the day this winter has been a train wreck both as a snow weenie and as someone attempting to forecast weeks out in advance, and I wish I had caught certain trends earlier. A borderline record strong +IOD followed by a stratospheric PV that’s been flirting with daily records at time this month are delivering strong and tough to stop results. As a snow weenie I hope it snows more, and I’m not ready for winter to be over…but a large part of me wants to launch this winter into the sun. Certainly some lessens to be learned.

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