Late Night Scribbles on June 3rd Mid Atlantic Severe Threat

ML L

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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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Active Sub-Tropical Jet Brings Interesting Snow Potential Late January

The pattern may not be record cold with a KU threat every 4 days, but there’s some interesting potential over the next two weeks, with 4 things to keep an eye on after this weekend’s sloppy system.

Threat 1: Tuesday/Wednesday of next week

EPS 138

I ultimately don’t think this one amounts to much except for maybe the Carolinas/southeastern VA.  A fairly potent sub-tropical jet shortwave slides off the Southeast Coast while a lobe of the PV gets displaced into southeastern Canada.  I don’t think the two can phase and as is, the trough axis looks too far east for anything to come up the coast without a phase.  But, it’s not horribly far off and if this was farther out I’d say to watch it closer.  As is there have been some hints that the southern piece may bring some snow to the Southeast near the coast, and that the northern piece may bring snow showers or squalls to parts of the Northeast, even without any sort of phase.

Threat 2: Somewhere around the 24h-26th

EPS 216

EPS 264

EPS 228 sfc

EPS 252 sfc

EPS 240 QPF

EPS 264 QPF

This one is fairly complex, but does have a low probability yet fairly high ceiling.

A robust sub-tropical jet shortwave will eject out of the southwest around the 23rd-24th, with what can be called bootleg (but persistent) blocking centered near Hudson Bay.

This is a robust piece of energy and some risk for an initial cutter does exist.  With the blocking and surface ridging over the top, there would likely be an eventual transfer to the coast after any cutter.  If there isn’t an initial cutter, the shortwave looks poised to go negatively tilted as it approaches the East Coast, which could favor a robust low developing offshore and moving up the coast.  There would be some needle threading involved for the coastal plain, but the lack of a SE ridge with a trough sitting off the coast ahead of this shortwave and blocking over Hudson Bay does increase the odds of this occurring…again, if there isn’t a cutter.  An initial cutter would warm our mid-levels quite a bit along the East Coast as the hour 240 QPF/850mb temp mean image shows…though, if there isn’t a cutter, or if there is a cutter but the Miller B takes over quickly enough, it is a plenty workable airmass.

How exactly this storm plays out will depend on the pattern in front of it over the Atlantic, the location/intensity of the Hudson Bay block, and what state the shortwave ejects into the Plains in.  Anything from a strong cutter with little snow or just some front end snow in the east, to a cutter with useful Miller B development, to no cutter and a much higher risk of a coastal, to perhaps a southern slider across the Mid-Atlantic seem to be in play based on the pattern and individual ensemble members.  Given how moisture-laden this system appears to be, if there is a coastal storm the potential exists for a significant snowfall somewhere.  Given the cutter risk and somewhat marginal airmass, at first glance I think this narrowly favors the interior Northeast for snow over the coast, but this is by no means impossible to pull off along the coast either.  If there is an initial low that comes out of the Plains and tracks towards the Ohio Valley or Great Lakes, some snow would certainly be possible in the Midwest and Great Lakes.

I am curious to see how quickly ridging may try amplifying over western Canada between the 24th and 26th…that has the look of something that may amp more than the ensemble mean suggests at this distance…if that occurred there could be more robust northern stream influence with this system which would increase snow potential.

Threats 3 & 4: January 28 – February 1

EPS 312

EPS 360

EPS 312 850

The ensembles suggest two more waves ejecting out of the southwest the last few days of January, with the polar vortex drifting towards the Davis Strait and allowing colder air to funnel into the central and eastern U.S.  With wave after wave moving off the East Coast through this period and potentially developing into a robust storm, the ensemble mean has a persistent 50/50 low feature.  With signs of more of a polar connection, a continued active sub-tropical jet, and a combination of an EPO cold press and 50/50 low ahead of any possible system keeping the baroclinic zone to the south, this period is very interesting for the East Coast.  However, it is way too early to determine which waves may amp, cut, be suppressed, etc, and it will take quite a while to sort out these wave spacing issues.

The EPO and NAO will be worth watching over the next week or two…I definitely can see a brief EPO tank during week 2, as there will likely be cyclonic wave breaks over the northern Pacific starting in about a week.  The NAO may dip if any of these storms deepens enough to get a cyclonic wave break over the NW Atlantic, though right now only a small number of ensemble members do that at some point.  Either occurring would make things more interesting and I do think we see the EPO try to go more negative than the ensemble means suggest right now during week 2.

Overall, the sub-tropical jet will give us plenty of opportunities over the coming weeks for snow across parts of the Midwest, Great Lakes, Ohio Valley, Northeast, Mid-Atlantic, and maybe even Southeast.  With still some uncertainty in the pattern and wave spacing issues that will take a lot of time to work out, along with some marginality to the amount of cold especially initially, it’s way too early to guess which wave may develop into a storm and produce snow as there’s a wide envelope of possible solution with every wave ejecting out starting around January 25th.  But, there’s definitely potential.

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Technical Long Range Discussion: Gradual Cooler Trend early-mid January

Quick Summary:

The very mild pattern that has developed to end December will slowly transition to a colder pattern through the first half of January…however, a positive AO and EPO, with a lobe of the polar vortex settling over Alaska to end December and start January, will slow the transition back to a colder pattern.  Any cold through the middle third of January is likely driven by improvements to the Pacific pattern, with a likely lack of a -AO or -NAO.  The Pacific pattern may become quite conducive to significant cold shots from late January into February, with the AO and NAO also likely trending more negative starting in late January.  Basically, winter will gradually return and then worsen through January if I’m correct, though don’t rush the transition in early-mid January.

Very detailed, technical discussion:

The fall and first few weeks of meteorological winter have been interesting…there have been periods of active convection in the western/central Pacific, resulting in relatively higher AAM orbits, some shots to the stratospheric PV, and a generally amplified ridge near the West Coast and into Alaska. There have also been periods of very little convection in the western/central Pacific and lower AAM when Indian Ocean forcing has dominated, during which we’ve generally seen the stratospheric PV strengthen with the Pacific pattern turning less conducive for cold weather in the central/eastern U.S.

The NAO has been variable and overall somewhat positive, though a period of –NAO in late November contributed to a Miller B off the northern Mid-Atlantic coast to start December that brought wintry weather from parts of PA and NJ into New England, and while an ongoing –NAO is temporarily staving off a significant Pacific-side-induced warmup. The NAO was staunchly positive for much of early-mid December. The AO has also been quite variable, with at least some of that variability tied to the evolution of the stratospheric PV.

analyzed 200 vp

We’ll start by looking at the analyzed 200mb velocity potential anomalies…the strong positive Indian Ocean Dipole is a constant feature, with uplift over the western Pacific much more variable. There was a distinct lack of Pacific forcing from mid-September through much of October, generally more forcing for most of November (though even then, it waxed and waned a bit), and then generally less during the first half of December.

gwo last 120

I’ve marked the periods of relatively inactive/active western Pacific convection on a plot of the overall global AAM anomaly over the last 120 days…note how generally, lower AAM has occurred when convection is quieter over the western Pacific, and higher AAM occurred when convection was more active over the Pacific. There are other factors influencing momentum, such as mountain and frictional torques, and positive East Asian mountain torque events occurred in early November, and again in late November/early December, helping to add momentum. Mountain torques are currently negative.

The generally more amplified Pacific pattern that has occurred during periods of more active western Pacific convection and higher momentum has featured a more +PNA/-EPO, and has also favored disruptions to the stratospheric polar vortex. The opposite has been true during periods of less western Pacific forcing/lower momentum, and this general evolution is clear when looking at the evolution of the annular mode/zonal wind anomalies with height since late October:

zonal wind anomalies

AM with height

On both charts, warm colors indicate a stronger polar vortex/annular mode/Arctic Oscillation. Note how during October and the first half of November, following a prolonged, deep period of low momentum and little western Pacific forcing, the strat PV intensified quicker than average. However, the period of increased Pacific forcing and higher momentum in November resulted in an amplified/blocky tropospheric pattern that favored disruptions to the stratospheric PV. These disruptions weakened the stratospheric PV in late November and early December, with that weakened PV/stratospheric -AO downwelling into the troposphere in mid-December and contributing to the recent shot of colder weather in the Northeast U.S. It should be noted that any vortex disruptions/warmings were considered minor, but it’s cool to see the troposphere -> stratosphere -> troposphere interaction on the above graphics, and tie it back to other processes (tropical forcing/AAM).

The recent period of a lack of Pacific forcing and lower AAM has again resulted in a strengthening vortex, with no sign of significant disruption into the start of the New Year.

Based on all of this, it’s possible that both camps in the little Twitter/forum disagreement about the AAM/stratosphere have merits to their argument. There have been periods of a lack of Pacific forcing and low AAM that have resulted in a stronger strat PV…there have also been periods of more notable western Pacific forcing, relatively higher momentum, a more amplified tropospheric pattern and resultant disruptions to the stratospheric vortex. At this point, I’d say the former has outweighed the latter over the course of the last few months, though both have occurred at times.

EPS 1-5 500

We’re getting our annual Christmas “torch” this week, mostly driven by a very poor pattern on the Pacific side…the PNA is negative with a strongly positive EPO as a large tropospheric polar vortex (TPV) anomaly sinks into Alaska. There is enough of a –NAO to induce confluence just east of New England, which keeps the warmest surface temperature anomalies west of the East Coast. Regardless, this isn’t a good pattern for snow for anyone in the central or eastern U.S.

RMM 40

As mentioned above, Indian Ocean forcing has largely dominated this month as a coherent MJO constructively interfered with the uplift over the western Indian Ocean that has been in place all fall due to the strong +IOD. This registered as a prolonged phase 2 RMM, moving into phase 3 after mid-month. The lagged phase 2 RMM composites for DJF show a propensity for much lower heights from AK down the west coast, as we’re seeing now, especially at lags 1-4. Each lag corresponds to about 5 days:

phase 2 lagged

The phase 3 composites show a similar pattern, except moved up roughly 1 lag:

phase 3 lagged

These lagged composites do show a strongly +NAO and AO, which does not match what we’re currently seeing. So, it’s likely that a combination of the Indian Ocean dominated forcing and low momentum has resulted in our very poor Pacific pattern, while the lingering effects of the late-November/early-December minor stratospheric warming along with other, smaller-scale features (such as cyclonic wave breaking over the NW Atlantic) are still influencing the pattern over the high-latitudes and Atlantic. The ultimate result is the torch isn’t quite as torchy for the East Coast, but both factors are worth consideration moving forward.

Moving forward, the easiest ways to dislodge the TPV anomaly over AK and overall unfavorable Pacific pattern are to bring a return of forcing to the western Pacific and to increase the AAM. Both start happening soon, but I’m not sure how much I want to rush this pattern change given 1) how long the phase 2-3 lags take to become more favorable 2) the strength of the TPV anomaly over AK and 3) how the lower AAM state with dominant western IO forcing has been the “base state” (though as November/early December showed, are not as overwhelming as they were earlier in the fall).

I’ll try to tackle the convection/tropical forcing portion of this equation first:

current VP

As the longer-running 200mb VP anomaly time-longitude plot above showed, we have recently seen a notable uptick in convection/forcing over the western Pacific. Another area of enhanced lift is approaching the central Indian Ocean, though is not quite as convectively active currently.

Multiple sources suggest that the activity over the Pacific right now is associated with higher-frequency forcing, while the activity over the Indian Ocean is associated with the slower-moving and more coherent MJO.

NCICS VP

Carl Schreck’s 200mb chi anomaly plots suggest that the MJO is still over the central Indian Ocean, and that the current uptick in western Pacific convection is as a result of an Equatorial Rossby wave and perhaps a convectively coupled Kelvin Wave juxtaposed over the lower-frequency uplift over the western Pacific. These plots suggest the MJO emerging over the western Pacific during the second week of January, when the MJO itself may then constructively interfere with western Pacific warm pool uplift.

Mike Ventrice MJO Kelvin

Mike Ventrice’s analysis plots also suggest the MJO is still somewhere over the Indian Ocean, with the bulk of the forcing over the Pacific a result of higher frequency processes. Along with the forecast plot two images up, the EPS, via multiple different plots, suggests the MJO emerging over the western Pacific during the first half of January:

Mike Ventrice EPS VP

Mike Ventrice’s EPS plots clearly show a coherent MJO progressing east, slowly, across the eastern Indian Ocean and Maritimes over the next two weeks. As this occurs it will destructively interfere with the subsidence from the +IOD, so it will not be that convectively active.

Weathermodels EPS VP

The Weathermodels VP anomaly charts also show a more coherent, slowly-propagating area of uplift emerging east of the subsidence in early January, and moving towards the Pacific, jiving with both the CFS forecasts above and Mike Ventrice’s plot.

EPS MJO

The multiple sources for lift and convection are likely causing the strange RMM behavior on the models over the next two weeks…there’s the shorter term western Pacific forcing from higher-frequency processes (that are resulting in more convection than the actual MJO over the Indian Ocean), with the MJO then emerging in the western Pacific likely during week 3, around or just after the end of these plots. That would likely result in a more coherent RMM propagation in early-mid January.

850 u anoms

Based on generally weaker convergence over the western Indian Ocean, along with strengthening convergence over the central Pacific, it seems that the +IOD is slowly losing its dominance while Pacific forcing is gradually starting to take hold. Given the waters over the western Pacific being warmer than average and the lower stratosphere remaining colder than normal over the tropics for the foreseeable future, it seems distinctly likely that once the MJO reaches the western Pacific during the first half of January that it becomes quite convectively active and propagates coherently through phases 5-8. While some affects from the +IOD will remain, I don’t think they’ll be able to dominate as much as they did in the fall. This has been my feeling for a while, though the general progression is a week or so slower than my thoughts in late November, which will likely slow the pattern’s evolution somewhat in January compared to my prior thoughts. As I mentioned in my last very long post, MJO propagation through the western and central Pacific tends to lead to blocking down the road at increasingly short lags from phases 4-8…for perspective, here’s phase 6’s lagged composites, where we may be somewhere around January 5-10th:

phase 6 lagged

So, tropical forcing should lend itself to a more favorable pattern for high-latitude (in particular, Atlantic) blocking during the second half of January, though the current convection over the Pacific is from higher-frequency forcing and may not have as significant of an impact in early January.

In terms of AAM/mountain/frictional torques…using the CFS for time/simplicity’s sake, though it’s not completely different from the EPS over the next two weeks…

CFS week 1 slp

There will be a very brief, weak +East Asian Mountain Torque during week 1 (so brief it doesn’t really show up in the mean forecast for the entire week on the CFS), a –Rocky Mountain Torque, and probably a modest increase in Frictional Torque due to the somewhat lower pressures just west of the Dateline than what we’ve seen. Overall, this will (and already is starting to) result in a very brief, modest increase in AAM…but not enough to really shake things up.

CFS week 2 slp

The CFS and EPS both have a much more substantial +EAMT during the first week to 10 days of January. This would add a more significant amount of momentum.

CFS week 3 slp

During week 3 the EPS and CFS suggest the +EAMT weakens (though does not turn as negative as it has recently been), though lowering pressures over the western Pacific likely result in some increase in FT. The CFS suggests a +RMT in week 3 as cold air descends east of the Rockies, though the EPS is more persistent with the +EPO and likely would maintain a –RMT in week 3.

The CFS and EPS weeklies both hint at a potentially more significant +EAMT in late January while FT remains positive.

In general, it seems like AAM will increase modestly this week but likely remain slightly negative through the end of the year…however, I think between an increase in FT and a strong +EAMT during the first week of January that we will see the AAM go modestly positive in early January. With FT increasing as lower frequency forcing returns to the Pacific and no signs of a prolonged –EAMT through January, along with some potential for RMT to increase if we develop a –EPO, the AAM will likely be neutral to modestly positive through much of January, with some potential for a more bonafide positive orbit the last week of January into early February.

CFS strat

As alluded to above, the stratospheric PV is intensifying after the displacement event earlier this month. The GEFS and EPS suggest a pattern the last couple days of December that may yield some disruption in early January, with hints of a stronger Aleutian low for a brief time along with a Scandinavian ridge. The Euro does show the first hints of some minor warming in early January. However, the pattern thereafter at the end of the EPS does not look conducive to further warming. The CFS, above, shows some modest weakening to the strat PV in early January, but has incredible spread thereafter, with a few members showing a more significant weakening in mid-late January while a larger portion intensify the vortex further. The anticipated propensity for more Pacific forcing by mid-January, and likely somewhat higher AAM state, may argue against the vortex continuing to intensify through mid-January, but it’s difficult to anticipate a significant warming event anytime soon.

Trying to sum it up…

The tendency this fall/early winter has been for a +PNA/-EPO to occur when there’s forcing over the western or central Pacific along with a relatively higher (near neutral or weakly positive) AAM…however, Indian Ocean forcing has been more dominant at times, resulting in multiple prolonged periods of limited/no Pacific forcing and lower AAM. One such period during most of December has resulted in our current +EPO/-PNA, lower AAM pattern along with a strengthening stratospheric PV.

A strong +EAMT in early January will attempt to amplify ridging towards Alaska…however, the current high-frequency forcing in the Pacific will dwindle by early January while the MJO remains over the eastern Indian Ocean. Along with the strong TPV anomaly currently over Alaska being tough to displace, this all will likely slow the flip to a much colder pattern in the CONUS. The AO should trend positive in early January beneath the intensifying strat PV, and the NAO also looks to trend positive too. So, we will need to rely on the Pacific side for cold. I do think we see a cool down over Canada in early January as the cold air with the PV over AK begins spilling southeast, but with a +AO and EPO (at least initially) it will likely struggle to surge south into the CONUS. We may see most of the CONUS average near or above normal for the first half of January with a north-south gradient, with places like the northern Rockies and upper Midwest likely averaging normal or somewhat colder and the STJ perhaps keeping the southern tier from completely torching. It’s not the warmest pattern ever, but with a +AO and EPO to start January it will be tough to see any large-scale negative anomalies over the CONUS for the first 10-15 days of January. Given the location of the PV and overall teleconnections to start January, I think there’s more risk to bust warmer than colder the first 10-15 days of the month…though for now I’m not forecasting a torch by any stretch.

As forcing returns more permanently to the western/central Pacific by the middle of January, I think we can more effectively improve the PNA and EPO and turn colder in most of the CONUS by late January. Increasing frictional torque (and perhaps Rocky Mountain torque) should make up for a likely downturn in East Asian MT around mid-January, keeping AAM from returning to negative values that dominated much of December. If East Asian torque increases more substantially again in late January as the CFS and to some extent the EPS weeklies suggest, the AAM may turn more significantly positive. If this occurs quickly enough, it may combine with any PV disruptions that occur in early-mid January for a more noteworthy weakening of the strat PV in late January. This could combine, along with the MJO progression I expect through early-mid January, to drop the AO and NAO late January into February.

In general, the pattern we’ve been in for the last few months with a tendency for amped ridging into the EPO region whenever we see an increase in Pacific forcing and AAM can be good for the central and eastern U.S., but we have gotten away from that recently and it likely won’t return immediately in early January. If this pattern continued through the winter, a 13-14 or 14-15 type look where we see PNA/EPO induced cold with less NAO help is doable. Assuming Indian Ocean forcing doesn’t overwhelm again for any extended periods and the strat PV doesn’t become too strong, that is a possibility. We have seen a tendency for cyclonic wave breaks and a –NAO over the NW Atlantic at times, and have to assume that without a major shakeup that tendency could return in later January/February when the tropical forcing becomes more conducive, and when the strat PV hopefully weakens a bit.

What I’m saying is for the central and eastern CONUS, we don’t need a significant strat warming event for a decent second half of winter in the central/eastern U.S. (the UK may be a bit different), as long as the vortex doesn’t remain uninterrupted well into January. A SSW would likely shake-up the pattern if it occurred, perhaps bringing a temporary +AO and warmup when it occurred followed by an increased risk for a –AO and –NAO. This currently seems less likely to occur, but given signs that the tropical forcing and AAM may become more supportive of disrupting the strat PV during the month of January, it’s something that can’t be at all ruled out. A SSW would change the evolution of things from late January through March, and I don’t want to speculate on how until one appears more probable.

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Technical Discussion: Trending Mild for December; Big Cold Looming for January and February?

This is going to be a long post discussing my analogs that I finally got around to updating along with how the next several weeks may play out…and attempting to tie those things together.

We’ll start with the analogs…after weighing a number of variables from tropical and extra-tropical SSTs, the solar cycle, QBO, low-frequency pattern in the tropics this fall and the general hemispheric pattern this fall, the larger set of analogs is:

1953-54, 1958-59, 1967-68, 1969-70, 1977-78, 1978-79, 1987-88, 1993-94, 1995-96, 2002-03, 2003-04, 2004-05, 2006-07, 2013-14, 2014-15, 2017-18, 2018-19

Here are the Nov-March 500mb composites:

All analogs NovAll analogs DecAll analogs JanAll analogs FebAll analogs Mar

For comparison, here is this November so far:

this Nov

Before diving in a bit more and analyzing features this month/trying to tie in to this year, I ran only certain sets of the analogs (based on factors such as QBO, low-frequency forcing in the fall, northern hemispheric fall pattern, and the stratosphere) and found some of them yielded a somewhat better representation of the November and likely December pattern than the full set (not that the full set is bad for November, it’s not)…I’ll post a couple of the more intriguing sets…

The closest set in November/perhaps December, likely not coincidentally, is the set where I only take years in the analog set that at least somewhat match this fall’s pattern…here’s those monthly 500mb composites:

fall pattern Novfall pattern Decfall pattern JanFall pattern febFall pattern mar

Here are the analogs with at least somewhat similar fall low-frequency forcing and QBO to this year:

VP QBO NovVP QBO DecVP QBO JanVP QBO FebVP QBO Mar

And to round out, the four analogs that had fairly perturbed stratospheric PV’s in December with at least some QBO similarity to this year:

strat qbo novstrat qbo decstrat qbo janstrat qbo febstrat qbo mar

I toyed with several other things, but I don’t want to post anymore analog 500mb composites right now and you likely don’t want to look at them, so I’ll stop.  A few takeaways are:

  1. Various composites do capture this month’s pattern respectably well
  2. Although a weak signal among the entire set, the various “enhancements” definitely strengthen the strong Aleutian low and generally warm North America look in December with a neutral to positive AO, and that look is quickly emerging on the EPS as we head into December after a chilly start. This is a strong signal for a mild December on the analogs for much of the continent.
  3. January shows a strong blocking signal in all composites, with –EPO blocking strongest with a decent –NAO signal too.
  4. February has the strongest –NAO signal in general, and is absolutely brutal for the Midwest and Northeast in terms of temperature anomalies on nearly every composite I did.
  5. March is not signaled to be particularly warm in any of the composites, though the QBO/December stratosphere set is not quite as chilly as some of the other sets.

So, tying everything into the pattern we have now and may see in December…

4indices

The start of December will likely be chilly for the central and eastern U.S. as a transitioning NAO, EPO-induced cold, and PNA spike work together to force cold into most of the CONUS.  However, the traditional teleconnections above quickly revert to opposite phases that are not supportive for cold as the AO also goes moderately to strongly positive in early December.  This suggests that the cold pattern to start December will moderate quickly, likely by the second week of the month.  A number of factors, aside from ensemble teleconnections, support this warm-up…

CFS slp 1-7

Trying first to look at the GWO, week one does feature a few sources trying to add momentum…a strong East Asian Mountain Torque, some Rocky Mountain Torque, and active western Pacific convection/tropical cyclone activity.  This will likely cause the GWO to rise modestly from its current weak negative value:

gwo last 120

However, week 2 sees the East Asian Mountain Torque weaken quite a bit while we lose frictional torque as pressures rise over the Equatorial Pacific, with at best weak positive Rocky Mountain torque.  We do gain some frictional torque back in week 3 if the models are right in some convection/lower pressure starting to return to the western/central Pacific, though the mountain torques are both progged to be strongly negative.  The EPS guidance is similar to the CFS in all of these areas, except it loses the +RMT quicker in week 2:

CFS slp 8-14

CFS slp 8-14

A general lack of momentum would argue for a less blocky high-latitude pattern, along with a weaker/more retracted Asian-Pacific jet and mid-latitude ridging over the Pacific, likely putting troughing near the West Coast.  For reference, here is the December correlation to AAM, with the color table reversed to depict what the correlation is when the AAM is negative:

Dec AAM

Some similarities to the pattern showing up later in the EPS runs, with a generally +AO, +NAO, and mid-latitude ridges over the western/central Pacific and across much of the Atlantic.

My general thought is that the sources of momentum that we should see over the next 7-10 days support the generally amplified pattern, and that the +EAMT through week 2 does support the +PNA that the ensembles re-develop during week 2 that would linger into week 3.  This added momentum would also support disruptions to the stratospheric PV.  However, there will be a clear loss of momentum most of weeks 2-3, which supports a generally +AO and poor Pacific pattern when combined with other factors.

200mb vp.png

Tropical forcing over the past few months has been dominated by a strong positive Indian Ocean Dipole, with essentially a standing wave over Africa and the western Indian Ocean.  The strong, displaced West Pacific warm pool has been somewhat active near and west of the Dateline, with frequent, strong subsidence in between in the vicinity of Indonesia.  Note that the AAM was at its lowest during the second half of September and first half of October when convection subsidence dominated much of the Pacific.

EPS MJO forecast

After a strong MJO propagation in early-mid November, the MJO has slowed considerably over the western Indian Ocean as it ran into the standing wave/enhanced lift that’s been there all fall due to the strong +IOD.  While a phase 2 MJO is not necessarily bad in the short term, it usually turns warm shortly after in December:

Phase 2 NDJ lagged

Each lag is roughly 5 days…note how with time, after a phase 2 RMM (which we’ve had for the last week or so and will have for another week or so), the pattern trends awfully close to what the EPS is showing developing within 2 weeks; a strong Aleutian low with troughing into the western US, a +AO, and anomalous mid-latitude ridges over the Pacific and Atlantic, and is not too different in some of the lags than the December pattern in a number of the “enhanced” analog packages.

In addition, Paul Roundy’s tool that incorporates the low-frequency forcing shows a similar pattern for a phase 2 MJO to what is likely for a time 10-15 days from now, and is an absolute torch in phase 3 (which is where some guidance tries taking it towards mid-December):

roundy 2-3

So, between analogs, ensembles, GWO, and tropical forcing, a number of factors argue for a +AO, lack of blocking, and unfavorable Pacific pattern taking hold in December.  Although the chilly start is very likely, it doesn’t last long, and the warm signal thereafter is very strong.  Note how the EPS, despite having a +PNA, generally is very mild at 2m at the end of its 12z Tuesday run:

EPS 11-15 temps

Now, despite the very strong (in my opinion) warm signal for a large chunk of December, there are some positive developments from above…

EPS 11-15 500

The added momentum from the mountain torques during the next 7-10 days, along with a subsequent strong Aleutian low, are favorable for disruptions to the stratospheric PV (in particular via wave 1 fluxes).  The GEFS and ECM both show very strong wave 1 activity ongoing now, and the GEFS suggests another surge towards mid-December (and the EPS pattern may allow for it).

ECM waves

It’s unclear whether or not this will cause a major Sudden Stratospheric Warming or not…while we have recently seen a minor warming, and another at least minor event is likely during the first half of December, only the extended range GEFS has been actually breaking down the polar vortex, while the op Euro, which only goes out to 10 days, has not yet shown this at 10mb.  While the vortex will likely be weaker than climo in December, whether or not we pull off a major SSW event or not is still very much up in the air.  That said, the PV will be fairly disrupted, and any downwelling for the upcoming disruption during week 2 would occur later in December into early January.

Attempting to tie the tropical forcing, GWO, stratosphere and analogs all back together now, here we go…

The stratospheric warming events will keep the tropical stratosphere anomalously cold for the foreseeable future, which should lead to enhanced tropical forcing.  The Western Pacific warm pool is somewhat warmer than normal displaced east towards the central Pacific:

global sstglobal ssta

This does argue for continued convection over the western and central Pacific, and the EPS weeklies and CFS both show that resuming mid-late December:

EPS chicfs chi

Although the eastern Indian Ocean and western Pacific are vaunted as the so-called “warm phases” of the MJO (and, at T=0 they do correlate to warmth for most of the CONUS), unlike phase 2, their lagged composites get considerably better with time.  So really, we do want the MJO to come out of the eastern Indian Ocean and propagate at some point in mid-late December…the improvements at later lags become apparent as early as phase 4, and improve through phase 8 for NDJ…I’ll post phases 4 and 8:

phase 4 NDJphase 8 NDJ

Given the cold stratosphere and general (albeit weaker) lower frequency forcing over the Pacific this fall, it seems likely we see another MJO propagation at some point.  Based on the EPS and CFS, that could happen at some point mid-late December.  The subsidence over Indonesia would suppress the MJO in phases 3-5, though the MJO would hopefully still progress through those phases and through the western Pacific, as the eventual result of that is a better pattern than what we’re seeing.

The GWO, after what may be a period of moderately negative values in mid-December, will likely climb later in December when/if convection returns to the western and central Pacific.  There are also some signs on the EPS weeklies and CFS that the –EAMT will weaken, and perhaps even become positive, in late December/early January.  This would also add momentum and support a more amplified Pacific pattern.  This would all occur in the general vicinity of when any stratospheric warming events in early December would try to downwell, and when the analogs all strongly suggest blocking becomes more likely in January.

So, while I think evidence is strong for a mild, if not full-blown “warm” period for a good chunk of December, I do think that if we see the MJO cooperate (propagate east by late December), along with at least a minor stratospheric warming event during the first half of December, that the factors would try to align at some point during the first half of January.  The type of pattern in late December, should the MJO propagate, may support further stratospheric PV disruptions should any warmings over the next couple of weeks remain minor.  The analogs are then very favorable through February for the eastern U.S., and don’t suggest a March torch either.

After all that, the tl;dr version is:

  • I think, with much more confidence than several days ago, that December winds up mild for much of the U.S., though there will be cold weather with snow potential to start the month, and I don’t want to rule out some improvement to the pattern towards the end of the month
  • Analogs strongly point to increased blocking in January, and I believe that if the (seemingly likely) upcoming at-least-minor stratospheric warming event does in fact occur, and the MJO is able to propagate east within the next few weeks, that this year would not be an exception
  • Per the analogs, once it gets blocky in January it can stay quite cold through February and perhaps into March.  This is seemingly supported by the QBO becoming more favorable for blocking through the winter, and by us likely becoming (in my opinion) farther removed from the very low AAM/GWO state we saw earlier in the fall.
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November 20, 2019: Quick Thoughts on Next Few Weeks

A few quick thoughts here on the next 3ish weeks.

After a pre-Thanksgiving cutter that will bring a warm-up to the Eastern U.S., what happens to open December seems to be up for debate. The ensembles agree on the potential for a legitimate, retrograding -NAO. While the attached images are pretty far out and the NAO is finicky, the initial block develops within the next several days and is of pretty high confidence to occur…the question is, how long does it maintain? There is more significant disagreement on the Pacific side, with the EPS developing a -PNA/+EPO, while the GEPS and GEFS have a more negative EPO and neutral to somewhat positive PNA:

EPS 11-15.png

GEFS 11-15

GEPS 11-15

There are a number of arguments to support the EPS’s -PNA/+EPO combo that would scour out the eastern U.S. cold as soon as the -NAO went away…in week 1, the combination of a negative East Asian Mountain Torque (note the anomalously low pressure over southeastern Asia), strong Indian Ocean forcing, and lack of Pacific forcing suggest a retracted Pacific jet and Aleutian ridging (a -PNA):

CFS slp 1-7

CFS chi 1-7

And not only that, but as has been pretty well-established by others on boards and on Twitter recently, a phase 1 MJO (western Indian Ocean forcing) does correlate to a -PNA/+EPO and ultimately, eastern U.S. ridging…the two images attached are the Paul Roundy tool which takes into account analogs that match the low-frequency forcing, and the other is the CPC lagged phase 1 composites (lags 0, 1, 2, and 3 pertinent for the next two weeks):

Roundy Phase 1

Phase 1 NDJ

So, plenty of evidence supports the EPS idea of a poor Pacific pattern being in place to start December (though it’s worth noting the -NAO would result in a non-zero wintry threat with that pattern for the eastern U.S., especially inland where it doesn’t take that much cold to snow in DECEMBER). It’s especially damning that the forcing remains in the western Indian Ocean for the foreseeable future once it gets there.

But, as you likely have realized by now I rarely post when the news is all bad, because I’m a complete and total snow weenie (despite how busy it gets at work when it snows). So, what’s the up-shot here? Well, it’s obviously the North American contingent of ensembles. How do they pull off that Pacific pattern, and is it realistic?

CFS slp 8-14

EPS SLP members

There is good agreement (and has been for a while) on a positive East Asian Mountain Torque occurring week 2 into week 3…the EPS (and the GEFS) has also been signaling some potential for a typhoon later in week 2 over the western Pacific. The +EAMT seems very likely to occur, but we’ll see about the typhoon at this junture. Either way, one or both would suggest the Pacific jet intensifying and trying to amp ridging into Alaska again…it’d also likely suggest a more neutral or even somewhat positive PNA and more negative EPO, especially if we get a typhoon feeding into the Pacific jet as a +EAMT occurs. If we amp the Pacific ridging again, although the cold likely would dump into the western or central U.S. initially, the -NAO would allow it to get into the east with no problem. If we don’t amp the Pacific ridging (so more like the EPS) it’s a closer to average pattern; the NAO keeps the eastern U.S. cooler than it would otherwise be, but the air is more Pacific than Arctic in origin with the EPS pattern. My gut is we do try to amp the Pacific ridging again to start December as the +EAMT and possible tropical cyclone suggest it, and we’ve amped that ridging whenever possible this fall. However, we’ll see if it’s closer to the GEPS/GEFS or more a compromise between that idea and the EPS (which wouldn’t be as cold). Regardless, if the the -NAO verified to start December it would suggest some snow potential for the eastern U.S.

Briefly moving ahead, I do think the persistent western Indian Ocean forcing eventually dominates and the eastern U.S. warms up…my guess is it happens at some point by mid-December. Once that happens I think we’re pretty mild for at least 2-3 weeks, and despite the interesting look we have to open December, I think it’s decently likely much of the eastern and central U.S. finish the month of December with a positive anomaly for temperatures. Exactly how warm the monthly departures are will be modulated by how the month starts, so a GEFS/GEPS outcome (colder start that takes longer to scour out) would temper the monthly warmth a good bit compared to the EPS (which I think is too mild to start the month, but may point to how the pattern inevitably ends up at some point).

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Quick Thoughts on November 11-12 Ohio Snowfall

Don’t have time write now for a super long write-up, but here are some quick thoughts…first the maps…

11-11 OH system snow

11-11 NE OH storm total

The thinking for NE Ohio is 1-2″ of synoptic snow near the lake and 2-4″ inland, highest in the hills. Temperatures marginal during the synoptic snow in the Cleveland area so hills should do better as they’ll be colder and also have some orographic lift to enhance the snow. Then the rest of the is lake effect.

Elsewhere, good setup for anafrontal snow with good right-entrance support, so everyone should get 1-2″…more in NW Ohio where they get into WAA snow Monday morning.

Parameters for the lake effect are very good, with a prolonged period of extreme instability and much better moisture than the last event. The flow is also fairly light and well aligned with decent synoptic support in a cyclonic flow under an upper-level trough. The temperature profile is ideal for high ratios with the lake effect. Question is band placement and persistence.

Winds swing NNE to NW late Monday night through Tuesday morning which should swing a Huron band from west of Cleveland east across the primary snowbelt, possibly getting into NW PA during the afternoon. Outside of this band, favorable parameters should allow decent orographic lift snow showers to allow for continued slower accumulations, especially in the hills. Both the NAM and RGEM show a vort max and surface trough dropping through late Tuesday afternoon or evening that may focus a more organized convergence band near Cleveland east into the snowbelt. This band may linger through most of Tuesday night as ridging builds south of the lake and turns winds more SW over land, with the snow eventually lifting NE and weakening by Wednesday morning. Any bands early Tuesday through Tuesday night can produce 1-2″ per hour rates in their core and perhaps some thundersnow.

The secondary snowbelt in southern Cuyahoga/northern Medina is odd…should get 2-4″ of synoptic snow, another 1-3 or 2-4″ through Tuesday AM as the Huron band swings east with orographic lift outside of it, and then not sure if the possible convergence band Tuesday evening/night sets up there or a little father north. Either way they should get several inches and could get 8″+ if they get the convergence band later Tuesday/Tuesday night.

For eastern Cuyahoga and Geauga, thinking 2-4″ of synoptic snow, another 2-4″ or so through Tuesday AM, perhaps up to 6″ if the Huron band is slow enough, and then likely at least several more inches where the convergence band sets up later Tuesday into Tuesday night. Either way, I think that higher terrain gets widespread double digits with well over a foot if / where the convergence band develops and is persistent.

Into NW PA, similar synoptic story, perhaps a tad more in the higher terrain, the Huron band may swing east into there and allow for beefier totals too, and even outside of that there will be some orographic lift. Think widespread 8″+ is likely in the higher terrain in southern Erie and northern Crawford Counties.

Let it snow!

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Final Thoughts on Thursday-Friday Snowfall in Northern Ohio into Northwest Pennsylvania

First, the maps…the nice one I posted on my FB page:

11-7 NE OH

I don’t have a nicer template that includes all of NW PA, but here’s a rough sketch:

11-7 wide view

And now an overly detailed analysis…

Synoptic rain / snow late Thursday morning/afternoon…

The back edge of a budding ana-frontal rain shield will mix with and change to snow across parts of central and northern Ohio on Thursday as colder air filters in.  This precip shield will be very progressive and only the back edge has a chance to briefly change to snow as the colder air catches it, so accumulations generally aren’t likely.  A few of the hills in north-central Ohio could see a coating on grassy and elevated surfaces.

BUFKIT 1

The forecast soundings for CLE and ERI actually aren’t bad with some signs of lake enhancement early in the afternoon, but it’s warm.  There’s moderate lift co-located with the snow growth zone and a respectably deep layer of lake-induced instability, with northerly winds that will hit the terrain south and east of Cleveland, along with in NW PA and SW NY well.  The issue is boundary layer temperatures in the lower elevations will be in the mid to upper 30s, so despite 925mb temps of -1 to -2C in this time period accumulating snow won’t occur in the low elevations (and along the lake it may stay rain).  The hills south of Cleveland may see a dusting and there could be up to an inch in the highest hills of Geauga County (and perhaps a dusting as far west as the heights suburbs).  The higher elevations of NW PA could see a quick, wet inch of snow with up to 2” in the higher elevations of SW NY as there’s more terrain and the precip shield is better-developed by then.  Again, no daytime accumulations in the lower elevations.

Lake effect Thursday night through Friday…

As a very deep cold airmass moves over Lake Erie Thursday night into Friday extreme instability will develop with a generally northwest flow.  The airmass is fairly dry and ridging starts building in from the west Friday morning making this a short-duration window for lake effect (especially in Ohio), so any decent accumulations will depend on any synoptic moisture / lift and upstream lake connections.

BUFKIT 2

There will be a few hour period in the evening behind the synoptic rain / snow when there’s little synoptic help and before any upstream connections establish.  There’s still moderate instability on the sounding and the winds in the lowest 5k feet are well-aligned, though the sounding is somewhat dry with a short fetch.  There will likely be snow showers in the early evening in this environment, especially in the higher terrain where there’s an orographic assist, but they won’t be organized and any initial accumulations won’t be especially good.

NAM 500 vort

We do get a shot of synoptic help with a decent vort max that moves over the lake from late evening into the overnight, moving east into Friday morning.  This adds some upward motion over the entire lake and does bring a brief shot of synoptic moisture, especially to the eastern half or so of the lake (extreme NE OH points east).

BUFKIT 3

CLE’s sounding improves as this vort moves overhead, with instability becoming extreme and moisture depth up to about 7k feet.  The flow is also moderate and fairly well aligned (a bit of shear but could be worse).  This may support a more widespread burst of snow with the vort max, along with locally heavy snow under any upstream connections that develop into NE Ohio.

BUFKIT 4

Erie’s sounding has even deeper moisture and instability, along with little shear beneath 10k feet.  This would support lightning with any more intense snow bands and greater than 2” per hour snow rates…NW PA also has more terrain to work with than NE Ohio and has a stronger upstream connection to a close-by Lake Huron.

NAM 925mb 1

The Lake Huron connection may start fairly far west Friday evening as winds behind the developing wave of low pressure will be north, but should quickly swing into the eastern half of Erie County or even extreme SW Chautauqua County NY ahead of the vort max before going back into more of Erie County towards early Friday.  The classic NW flow connection from just east of Marquette to near Traverse City to Lake Erie likely goes into the eastern suburbs of Geauga County ahead of the vort max with a slightly W of NW wind (growing up in Solon that’s where that connection goes with a 305ish flow)…it may back west a bit into early Friday behind the vort as winds veer a little more.  There may be another connection from Lake Michigan west of Cleveland but is likely less organized.  These will be the connections to watch for the best accumulations…how stationary they are or aren’t will be important to how high the highest localized totals are…the Lake Huron one is most interesting as it’s obviously the closest connection and also has deeper synoptic moisture than the NE Ohio ones will.

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Ridging builds in quickly towards Friday morning and by 10 AM, the Cleveland sounding is pretty dry.  With a lake induced EL still over 10k feet and a well-aligned flow there may be some lingering light to moderate snow showers with any left-over band that’s drifting around the metro or Snowbelt south or east of Cleveland, but it likely won’t be organized or accumulate all that much more by this point.

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Erie’s soundings still remain very impressive through the morning and half decent into the evening, so the Lake Huron connection (which may briefly drift into Ashtabula County around noon Friday before swinging back east) could still rip pretty well into Friday afternoon.  While the inversion heights and moisture finally start diminishing through the afternoon, there’s moderate instability and a reasonably well aligned flow through the evening, so as the winds go W and then SW Friday evening there may be one last flareup over NW PA that quickly lifts up the lakeshore but drops a little bit more snow as it goes.

Thoughts on band evolution and accumulation from the lake effect…

There’s likely an uptick late evening as the vort approaches over NE Ohio and NW PA.  Upstream connections will be important and eastern Erie County PA/Chautauqua County NY, along with eastern Cuyahoga/Geauga in Ohio (and perhaps a weak one southwest of Cleveland) may be hot spots.

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As the vort moves through it brings a subtle surface trough that may bring a more general burst of snow from Lorain and Medina Counties points east given extreme instability and enough moisture after midnight.  With winds briefly going close to WNW ahead of it, it’s possible a more organized west-northwest to east-southeast oriented band briefly develops over Cuyahoga and Geauga Counties and then swings southwest into parts of Lorain, Medina and Summit Counties as the vort moves through and pushes the surface trough south.  This may coincide with the upstream connection into the east side of Cleveland.  The Lake Huron band will likely swing west into much more of Erie County PA as the vort passes overnight Thursday night into early Friday.

Behind the vort into Friday morning winds don’t move much for a few hours, so the lingering convergent band into the Secondary NE Ohio Snowbelt (parts of Lorain / Medina / Summit) may last into the morning in some form, and the upstream connection into the eastern suburbs also lasts into the early morning.  The big show will be the Lake Huron band in NW PA that should go straight into the morning.

Winds do go more northerly Friday morning as the flow turns anti-cyclonic.  This should end any substantial snow in the Cleveland area, though whatever is left of the bands that establish overnight may continue to drift west and produce snow showers that might drop very light amounts through Friday morning.  This likely brings the Lake Huron connection briefly into Ashtabula County (and it should still be moderate to heavy) before it slowly swings back east Friday afternoon and evening across NW PA and into SW NY as winds slowly back to the W and then SW.  As it swings east enough instability remains for it to drop a little more snow as it goes.

For daytime Thursday accums I’m assuming little to none except for the highest hills in northern Geauga that could see an inch…along with inland NW PA and SW NY that could see an inch or two above 1000 feet.

In NW PA I except any Lake Huron connection to have 1-2”+ per hour snow rates from late evening through Friday morning, a period of 12-15 hours.  With that said, it likely is gradually moving most of the time so areas may only be under it for a couple of hours at a time.  Outside of the connection, the soundings do support snow showers due to orographic lift for about the same period which will keep accums going, albeit at a slower pace, in the hills outside of the narrower heavy band.  Ratios will become high away from the lake as well which will help with accumulations.  There could be a quick dusting to 2” Friday afternoon or evening as the remnant band lifts back up the shore.  All in all I don’t see how a lot of the higher elevations in NW PA don’t see at least 4-8” of snow, with locally higher possible if the band is at all persistent in any area.  My gut says someone gets over 8”, perhaps a foot, but with the band perhaps not locking in it’s hard to explicitly forecast that.  Also went 2-4” into eastern Ashtabula where there’s some terrain and where the band may drift into for a time around noon Friday.

In NE Ohio I’m going with a general inch or so away from Lake Erie for the uptick that occurs when the vort max and trough push through.  The eastern suburbs into Geauga County I went 2-4”…the thought is with a possible WNW to ESE band developing for a time ahead of the vort max and a likely upstream connection that keeps a narrow band going into Friday morning, along with enough instability for 1” per hour rates under any focused bands, that over 2” is pretty likely.  If any banding is more persistent someone may get more than 4”, but I think that’s very localized.  The higher hills in northern Geauga may be a little northeast of the banding Thursday night into early Friday but will see some orographic lift snow showers anyways, and could see a light accumulation during the day Thursday, so over 2” seems likely there.

The other interesting spot in NE Ohio is the “Secondary Snowbelt” as the remnant convergent band may sit in that area into Friday morning behind the vort max.  There isn’t a well-defined upstream connection into that area, I’m not confident they get any accumulation during the day Thursday, and ridging builds in fairly quickly so I’m not confident many areas get over 2” there, but a few may in the area I outlined.

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