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



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.


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.


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:


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…


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:


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.


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.


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).


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.


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.


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.


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.

NAM3 32

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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Early Thoughts on Thursday Night – Friday Lake Effect Snow off of Lake Erie

It seems that a brief period of light synoptic snow is fairly likely for most of northeastern Ohio Thursday afternoon. It probably struggles to stick in most areas, but some hilly areas may see a slushy coating.

As for lake effect, the air coming down at the end of the week is much colder than the airmass we saw last weekend when accumulating snow was mostly confined to the highest terrain in SW NY, however, the fetch is short which adds different complications.

Water temps are still fairly toasty in areas that we care about for a NW flow event, though the waters over western Lake Erie are into their yearly free-fall…

lake erie temps

The airmass Thursday night into Friday features 500mb temps of -32 to -35C (even colder near BUF), a difference of 45C or so over central and eastern Lake Erie. 700mb temps will be -22 to -24C, a 35-38C difference, while even 850mb temps will be -11 to -12C, a 24-26C difference. These values are all extreme. The issue is fetch and how long any lake effect lasts.


The Cleveland sounding at its peak is perfectly fine, with extreme instability and EL heights over 15k feet, little shear, and moisture depth to almost 10k feet. Unfortunately, this look only lasts a few hours with moisture depth and inversion heights falling quickly towards morning.


The Erie soundings are nuts, and last longer, well into Friday morning. There is a bit more shear, but that seems to be partially due to the model trying to simulate an intense lake effect band off of Lake Huron that is messing with the wind fields.

With our short fetch events, some sort of synoptic lift and/or upstream lake connections seem to be mandatory for good snow…

NAM 925.png

The Lake Huron connection should go into extreme SW NY or NW PA (the NAM does swing it west after this image). As usual with a NW flow, there will be some sort of Lake Superior/Michigan connection into the Cleveland area that can bring a narrow band or two of enhanced snow, especially in the hills. This connection likely dies down early Friday as the ridge builds in, though the Lake Huron one may last through a good portion of the day Friday.

NAM 700mb

The models do swing a weak shortwave through late evening into the overnight Thursday night. This would help for a few hours, though after this passes the ridging starts building in from the west and the environment becomes less conducive in NE Ohio (again though, downwind of Lake Huron will be conducive into a good chunk of Friday).

ERI overview

The above image is the NAM BUFKIT “overview” for Erie…the red contours are omega, greater than -15 is considered to be strong. The fill is RH (height is on the right), the snow growth zone is the pink/yellow outlined area that is sitting from 4-10k feet Thursday night through Friday, the low-level wind direction is plotted by the dotted lines (direction in degrees on the left), and time is on the bottom (in Z time). A busy image. But, what it shows is a classic “cross hair” signature (strong lift in the snow growth zone) for about 12 hours late Thursday night into Friday, indicative of heavy snow potential, with the winds only shifting slowly…moisture depth doesn’t really decrease until Friday evening. It is really hard not to get warning criteria snowfall with this type of signature.

My overall guess for NE Ohio is 1-3″ for most areas (perhaps little to none right along the lake) from Lorain and Medina Counties points east, even as far south as Akron/Canton and Youngstown. Short duration, short fetch events usually underwhelm, though with a NW flow many could see that light accumulation of snow. If the upstream connection parks in the hills south or east of Cleveland for several hours late Thursday night into Friday morning someone could see locally several inches, but I think that’s the exception and most see an inch or two.

For NW PA this seems like a very favorable setup, though whether or not the band slowly moves around or sits somewhere may determine how high the totals get. I think a widespread 3-6″ in interior NW PA (and extreme SW NY) is a decent bet, but locally up to or over a foot if the band off of Lake Huron is organized enough. The lakeshore will see less, but under the Lake Huron band can accumulate decently.

Let’s see how this trends over the next couple days…and probably more chances coming up with even colder air next week. I’m in NE Ohio for a few days around Thanksgiving, let’s not freeze the lake before then!

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Winter 2019-2020 Forecast Thoughts, A Technical Discussion

These maps have been released publicly via video, so I’ll go ahead and post them here to go along with a more technical explanation and commentary on what I’m thinking and where I’m most worried:

Temperatures (December-February):


Precipitation (December-February):

precip copy

Snowfall (first to last flake):


Since I’m using the maps I created at work, for clients on company time, I’ll “plug” our YouTube video…I take no responsibility for the extra glamor (the thumbnail and emojis), but the maps, anything in this post, or anything I verbally say in the video are all fair game to comment about.  Here’s the link to the video (the intended audience is the general public, so very minimal technical talk)…

Before getting any farther, a few of the odder acronyms used multiple times:

PMM: Pacific Meridional Mode (off-equator water temperatures)

IOD: Indian Ocean Dipole (a positive IOD has warmer waters in the western Indian Ocean and cooler waters closer to Australia and Indonesia)

QBO: Quasi Biennial Oscillation (a pattern of westerly and easterly winds in the stratosphere that impacts the polar vortex in winter, and also can interact with tropical thunderstorms by influencing the temperature at the tropopause and modulating instability)

“Low Pass” signal: The longer-duration tropical forcing pattern (standing waves as opposed to something that propagates like the MJO)

MJO: Madden Julian Oscillation, an eastward propagating pattern of enhanced thunderstorms that can influence the pattern globally.

As for the discussion, the general ideas used earlier in September to gather some analogs still largely hold…those ideas were:

  • ENSO: Neutral, though can allow for weak El Nino or weak La Nina if the year featured warmer waters near the dateline. Extra preference if coming off of a weak or moderate El Nino the prior winter-spring.
  • PDO: Neutral or positive (it’s positive right now though not strongly)
  • QBO: Positive trending negative, expecting the 30mb winds to flip to negative at some point in the early to mid-winter
  • Solar: Minimum
  • Indian Ocean: Positive IOD in the fall, can trend downwards during the winter
  • Off-equator Pacific SSTs: A positive to strong positive PMM (it’s very positive right now), with more weight given if it stays positive through winter
  • Atlantic: Neutral or positive AMO (it is positive right now though not strongly)
  • Tried to find matches based on similar tropical forcing in mid-late summer as this year, though not as strong of a weight.
  • Tried to find matches based on years with significant spring/summer high-latitude blocking, though was not as strong of a weight.

As a reminder, the analogs were: 1958-59, 1966-67, 1969-70, 1978-79, 1980-81, 1985-86, 1990-91, 1993-94, 1995-96*, 2003-04, 2004-05*, 2013-14*, 2014-15, 2017-18, with starred representing the highest scoring matches when considering all of the above and getting double weight.  That 500mb composite map for DJF looked like this:

Analog 500 sept

The big stand out features in the analogs were: significant potential for a –EPO, a somewhat +PNA favored with the Aleutian low displaced to the south, mixed NAO signal though at least some potential for a –NAO, and decent potential for a –AO over the course of the winter. The strong signal for a –EPO along with some signal for a –AO does imply increased potential for a cold winter in much of central and eastern North America, with the +PNA favoring a mild winter along the West Coast.

Just looking at the various drivers on their own…a neutral or weak ENSO (likely warm neutral with warmer waters near the Dateline), very +IOD this fall, +PMM, +PDO (that has seemingly trended more positive in September, we’ll see what the monthly value is), descending –QBO with a lingering +QBO in the lower stratosphere, solar min, and the “low pass/lower frequency” signal this summer looking somewhat Nino-ish still with the strongest chi anomalies over the central/eastern Pacific, you get an interesting picture. The lingering +QBO and strongly +IOD/Modoki-ish look, along with the ongoing signal in the tropics late summer/early fall, support the most active convection occurring outside of the West Pac warm pool (so, outside of phases 3-6 of the MJO) heading into winter…meaning it would be most favored/active in phases 8, 1, and 2. We already are seeing this now, with tropical forcing most active over Africa and the western Indian Ocean, with some of the longer range models (EPS and CFS) showing signs of life over the central Pacific in a few weeks. Here is a look at the “low pass” signal via velocity potential/chi anomalies since July 1st:

Jul-Sept ChiHere are the CPC’s 200mb height composite anomalies for phases 8 and 1 of the MJO in December-January-February:

MJO 200mb djf

During winter, phases 8/1 of the MJO are both cold. The analogs are overall somewhat chilly for December in the central/eastern U.S. (though in general, January and February are the coldest months compared to normal in the composites), and this is generally done via PNA/EPO ridging in December in the analogs with a +NAO for December in the mean. Interestingly, a Phase 8 MJO in DJF strongly favors a –NAO…given the lower frequency signal appears to involve over the western Indian Ocean, Africa, and the central/eastern Pacific, perhaps that’s why Paul Roundy’s low pass analogs show a strong –NAO signal (and somewhat of a -EPO signal) heading into December:

roundy low pass

Phase 1’s composite looks more like the analog composite for December, with any cold coming from PNA/EPO ridging more so than a –NAO. Given the propensity for a –NAO over the last several months, SSTs in the tropics potentially favoring convection in an area that teleconnects to a –NAO in winter, and current low-pass signal analogs also suggesting the same, it seems there are multiple signals pointing to the possibility for NAO help as early as later in November and December. The analogs and tropical SSTs, along with to some extent the same low-pass analog posted above, also suggest potential for a +PNA/-EPO as early as later November or December. Quick Siberian snow cover advance, favoring a stronger Siberian high and positive East Asian Mountain Torque events and subsequently an Aleutian low may also enhance the potential for a +PNA/-EPO to start winter.

Several other well-respected long range forecasters have mentioned the potential for winter to “start early” this year in the eastern U.S. (and if the NAO is involved, Europe) compared to most recent winters…if the tropical signal (driven by the strong +IOD, lingering Nino 4 warmth, and to some extent the +PMM and AMO) is not strongly disrupted at the wrong time by intra-seasonal variability (such as an amplified MJO passage through phases 3-6, or a negative East Asian Mountain Torque), then there appear to be multiple reasons for optimism as early as late November or December, with some signal for both +PNA/-EPO ridging and perhaps a –NAO. Assuming the lingering +QBO in the mid and lower stratosphere, strong +IOD, Nino 4 warmth/+PMM, and cooler waters near Indonesia persist over the next few months, the “ingredients appear to be in place” for less convection in the West Pac, suggesting a lessened risk for a destructive MJO passage. We’ll see how the tropics signal plays out as we head deeper into fall.

Heading through winter, a big potential source of uncertainty will be what happens as the –QBO continues to descend in a deep solar min and the +IOD presumably weakens (which is climo for northern hemisphere winter). Unless a stronger move back towards El Nino occurs this fall (still not ruled out with the warmth near/west of the Dateline, though if this occurred it’d very likely only be a weak El Nino), a weakening +IOD and any strat warming events (which may become favored during mid-late winter assuming the –QBO continues to descend when combined with the solar min) may increase the risk for a more amplified MJO in the “unfavorable phases.” So, there likely remains some warmer risk in the eastern U.S. in particular for mid-late winter.

A hat-tip to “Snowy Hibbo” on the 33andrain forum (website: (twitter: @longrangesnow) for inspiring some further discussion on the current disconnect between the tropics and extra-tropics, and how this may tie into the cold December idea in the central/eastern U.S. and also the potential warmer risk later in winter…


The extra-tropics have been firmly in a more La Nina-like state since late June/early July…the above shows the Global Wind Oscillation since the end of June, and negative values are more characteristic of a La Nina atmosphere.  This contradicts the more El Nino like tropical signal (that is evidenced by the chi anomalies and other indices such as the persistent -SOI).

Anecdotally, La Ninas tend to be colder in December, milder in February in the eastern U.S., with El Ninos the opposite.  With all of the other factors above, driven by potentially favorable tropical forcing for cold occurring heading into early winter with a La Nina like extra-tropic base, does that add confidence to winter actually starting in later November/December in the east?  Also, as the +IOD presumably weakens in winter per climatology as discussed above, does the lingering El Nino state in the tropics also weaken, opening the door to enhanced convection in the West Pac during the mid-latter portions of winter?  This would bring that warmer risk to the eastern U.S., and may occur at a time when stratospheric warmings are more favored, which already increases that warmer risk at least temporarily before any SSW/vortex disruption downwells into the troposphere.  It seems the interplay between the Nina-like extra-tropics, and Nino-like tropics, and how long the latter hangs on, is a large point of uncertainty for winter forecasts.

It’s worth noting that La Ninas are, on average (with some exceptions to both cases), much colder as a whole across Canada and the northern U.S. than El Ninos in winter, and the forecast temperature map as is would probably fall closer to La Nina climatology than El Nino…so this La Nina element to the pattern may not be the worst thing for a lot of the U.S. if it continues into winter.  Many analogs had a similar interplay between a more La Nina-like pattern in the extra-tropics when coming off of an El Nino (or otherwise had Nino 4 warmth), and had cold winters, so this wrinkle isn’t necessarily bad if you want cold and snow in the U.S.

If the Nino base state holds on in the tropics through the winter, the warmer risk in the eastern U.S. may be mitigated a bit.  If a strat warming event occurred, there may be additional risk for –NAO blocking later in winter. It bears noting that the monthly analog composite mean has a +PNA/-EPO through March (strongest in January) along with a more –NAO in all three months (compared to December when the composite NAO is positive), with January also having the most –NAO out of JFM (February a close second).

Beginning to tie this into the maps…comments on the temperature: It seems very likely that the Great Lakes and Upper Midwest see a cold winter based on the analogs and presumed tropical signal favoring a –EPO and +PNA this winter. It conversely seems very likely that the West Coast is mild. The East Coast is a mixed bag…perhaps due to being snake bitten last winter, and remembering mid-January-February of 2018, I had a hard time bringing the below average temps for the winter too far southeast. I’m worried a somewhat brief (a couple-few week) period of very mild temperatures is possible as extensively discussed above. However, it can conversely be argued that barring an intra-seasonal very mild pattern that something can argue for colder weather at nearly all points in the winter (and early spring) in the eastern U.S…so if the cold signals trend stronger heading into fall, I may end up bringing below-average temperatures farther southeast. As for the Southeast U.S., due to similar reasoning I currently have somewhat mild temperatures for the winter as a whole, though there is a strongly implied risk for EPO-induced cold shots that can bring wintry precipitation into the Deep South even if the winter averages a bit warmer than average.

As for snow/precip…the analogs strongly suggest a wet/active winter from the Ohio/Tennessee Valleys into the northern Mid-Atlantic and New England. I couldn’t think of a good reason to deviate, though if things trend much colder in the east it would inherently trend drier. The analogs are also generally dry to very dry across much of the southern U.S., especially in Texas. A –EPO would suggest an active northern stream into the Plains, keeping precip from ending up too far below average and likely favoring above-average snow in the Upper Midwest, parts of the Plains and the eastern slopes of the Rockies. The analogs are very dry in the Pacific Northwest and couldn’t find good reason to deviate. Conversely, they lean dry for California and the Southwest but not as strongly as the Pac NW…with a lingering +QBO and El Nino influence to start the winter, along with a very +PMM, there is definitely potential for precipitation/mountain snow in California and the Southwest, especially through the first half of winter. For now went near normal precip/mountain snow in that area…may need to trim the below normal in the Dessert Southwest and perhaps consider above-average for parts of California if the lingering El Nino signal persists through the fall.


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Mid September, 2019 Winter Analog Notes

I posted this the second week of September on a couple of forums, just posting it here for the record/reference later…

Playing with some analogs right now…here are some analogs, discussion, and caveats, focused mainly on the U.S….FWIW, my temp map for work is not as cold as this analog composite would imply and allows some risk for a SE ridge to crop up should things trend too far in the wrong direction (read below).

Here are my key assumptions when looking for analogs right now…the analogs used to compile the attached 500mb map were found by subjectively rating every possible analog on each of the following criteria. Ultimately I give ENSO, PDO and QBO the most weight along with the solar cycle, though all of these are considered.
  • ENSO: Neutral, though can allow for weak El Nino or weak La Nina if the year featured warmer waters near the dateline. Extra preference if coming off of a weak or moderate El Nino the prior winter-spring.
  • PDO: Neutral or positive (it’s positive right now though not strongly)
  • QBO: Positive trending negative, expecting the 30mb winds to flip to negative at some point in the early to mid-winter
  • Solar: Minimum
  • Indian Ocean: Positive IOD in the fall, can trend downwards during the winter
  • Off-equator Pacific SSTs: A positive to strong positive PMM (it’s very positive right now), with more weight given if it stays positive through winter
  • Atlantic: Neutral or positive AMO (it is positive right now though not strongly)
  • Tried to find matches based on similar tropical forcing in mid-late summer as this year, though not as strong of a weight.
  • Tried to find matches based on years with significant spring/summer high-latitude blocking, though was not as strong of a weight.
The analogs I went with for this exercise are…1958-59, 1966-67, 1969-70, 1978-79, 1980-81, 1985-86, 1990-91, 1993-94, 1995-96*, 2003-04, 2004-05*, 2013-14*, 2014-15, 2017-18. Stars denote highest scoring matches when considering all of the above and double weighting.
Analog 500 sept
Obviously some of these are very cold winters for the eastern U.S. and the composite look is cold. There is a strong signal for Alaskan ridging and a -EPO, which is not a warm pattern for the central and eastern U.S., though where exactly the cold dives in can make a difference for the eastern U.S. with more mixed signals on an NAO. A neutral-ish ENSO and +QBO to start winter aren’t great signals for a -NAO, though the deep solar min is and the QBO will be improving through the winter. The SSTs up there support a -NAO, but aren’t a strong forcing mechanism on their own. There is not a strong correlation between negative summertime NAO and subsequent winter NAO and the forcing mechanisms are different, so the persistent -NAO this summer doesn’t really help or hurt.
In terms of what to watch for in the eastern U.S. in terms of swinging warmer or locking in cold…I’ll be watching to see if we hang on to the warm waters near the Dateline in the Equitorial Pacific and the +IOD as we head into fall. If we keep those we are more likely to see convection near the Dateline this winter which usually forces an Alaskan/western Canadian ridge. If we see a stronger push towards La Nina and lose the warmth near the Dateline, the risk for a more amped SE U.S.  ridge increases…we don’t need the Nino region 3.4 anomaly to be above 0C to have a cold winter in the east, but region 4 is pretty important and needs to stay warmer IMO.
Based on the persistent -SOI and forecast generally weak trades over the central and western Pacific over the next week or so, there won’t be a big La Nina push in the near-term, though the recent easterly trade surge did nudge things in that direction over the last couple of weeks and there’s still a lot of time for that to resume.  The waters near the Dateline and just west remain fairly warm both at and below the surface, and until that goes away some move back towards a weak Modoki El Nino also can’t be ruled out.  As we saw last winter, a SSW can really enhance tropical forcing/convection over the West Pac warm pool (which is usually warm for the eastern U.S.), so an initially +QBO and seemingly low risk for an early SSW may give some margin for error…but if the SST pattern becomes unfavorable between Australia and S. America for convection near the Dateline the pattern more likely supports eastern U.S. warmth this winter.
The analogs that have an Aleutian low in October generally had much colder subsequent winters than the ones that have an Aleutian high and subsequent trough over western Canada in October. I’m aware of what the longer range guidance hints at to start October up there, but wouldn’t lock it in yet. A continued drop of the QBO heading into the fall is also important for increased high-latitude blocking prospects as we head into winter and a lower risk for the Pacific jet to be too strong/zonal into the west coast, which would likely result in quite a bit of warmth for North America given how mild the entire Pacific is.
An additional caveat is the northern hemisphere water temperatures from the tropics up to the polar regions where there was another near-record sea ice melt are ON FIRE. Do older analogs break down as we continue to warm? If so, how do things change? The warming baseline gives less margin for error (you probably don’t luck into below-average or even average temps, you either have cold signals or you torch).
Lots of food for thought…I definitely don’t hate the prospects for a snowy winter from the Ohio Valley and Great Lakes into the Northeast, and the warm PMM and any remaining warm ENSO influence can bring some snow prospects to the mountains out west, but the pattern can turn warm quickly if we lose the warm equatorial waters closer to the Dateline.
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8/27/19: The Uncertain Case of Where Dorian Goes, how Strong it Gets

Dorian IR

Infrared satellite image of Dorian as of early Tuesday evening

(When dealing with tropical cyclones and other adverse weather, always follow the advice of local officials for your safety)

Dorian, a sputtering Tropical Storm between the Lesser Antilles and Puerto Rico over the eastern Caribbean that has its eyes set to the west-northwest towards Puerto Rico, the Dominican Republic, parts of the Bahamas and eventually parts of the Southeastern U.S.  A lot of uncertainty is involved in both the track and intensity forecast for Dorian over the next several days, meaning potential U.S. impacts range from very little to potentially significant, so let’s try to break it down.

The satellite appearance of Dorian continues to leave something to be desired, with intermittent bursts of deep convection that struggle to maintain long enough for substantial intensification due to very dry air surrounding the storm that continues to get entrained into the circulation.  Outflow is decent especially given the storm’s small size.

Dorian radar

Radar image of Dorian from early Tuesday evening

Dorian was terribly disorganized earlier Tuesday with a significant SW to NE tilt to its vortex…since then, aircraft reconnaissance and eastern Caribbean radar suggest the surface center re-formed north, closer to the more persistent convection.  Radar has shown some formative attempt at some inner core structure with a curved band wrapping partially around the circulation, though for now, the continued intermittent/bursting nature of the convection will likely keep short-term intensification from getting out of control, and current aircraft recon finds that Dorian has remained rather steady (perhaps modestly intensified) since the last plane was in there earlier this afternoon.

Dorian WV

Water vapor imagery from early Tuesday evening showing Dorian, dry air surrounding it, and an upper-low in front of the Tropical Storm

As for what lies ahead in the short term…dry air will continue to dominate the environment around Dorian until it gets north of the Greater Antilles, and shear will increase by Wednesday as Dorian encroaches on an upper-low and enhanced southerly upper-level winds to its west.  This upper-level dropsonde from around 5:30 PM EDT shows a notable amount of shear above 500mb along with a very dry environment near the inner core of Dorian…the shear will only increase more into Wednesday.

Dorian sonde

Upper-level dropsonde near Dorian, showing very dry air (separation between red [temperature] and green [dew point]) and some wind shear (barbs on the right-hand side)

Dorian’s increased organization today may make it slightly more resilient to these potential negative environmental influences, though it’s still a small, fairly weak storm, so there will likely be some weakening Wednesday into Thursday.

Dorian steering

Mean low-mid level flow as of early Tuesday evening

In the short term, Dorian will turn northwesterly as it approaches a fairly large weakness in the sub-tropical ridging to its north.  This northwest turn, along with Dorian’s center reforming farther north early this afternoon, essentially guarantees that Dorian will not track over or even that close to the Dominican Republic…however, a track over or extremely close to Puerto Rico seems very likely now.

Between Dorian’s small size/fairly weak nature…moderate shear and continued dry air intrusions over the next couple of days…and land interaction with Puerto Rico on Wednesday (and perhaps some indirect interaction with Hispaniola), there may very well be a net weakening through late Thursday.  Many global and dynamic hurricane models suggest this.  However, with much less interaction with the very mountainous Hispaniola, some increase in organization since Tuesday morning, and only 24-36 hours of markedly stronger shear, it seems considerably more likely than not (though still not 100% certain) that Dorian makes it north of the Greater Antilles as a tropical cyclone.

GFS 200 96

Forecast upper-level winds over the western Atlantic on the GFS model, valid Saturday morning

Should Dorian survive the next 48 hours, conditions will be considerably more favorable for intensification beginning Friday and through the weekend over the Bahamas.  The upper-low bringing increased shear over the next couple of days is expected to dissipate as an upper-level anti-cyclone develops over Dorian, bringing very light shear.  In addition, multiple outflow channels may develop, with a potential jet streak to the north being particularly concerning.  I find it worrisome that global models are developing this favorable environment before they really deepen Dorian, leading me to believe there’s a good chance this environment isn’t strongly/perhaps incorrectly influenced by Dorian on the models.

Dorian GFS 96 RH

Mid-level relative humidity forecast from the GFS model, valid Saturday morning

In addition to reduced shear, Dorian will find much less dry air north of the Greater Antilles this weekend.  While it may take some time to mix out the already-entrained dry air Friday-Saturday, the much moister ambient environment and lower shear, along with Dorian’s small size, should ensure that the process happens reasonably quickly.

Dorian SSTs

Water temperatures across the western Atlantic Ocean

The waters beneath Dorian will also be quite warm this weekend (and are warm to a sufficient depth).  The combination of a small storm, low shear, strong outflow, a moist environment and plenty warm waters could result in significant intensification starting Friday or Saturday as Dorian tracks northwest towards the northern Bahamas if the system can maintain a closed circulation through Thursday.  A high-end hurricane would be possible if all goes properly, and some recent modeling is pointing towards that solution.  There’s likely a middle ground solution between the (unlikely) total dissipation over the Greater Antilles and the much more troubling “holds together now, major hurricane later” that involves Dorian opening up into a trough over the next couple of days, but re-developing into a tropical cyclone this weekend.  That would likely put some sort of a cap on the future intensity of Dorian as it nears the northern Bahamas and threatens Florida, though still may have impacts if the storm hits land.

It’s ultimately still a little too early to know with much confidence how strong Dorian will be by Sunday or Monday, with that answer becoming clearer by Thursday when we see how Dorian fairs in a more hostile environment with some land interaction starting on Wednesday.  The environment and some modeling over the weekend is alarming, but we should not over-look the shorter-term questions and sound too many alarms yet…a heightened state of awareness in areas potentially affected is warranted though.

Dorian EPS 96

Large-scale weather pattern predicted by the European ensemble Saturday morning

Dorian will continually generally on a northwest track through the end of the work-week.  By Saturday, a trough over the Northeast U.S. will lift out and sub-tropical ridging will build back in over the Southeast, pushing Dorian more “left” and causing a bend back to closer to a westerly than northwesterly motion.

Dorian’s northwest motion in the meantime may take it largely north/east of the Turks and Caicos (perhaps grazing the eastern islands), though the turn back to a closer-to-west motion this weekend may bring it very close to or into the northern Bahamas.

The next item of interest is of course Florida, as the pattern as shown on Saturday above would push the storm towards the eastern Florida coastline as a strengthening storm.  This is where uncertainty increases…it is very likely Dorian at least approaches the Florida peninsula due to the ridge build in to its north to start the weekend, but does it keep going until hitting the coast?

Dorian EPS 144

Large-scale weather pattern predicted by the European ensemble Monday morning

There is still uncertainty regarding the final approach/potential impact to Florida, as a weak, progressive shortwave trough is expected to zip through the Northeast Sunday into Monday, eroding the ridging north of Dorian a little bit.  The exact speed and amplitude of this trough along with Dorian’s location, size, and intensity will determine if this shortwave has much/if any tug on Dorian and potentially allows it to recurve just east of the Florida coast.  The window for this occur is small as the trough zips east and ridging builds back in Tuesday into Wednesday next week.

Dorian EPS N Hem 144

Hemispheric forecast weather pattern from the European model valid Saturday morning

The hemispheric view shows the delicate/uncertain nature of the exact evolution of the fairly flat/progressive shortwave that may open up a weakness in the ridging Sunday/Monday north of Dorian.  The exact amplitude of that shortwave will be influenced heavily by a ridge over the western US (and to some extent by a large trough over eastern Canada).  That ridge out west is influenced by a number of features over the northern Pacific…so a lot can still change.  It’s a narrow window to recurve before hitting Florida, with a number of features causing a lot of uncertainty this far in advance.

Dorian GEFS trend

GFS ensemble pattern forecast “trends” over the last two days’ worth of runs, valid Sunday morning. Reds indicate a trend towards more ridging, blues towards more troughing

The GFS ensembles don’t show an encouraging trend in that regard, with the ensembles trending towards the trough hitting the Pacific NW earlier/stronger, raising heights over much of the northern/central U.S. and likely dampening out our shortwave even more.

So, long story short, Dorian if it holds together the next two days will likely get very close to or hit Florida Sunday or Monday as what could be an intense hurricane.  However, uncertainty over its fate over the next couple of days, along with a very delicate forecast for the pattern steering Dorian near the Florida coast, makes it way too early to make any sort of proclamation about how this story ends.  Needless to say, Florida has a storm to definitely watch closely, with nearby areas such as the Southeast (should it turn north and just miss Florida) and the Gulf Coast (should it not turn north and continue west) also needing to closely monitor Dorian.

Dorian EPS track

European ensemble forecast tracks for Dorian through the next 10 days

The European ensemble tracks show the risk to many areas (and huge uncertainty) well…the storm will make a WNW “turn” towards Florida this weekend (assuming it holds together)…does it keep going west-northwest into the peninsula (and possibly into the Gulf), does it slow down and turn just offshore (possible threatening Georgia or the Carolinas), or is something else on the table?  And how strong is it?  Lots of questions…as is often the case at this distance out not many answers, but plenty of reason to closely watch this one.

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