My Winter Outlook 2020-21

[Isotherm]

My winter outlook for the 2020-21 season can be read on my website, linked here, for those interested:

https://www.lightinthestorm.com/

[Allsnow]

4 minutes ago, Isotherm said:

My winter outlook for the 2020-21 season can be read on my website, linked here, for those interested:

https://www.lightinthestorm.com/

Very nice outlook Tom. Hard to disagree with any of this currently. 

[weathafella]

14 minutes ago, Isotherm said:

My winter outlook for the 2020-21 season can be read on my website, linked here, for those interested:

https://www.lightinthestorm.com/

Thank you as always for the detailed work. Looks like a pike north winter.

[michsnowfreak]

Nice work! I am no forecaster, just a climo guy.  I am really seeing a lot of roller coaster weather this Winter, would you agree? Seems as though while it can happen any year, and usually does, nina is extra susceptible to having wild swings. For instance, a +2ﾟ nina January could easily have a Max of 62 and Min of -12 whereas a +2° nino January could have a Max of 49 in a Min of 9. how we arrive at the final on numbers may be a wild ride.

[40/70 Benchmark]

Great work, as usual. We are largely in agreement, but differ a bit on how we arrive there. 

We agree that the seasonal PNA will be near neutral or even positive in the mean, but I favor PNA more in the second half. You also favor the most wintry period in January, and I am more December. I would not be surprised if you have the better idea there, as I was torn on whether to favor Dec 15 to Jan 15, but ended up highlighting December, while acknowledging that the favorable pattern could linger into January. Caveat being that you aren't that hostile for SNE in December....we both agree that the mid atl will struggle early.

February will be terrible. Agreed. I mentioned some hope for March, but it is low confidence and did not bake in into the snowfall forecast.

[40/70 Benchmark]

Was you primary analog 2007 or 1975?

[WEATHER53]

2 hours ago, Isotherm said:

My winter outlook for the 2020-21 season can be read on my website, linked here, for those interested:

https://www.lightinthestorm.com/

Great stuff ISO and we agree perfectly on Jan and Feb but I do have Dec as cold. Went for 11” at DCA which is a bit higher than you .

Have you seen Dec Pic or Raleigh’s yet?

[Roger Smith]

My main disagreement would be early in the winter for interior western regions, I suspect it will turn out a lot colder than shown. That would only have the effect of shifting your depicted warm core a bit further east to the mid-Atlantic states possibly (with more of an up and down regime in the central plains states). Otherwise I could certainly see the scenario of a relatively mild winter getting milder towards February. 

This may be one of those winters like 2015-16 with a lot of mild weather but one great winter storm anyway (for the east). 

[raindancewx]

I'm on board with what Roger said - there should be a storm that comes through the West like the October system that runs up the coast at some point, but it would be cold enough for most areas to see snow.

It does look like he is using 1975-76 as the main year. Would be cool to see that Spring, pretty sure it snowed down to El Paso last week of April 1976. That was my issue with 1975-76 when I looked - I don't know that we'll transition into an El Nino next year. I don't have a good feel on that at all. The MJO in 1975 isn't terrible compared to this year, but its off by 2-3 weeks timing wise I'd say heading into December. That's one of the reasons I didn't give 2007 full weight in my outlook, it's been off for the MJO timing for a while now. The little heat core max over Nebraska and Missouri is very 1975-76, it's usually warmest in a La Nina in Texas, the Southwest or the Southeast, so it's likely that is the year used at least in part. December is probably the easiest to see as 1975, he's got Florida/New England near average with the heat in Montana like here -

The winter of 1975-76 actually did have one very cold period nationally though. With the north Pacific and north Atlantic warmer than in 1975-76, I'd imagine this period would be pretty snowy in some places with more moisture available.

I think 1975-76 is probably the right idea (though I don't buy the core of the heat over NE/MO), but with the MJO different I'd expect at least a somewhat different progression. You can see for instance October 1975 was pretty warm in the middle of the country, while it was pretty cold this year. October finished looking a lot like 1954 or even 1959 if you try to find a one year match, and those years do get pretty cold out here like Roger said.

 

[BrittanyM0508]

Great work and insight indeed! I am not a weather forecast expert either (I rely on apps like climacell), but I would have to agree with michsnowfreak. The hurricanes on the east coast and the sudden winter storm in the west are proof of that.

[40/70 Benchmark]

6 hours ago, Roger Smith said:

My main disagreement would be early in the winter for interior western regions, I suspect it will turn out a lot colder than shown. That would only have the effect of shifting your depicted warm core a bit further east to the mid-Atlantic states possibly (with more of an up and down regime in the central plains states). Otherwise I could certainly see the scenario of a relatively mild winter getting milder towards February. 

This may be one of those winters like 2015-16 with a lot of mild weather but one great winter storm anyway (for the east). 

I think the one great storm is more of an el nino phenomenon.

[Isotherm]

Thanks all for the comments. Ray - I just posted a full response to your above comments on your thread. Weather53, I have not seen either of their forecasts yet, though I think I read CWG was coming out with their outlook this week (or soon). Michigan - I think you're correct on that. I anticipate a fairly cold picture across Canada this year, not atypical for strong La Ninas, and so, any spasmodic perturbations w/ short wave trough slipping through the Lakes/Northeast could ephemerally plunge temperatures to very low values, before rising again. I especially anticipate this volatility later in December and January.

 

[griteater]

Iso - good read as usual.  Thanks for continuing to post these (same with others posting outlooks).

[Isotherm]

Brief update with some thoughts on the storm threat & overall winter outlook. The provenance of this week's storm threat for the Northeast corridor can be traced backward in time to the robust poleward and eastward transfer of angular momentum, which is large part mediated this fairly unique (given various background indicators) episode of high latitude blocking. Most of the diminution of negativity is focused in the AO domain space, but the NAO has turned negative and will remain there for a period. It is this poleward momentum deposit and associated +GP height/50-50 anomaly which will act to largely countermand a less than optimal Pacific regime. 

This projected storm represents the most auspicious precondition set-up in a few years for the Northeast coast. It is highly likely that Baltimore, Philadelphia, Boston and NYC will receive at least 5-6" of snowfall, possibly DCA as well. The heaviest axis of snow - location indeterminate - will likely feature 8-16" on average. The most accumulation will occur approximately 1.5 degrees W/N of the path of the z850 low, and slightly W/N of the path of the z700 low, as evidenced by optimal uplift/DGZ confluence. This zone may occur anywhere from the northern mid Atlantic coast to north of NYC, depending upon the z500 evolution. It's nigh impossible to ascertain this zone right now.

 However, more importantly, from my perspective, is the storm opportunity provides a quasi-indicator for forthcoming pattern relapses.

December's temperature departures thus far are according with my winter outlook: generally warm nationwide with the core of the warmth in the W-C plains, and cooler in the SE US. The z500 of +EPO, neutral-positive PNA is also verifying. The December AO and potentially NAO will be more negative than expected (hence the larger storm threat this week). However, this episode provides insight to me, that tropospheric receptivity is heightened for another pulse(s) of momentum transfer, and increased favorability, for my target winter month in the Northeast (January). If anything, the events of this week suggest to me that my January ideas are on track (for NYC: slightly colder than normal, and snowier than normal). A technical SSW is unlikely this winter, but the SPV should be weaker than normal for early-mid January.

My expectation is the NAO will neutralize beyond this week, along with the AO, as the SPV reconsolidates and momentum alters. Thereupon, the Pacific should reorganize into a more conducive structure again, by the end of December. Thereafter, there will be another decline opportunity in the NAO/AO in the month of January, potentially operating contemporaneously with a +PNA and even near neutral EPO. My thoughts remain the same for beyond January. However, this week's storm will increase the probability that NYC's seasonal snowfall ends in the upper portion of my range, assuming the winter pattern continues to proceed as I anticipate.

My projection at this stage for snowfall this week is 6-12" for NYC, a significant snowstorm. Again, the heaviest axis will be a function of z700/z850 proximities.

 

[Isotherm]

The GFS based guidance is likely rushing the genesis/realization of the auspicious pattern; the ECMWF/EPS are more congruous with the physical drivers. z50 geopotential heights favor a positive NAO from now through circa Dec 28th; as the Canadian 'warming' occurs in the stratosphere and z50 restructures w/ a wave-2 paradigm, geopotential heights will begin to respond within a few days over the NAO domain, with diminution there in the final 1-2 days of December/beyond. The classical Siberian high descent initiates December 26th +/- a couple of days which will induce a +EAMT and jet extension event at the very end of December, and thus ameliorating the downstream Pacific/PNA/EPO domain pattern by the early days of January. Therefore, the December 28th-January 2nd period should feature expeditious amelioration of the hemispheric pattern, first, with improvement in the NAO domain at the very end of December, then, shortly followed by the Pacific. If there is a storm threat of significance in late December, it would likely favor the interior Northeast/New England, in my view, with a threat for I-95 beyond that time-frame (sometime in the first 10 days of January), but we'll see how it evolves.

[griteater]

@Isotherm - thanks for posting your updates in here.  I hope you will continue to do so.  Yeah, I'm not sure what to make of this big incoming +EAMT.  Following the descent of the Siberian High, I would expect to see a strong low kick out into the northwest Pacific, with a downstream large -EPO and/or +PNA ridge...but the current modeling acts like it wants to somewhat flood AK with low pressure.  I wasn't sure if that's a situation where models will correct in time, or if the large nature of the +EAMT could lead to that type of result.  I'd also expect to see solid split flow develop following the development of the -EPO/+PNA ridge.

Do you recall other scenarios / analogs with this type of large Siberian High descent? 

[Isotherm]

13 hours ago, griteater said:

@Isotherm - thanks for posting your updates in here.  I hope you will continue to do so.  Yeah, I'm not sure what to make of this big incoming +EAMT.  Following the descent of the Siberian High, I would expect to see a strong low kick out into the northwest Pacific, with a downstream large -EPO and/or +PNA ridge...but the current modeling acts like it wants to somewhat flood AK with low pressure.  I wasn't sure if that's a situation where models will correct in time, or if the large nature of the +EAMT could lead to that type of result.  I'd also expect to see solid split flow develop following the development of the -EPO/+PNA ridge.

Do you recall other scenarios / analogs with this type of large Siberian High descent? 

 

@griteater, thanks for your reply. Regarding the evolution: the amount of momentum which ultimately will be added to the system via the torque processes should not be superfluous/overpowering to the extent that all semblance of +GP heights are extirpated in W North America. January 2016 is an exemplar of what one might consider the epitome of an extreme +AAM regime, complete with anomalously coherent GWO circuits. Even in that paradigm, the PNA domain was positive (actually, strongly so), and even the EPO neutral to slightly negative, w/ a GOAK trough immediately west of the W North American Coast. Of course the +ve momentum regime extant is much less anomalous, and the z500 is multifactorial, but it's an example of what can happen. I think modelling (e.g., EPS depiction for early January) should eventually morph into a depiction wherein we see positive geopotential heights in the PNA domain, and close to neutral or positive heights extending northward on the W Coast of North America. We should see the lower GP heights retrograde somewhat, but I'm not yet sold on a massive -EPO in conjunction with a +PNA/-NAO. My original thinking for January was +PNA/-NAO and EPO closer to neutral or pos., as preseason factors did not appear favorable for prolonged -EPO. One factor to aid the EPO heights, possibly, is stratospheric alterations - however - will need to monitor. 

So, in short, I think we'll see data back off somewhat on driving the Pacific jet through W NA, and eventually by early January, begin to see the +PNA reform with at least near neutral EPO. That pattern along with the Atlantic changes should be sufficient to make the pattern interesting for many. It's far out, but the end of the EPS seems to begin these changes to which I refer in the PNA domain.

[Isotherm]

Mid-Winter Checkpoint:

December = Thus far, the winter is progressing mostly in accordance with expectations. December finished +1.7 in NYC (within the range provided for in the outlook, +1.5 to +2.5). Most of the Northeast/Mid-Atlantic finished similarly. The core of warmth centered over the Rockies/NW Plains as anticipated, with a cool zone near Florida.

The +PNA verified as expected, along with the +EPO. The NAO for December was merely slightly negative. The principal error was the AO domain, which finished -1.7 (negative); this aided in permitting the significant snowfall for the NE corridor.

January = the forecasted mean +PNA/-NAO looks to verify. I see no reason to depart from the temp. anomalies forecasted. The highest probability for medium+ impact events of the winter exists in the coming multi-week period. The occurrence of the SSW is immaterial to the overall forecast, and a byproduct of the antecedent tropospheric forcing. This will likely be a significant, not major, SSW, and a non-vortex destruction event. This, too, has implications. The EPO may briefly move negative in the latter part of the month. The most conducive period for the SE US/Mid-Atlantic will likely be 12th-20th. Beyond the 20th, more resistance should develop in the far SE US via PNA diminution, implying the target zone may shift northward along the East Coast. The details are indeterminate and will be predicated upon wave/timing/propagation. Therefore, January 10th-31st should provide for greatly enhanced probabilities of wintry weather.

February = I expect a rapid disintegration of the coming multi-week pattern, with the genesis of +NAO/AO in concert w/ -PNA/Nina-esque Pacific structure. The delay of any negative PNA phase until February was/still is expected from my standpoint. It's possible the PNA may beginning transitioning as early as very late January. Sub-seasonal forcing should constructive interfere w/ low frequency forcing near the Maritime Continent, retracting the jet and forcing a more canonical Aleutian High/NW US trough paradigm. The stratospheric vortex should expeditiously redevelop to at least near average intensities by early February, and subsequently couple rapidly. Thus, I continue to think February will favor locations further north [N/C New England] for meaningful snowfall.

 

 

 

 

 

 

 

[40/70 Benchmark]

2 hours ago, MJO812 said:

Meteorologist  Jeff Berradeli  just wrote this

 

https://www.cbsnews.com/news/stratospheric-warming-winter-weather-coming/#app

Hey @Isotherm, why is it that this article views the SSW as major, and you do not?

Thanks.

[40/70 Benchmark]

I have always been in your camp that the PV recovers in Feb, but Cohen is saying this is indeed a major event by definition. 

[LibertyBell]

On 1/6/2021 at 10:45 AM, Isotherm said:

Mid-Winter Checkpoint:

December = Thus far, the winter is progressing mostly in accordance with expectations. December finished +1.7 in NYC (within the range provided for in the outlook, +1.5 to +2.5). Most of the Northeast/Mid-Atlantic finished similarly. The core of warmth centered over the Rockies/NW Plains as anticipated, with a cool zone near Florida.

The +PNA verified as expected, along with the +EPO. The NAO for December was merely slightly negative. The principal error was the AO domain, which finished -1.7 (negative); this aided in permitting the significant snowfall for the NE corridor.

January = the forecasted mean +PNA/-NAO looks to verify. I see no reason to depart from the temp. anomalies forecasted. The highest probability for medium+ impact events of the winter exists in the coming multi-week period. The occurrence of the SSW is immaterial to the overall forecast, and a byproduct of the antecedent tropospheric forcing. This will likely be a significant, not major, SSW, and a non-vortex destruction event. This, too, has implications. The EPO may briefly move negative in the latter part of the month. The most conducive period for the SE US/Mid-Atlantic will likely be 12th-20th. Beyond the 20th, more resistance should develop in the far SE US via PNA diminution, implying the target zone may shift northward along the East Coast. The details are indeterminate and will be predicated upon wave/timing/propagation. Therefore, January 10th-31st should provide for greatly enhanced probabilities of wintry weather.

February = I expect a rapid disintegration of the coming multi-week pattern, with the genesis of +NAO/AO in concert w/ -PNA/Nina-esque Pacific structure. The delay of any negative PNA phase until February was/still is expected from my standpoint. It's possible the PNA may beginning transitioning as early as very late January. Sub-seasonal forcing should constructive interfere w/ low frequency forcing near the Maritime Continent, retracting the jet and forcing a more canonical Aleutian High/NW US trough paradigm. The stratospheric vortex should expeditiously redevelop to at least near average intensities by early February, and subsequently couple rapidly. Thus, I continue to think February will favor locations further north [N/C New England] for meaningful snowfall.

 

 

 

 

 

 

 

ISO focusing a bit more directly (on snowfall rather than temps which are always going to be mild, we know this already) do you see another 15-20 inches for NYC from the middle to end of Jan?  That would get us to near normal seasonal snowfall with the 10" we have already.  While I know Feb is forecast to start out mild and dry, in many similar winters, we've returned to another, albeit smaller window of snow towards the end of Feb and beginning of March, which could land us another 5-10 inches.  Would you say the odds are about even for NYC to have around 30 inches of snow by the end of the season?

 

[Isotherm]

@40/70 Benchmark, the definition of sudden stratospheric warmings has been elusive, subjective and often capricious throughout the decades (see this paper, at least 9 different definitions): https://www.arl.noaa.gov/documents/JournalPDFs/Butler_et_al_2015_bams-d-13-00173.1.pdf  You are correct that the common WMO definition of 'major' will be reached [mean zonal wind reversal to easterly at 10 hpa / 60N]. Other definitions, one of which I prefer for major ascertainment, is 60N/10hpa mean zonal wind reversal to easterly for a minimum of 5 consecutive days (see Simon Lee's blog: https://simonleewx.com/2019/01/24/the-january-2009-ssw/).

I've been monitoring this event, and it seems we may have an interruption in the zonal wind reversal in the coming 1-2 days [see ECMWF below]:

[note that the 10 hpa/60N zonal wind will probably hit or exceed 0 in the coming 48 hours, before slipping back easterly again].

https://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/ts.php?plot=fluxes&alert=1&lng=eng&hem=nh

So, we'll have to watch that interruption.

 

But regardless, the technical, subjective definitions don't matter much to me, as what is more integral is the tropospheric receptivity, downwelling potential, and resultant tropospheric response. The existing z30 westerly stress from the +QBO will tend to promote a more robust post-SSW recovery in my view, as well. 

@LibertyBell, I believe there's high probability of NYC finishing in the upper portion of my snowfall range (low/mid 20s) with some potential for higher/toward average, depending upon the outcomes of the next few weeks (which I think provide the best opportunities for snowfall). In March, wavelengths are much shorter, and thus typically unfavorable winter patterns can become more serviceable - so I wouldn't be surprised if we had an event in March. I would still lean toward around 30" total or lower for the seasonal total. We shall see how it evolves.

[40/70 Benchmark]

16 minutes ago, Isotherm said:

@40/70 Benchmark, the definition of sudden stratospheric warmings has been elusive, subjective and often capricious throughout the decades (see this paper, at least 9 different definitions): https://www.arl.noaa.gov/documents/JournalPDFs/Butler_et_al_2015_bams-d-13-00173.1.pdf  You are correct that the common WMO definition of 'major' will be reached [mean zonal wind reversal to easterly at 10 hpa / 60N]. Other definitions, one of which I prefer for major ascertainment, is 60N/10hpa mean zonal wind reversal to easterly for a minimum of 5 consecutive days (see Simon Lee's blog: https://simonleewx.com/2019/01/24/the-january-2009-ssw/).

I've been monitoring this event, and it seems we may have an interruption in the zonal wind reversal in the coming 1-2 days [see ECMWF below]:

[note that the 10 hpa/60N zonal wind will probably hit or exceed 0 in the coming 48 hours, before slipping back easterly again].

https://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/ts.php?plot=fluxes&alert=1&lng=eng&hem=nh

So, we'll have to watch that interruption.

 

But regardless, the technical, subjective definitions don't matter much to me, as what is more integral is the tropospheric receptivity, downwelling potential, and resultant tropospheric response. The existing z30 westerly stress from the +QBO will tend to promote a more robust post-SSW recovery in my view, as well. 

@LibertyBell, I believe there's high probability of NYC finishing in the upper portion of my snowfall range (low/mid 20s) with some potential for higher/toward average, depending upon the outcomes of the next few weeks (which I think provide the best opportunities for snowfall). In March, wavelengths are much shorter, and thus typically unfavorable winter patterns can become more serviceable - so I wouldn't be surprised if we had an event in March. I would still lean toward around 30" total or lower for the seasonal total. We shall see how it evolves.

Tom, same page...that is the rationale that is utilized in the outlook back on 11/5. While I would like us to verify, I would also like a February 2015 redux, so forgive me for being a bit torn. lol

I feel as though even if we are correct about February, there could still be a substantial phase change event early in the month.

@LibertyBell I am also with Tom as to having some hope for March...hopefully its not like last season where the it happens too late.

[Isotherm]

53 minutes ago, 40/70 Benchmark said:

Tom, same page...that is the rationale that is utilized in the outlook back on 11/5. While I would like us to verify, I would also like a February 2015 redux, so forgive me for being a bit torn. lol

I feel as though even if we are correct about February, there could still be a substantial phase change event early in the month.

@LibertyBell I am also with Tom as to having some hope for March...hopefully its not like last season where the it happens too late.

 

Ray, indeed and I hear you re the internal conflict going forward. Certainly after the past few benign winters, it would be nice to have a longer wintry period. February's more canonical Nina z500 structure should tend to favor the northern and potentially even central New England at times for snowfall. So I don't think your area has as much concern to maximize the upcoming 2-3 weeks of potential (though the coming period should offer best opportunities).

[40/70 Benchmark]

6 minutes ago, Isotherm said:

 

Ray, indeed and I hear you re the internal conflict going forward. Certainly after the past few benign winters, it would be nice to have a longer wintry period. February's more canonical Nina z500 structure should tend to favor the northern and potentially even central New England at times for snowfall. So I don't think your area has as much concern to maximize the upcoming 2-3 weeks of potential (though the coming period should offer best opportunities).

It will probably be north of me, but those front enders may stack up more here near the NH border, than say CT and RI.

[raindancewx]

I really can't see Northern New England ending up as the only cold spot in the US as you have it for January.  Utah is the place that has been consistently cold this winter and you had it solidly warmer than average this month. It really doesn't look that warm out there in the long range, you'll keep seeing inversions with cold lows even if there is high pressure and little moisture. The -NAO doesn't really favor cold in northern New England in January, so your idea never really made any sense to me. By the time we get to March, it's actually a warm signal for a lot of the Northeast, and even by January it's kind of Neutral. It favors storms more than cold really. You've got a lot of places up there in Northern New England 10+ above normal so far this month. My guess is they'll cool off later in the month with more normal warmth and snow, and still finish 3-8 above normal in some spots.

It's also going to be real hard to get big systems for Northeast snow until the SOI dies off, the +20 ish readings of late will keep the subtropical jet weak and way south of where it would be in an average year. You were looking in the winter thread when I posted that a -NAO never follows a sufficiently high SOI in December, so you seem on board with that.

You missed the WPO & NAO combination for December, which is why you couldn't see the cold in the West from what I can see. It's not like the PNA favored cold in Utah, it doesn't do that. Your forecast this year was actually pretty strange, its pretty rare to have Missouri as the hottest place in the US in a winter. If you think the South will be warm, the warmest areas in a La Nina will almost be the Southeast coast, south Texas, or the southern areas of the Southwest.

 

[LibertyBell]

On 1/8/2021 at 9:25 AM, 40/70 Benchmark said:

Tom, same page...that is the rationale that is utilized in the outlook back on 11/5. While I would like us to verify, I would also like a February 2015 redux, so forgive me for being a bit torn. lol

I feel as though even if we are correct about February, there could still be a substantial phase change event early in the month.

@LibertyBell I am also with Tom as to having some hope for March...hopefully its not like last season where the it happens too late.

Thanks Ray, I think you, Tom and I are on the same page, also I seem to recall seasons in which we have lackluster Februaries, winter usually makes a little comeback or at the very least, a last stand in March.  

[40/70 Benchmark]

28 minutes ago, LibertyBell said:

Thanks Ray, I think you, Tom and I are on the same page, also I seem to recall seasons in which we have lackluster Februaries, winter usually makes a little comeback or at the very least, a last stand in March.  

My analog composite had quite a robust signal for blocking in March, but my forecast was more conservative....I gotvspooked by the QBO, intensity of la nina, and the tendency for mate blocking to delay until spring of late....like last year, which burned me.

[LibertyBell]

2 hours ago, 40/70 Benchmark said:

My analog composite had quite a robust signal for blocking in March, but my forecast was more conservative....I gotvspooked by the QBO, intensity of la nina, and the tendency for mate blocking to delay until spring of late....like last year, which burned me.

Yep there seems to be a trend of late for May to be our blockiest month.  That did result in the latest snow I have ever seen (I was too young to remember May 1977).

[40/70 Benchmark]

Tom, are you getting uneasy about your Feb call? Starting to look like the PV may not recover for at least through early Feb....