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January Medium/Long Range Discussion

nj2va

By nj2va,
in Mid Atlantic

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Maestrobjwa

Maestrobjwa

Posted
Posted
5 minutes ago, Bob Chill said:

12z gefs pretty unanimous with a strong storm signal d8-10. The faster solutions are mostly rain and the slower ones are either mixed or all snow. Timing appears to be key here. Some solutions have multiple waves as well. We're in shotgun mode with large spread so there's just no way to know how it will shake out yet. 

And cranky alluded to something else...that even if next weekend is a cutter, that it could open up the door to something else afterward!

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Maestrobjwa

Maestrobjwa

Posted
Posted
Just now, Weather Will said:

Bob, Do you or any anyone else know if there have ever been any studies that show that one perturbed member of GEFS has been more reliable than another?  Or am I not understanding how the ensembles work?

 

I'll let Bob give ya a fuller answer, but just from my amateur observations on here...it seems that it is better to look at the ensembles as a whole, rather than individually. Of course...if ya have s few that have a vastly different outcome, doesn't hurt to speculate as to why...but yeah, I'd say look at them more as a group. Back to you, Bob! :lol:

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frd

frd

Posted
Posted

I am bringing this great update over to the long range thread from @Isotherm  recently posted from 33andrain. 

Some of what Tom brings up is surprising to me, I admit.  

As always I appreciate the time needed for an update such as this. 

Posted 14 minutes ago

 

Significant caution needs to be exercised here, initially. The NAM modality shifted this past week courtesy of the strat-modulation [circa -2 SD now], and to a lesser extent, NAO diminution; however, there's been systemic, countervailing destructive interference, spasmodically this winter from the tropical Pacific which must be thoroughly recognized. The AO and NAO are modestly negative, but the NAO is not yet fully responding w/ a retrogressive Greenland block, due largely to unpropitious tropospheric receptivity via repeated diminutions in angular momentum and intermittent tropical forcing interference.

 

This present +ENSO event is now virtually gone -- SST's approaching neutral in ENSO region 3.4. Part of the reason why SOI responses have been meager, and the canonical walker cell has not been as robust as expected.

 

Further, the z50 QBO easterly shear stress and its influence on tropical forcing, coupled w/ the weaker El Nino event, and the SSW forcing, has allowed the MJO to circuit Nina-esque phases more frequently than I think most anticipated so far this winter.

 

Going forward, it's incontrovertible that the SSW-split has rendered the NAM altered, but lack of cooperation on the Pacific side has delayed significant winter storm opportunities on the East Coast. We are missing a favorable window this weekend - which is unfortunate - this weekend, w/ GWO/MJO circuits out of phases 8, 7, however, at least the Mid-west and Mid-Atlantic will be profiting from this recent pulse of felicitous tropical forcing.

 

Angular momentum declined once again over the recent week; however, we are reversing that now, with increase +EAMT, and we will see another modest +EAMT pulse this coming week which will further extend the Pacific jet January 15th-20th. Contemporaneously, the MJO will circuit back through phases 5-6-7 quite rapidly. Due to the torque and tropical forcing contributions, I would also anticipate the GWO to expeditiously circulate back toward phase 5/6 over the next week, and eventually back into 7 later this month.

 

This increase in equatorward momentum transport will alter the jet structure as we near January 20th. One will note on the h2 zonal wind maps on the EPS, the canonical "split" jet stream structure becomes much more evident after January 20th, and especially by Jan 22-24.

 

The significant Greenland blocking and concomitant stable PNA amplitude will occur when constructive interference finally realizes between the SSW after-effects, the GWO/AAM and the MJO. The pattern is now colder than normal, but this pattern truly elucidates strongly the importance of a semi-stable PNA structure for increased probabilities of the significant east coast winter storm. We are evolving toward a stable PNA structure, but not yet.

 

The SSW is already inducing benefits, but the recent AAM plummet implicates disconnect again right now (ephemerally). However, the AAM will increase and constructively interfere w/ the SSW going forward as extratropical torques improve. The MJO will adjunctively increase momentum over the next two weeks, but initially, as it traverses phase 5 in particular, energy may continue to bombard the West Coast such that it destructively interferes w/ stable PNA. Once the MJO pulse moves into phase 6, this will change - and MJO/GWO and SSW will all be acting in concert. This, to me, is probably after the 20th threat (closer to 22nd/23rd and onward).

 

To recapitulate:

 

-We miss another fairly conducive window this weekend, but other regions of the East receive a snowstorm

-The systemic issue so far this winter has not been the polar domain, but the Pacific domain, due in part to enhanced intraseasonal interference via a weaker +ENSO event than anyone anticipated [I thought about +0.8c trimonthly but it will be weaker], SSW enhancement, and less than optimal momentum responses

-the NAM/NAO have been and will remain negative, but the lack of a stable PNA implies a colder than normal pattern with reduced significant snow chance in the medium term initially

-the threat on the 20th may or may not produce, but there's a strong argument it doesn't, because the NAO has not properly retrogressed yet to countervail an unstable PNA, induced via unfavorable MJO passage

-equatorward momentum transport w/ increasing AAM, split flow regime begins after the 20th, and the MJO will thereafter constructively interfere too

-the stable PNA structure truly stabilizes 22nd+ 

 

Everything remains on track for a prolonged period of colder than normal and enhanced snow opportunities henceforth through winter. However, since we miss another favorable period this weekend, we're going to need to wait until revitalization of constructive interference once again. This may mean the initial short wave on the 20th/next weekend is rain followed by bitter cold. The 20th wave could set-up up the 50/50, further pump / retrogress the NAO as trop. forcing becomes improved, and PNA structure amplifies/stabilizes for the 22-23rd threat.

 

We've had a lot of poor luck snow wise this winter, and it's principally b/c of the Pacific domain, reverting frequently to Nina-esque proclivities.

 

 

 

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Ralph Wiggum

Ralph Wiggum

Posted
Posted
18 minutes ago, Bob Chill said:

12z gefs pretty unanimous with a strong storm signal d8-10. The faster solutions are mostly rain and the slower ones are either mixed or all snow. Timing appears to be key here. Some solutions have multiple waves as well. We're in shotgun mode with large spread so there's just no way to know how it will shake out yet. 

We take

gfs-ens_mslpa_us_38.png

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Bob Chill

Bob Chill

Posted
Posted
10 minutes ago, Weather Will said:

Bob, Do you or any anyone else know if there have ever been any studies that show that one perturbed member of GEFS has been more reliable than another?  Or am I not understanding how the ensembles work?

 

I highly doubt any one ens is better. That's not really what they're intended for (picking one out of the bunch). Since the global atmosphere can't be perfectly modeled at any time, all ens members and ops will have some errors embedded at initialization and these errors compound over time. However, the average (or ensemble mean) of all the members gives you the most accurate (and still not perfect) forecast of large scale hemispheric features. It's still not perfect at longer leads and I'll be long dead before they are. 

Ensembles are run for the sole purpose of coming of with an average in an effort to smooth out errors and improve forecasting accuracy in the medium to long range. We like to look at individual members because it gives us a clearer picture with the spread though. When you see similar solutions through the ens members it gives confidence in a specific forecast. When spread is large it reduces confidence in any one outcome. 

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psuhoffman

psuhoffman

Posted
Posted

@frd The way he worded it sounded more negative than it really was.  In essence he was only (in a much more technical way) reiterating the progression we have been discussing for a while.  After this weekend there will be a pullback in the pattern to an average looking one with some good and bad features as the transition period takes place.  We discussed the coming rise in the SOI before another crash which is related to and coincides with a possible weak/fast traverse of the warm mjo phases.   The January 20th period is variable based on the pacific.   There is a risk that becomes a rainstorm if it times up poorly with the transition in the trough/ridge alignment out west.  There is the risk the trough digs into the central US initially.  But we all see how the pattern evolves after January 22.  So it is very possible we don't score something by then...but the core of this has always looked to be after that.  These threats before then are kind of a bonus during the transition period.  I think our best chance for a significant snowstorm or several of them... will be the very end of January and February.  That seems to be what he is saying...in a nutshell.  

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Isotherm

Isotherm

Posted
Posted
6 minutes ago, psuhoffman said:

@frd The way he worded it sounded more negative than it really was.  In essence he was only (in a much more technical way) reiterating the progression we have been discussing for a while.  After this weekend there will be a pullback in the pattern to an average looking one with some good and bad features as the transition period takes place.  We discussed the coming rise in the SOI before another crash which is related to and coincides with a possible weak/fast traverse of the warm mjo phases.   The January 20th period is variable based on the pacific.   There is a risk that becomes a rainstorm if it times up poorly with the transition in the trough/ridge alignment out west.  There is the risk the trough digs into the central US initially.  But we all see how the pattern evolves after January 22.  So it is very possible we don't score something by then...but the core of this has always looked to be after that.  These threats before then are kind of a bonus during the transition period.  I think our best chance for a significant snowstorm or several of them... will be the very end of January and February.  That seems to be what he is saying...in a nutshell.  

 

 

@psuhoffman, thanks for the summary - you are correct. The polar field has already rapidly improved c/o of the SSW, and we have a nice Pacific favorability window this weekend c/o of the MJO/GWO 7-8 circuit which will benefit you all in the Mid-Atlantic w/ a nice snowstorm. As the AAM starts rising again this week, and MJO re-circulates, before we arrive at the period in which all factors are "working together perfectly", the PNA may be a bit unstable. This makes the 20th threat a bit risky, with unstable PNA and non-retrogressed -NAO, but that will rapidly improve thereafter, which is why I think 22/23rd--> could bring the lower-risk / higher ceiling opportunities. Everything is progressing as expected atmospherically, but as PSU said, that post was more directed toward the NY forum (applicable to BOS too) which has rolled snake eyes in every favorable window thus far.

@frd

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