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December 18-20 Winter Storm Discussion


Chicago WX

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The GFS and ECM suites remain at odds (and, it wouldn't be fun if they agreed at this point, would it? I want to earn my snow, and don't want to sit here 7-8 days out knowing I'll get it...ok, enough of me rambling) with how they handle the energy moving into the western US. However, there have been some semblances of coming to a consensus on this weekend's system as well as the development of a strongly west based -NAO this weekend.

First off, the GFS and the first wave:

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Initially, the GFS showed a strong shortwave over the upper MW Friday into Saturday that suppressed the first southwestern shortwave ejecting east over the weekend. This resulted in the model likely becoming too zonal across the central and eastern US into early next week. Now, the model is closer to the Euro in the handling of the first wave, driving it farther north and maintaining a bit more of an amplified pattern heading into next week:

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An interesting change is the now relatively well agreed upon development of a west based -NAO as mentioned above this weekend. Assuming the first wave comes out in one piece, it should do its part to lower heights and bring in marginal cold for the Lakes/upper Ohio Valley, and with a potential west based -NAO on the table, this should allow any second storm to track farther south than the first storm and have a bit more cold to play with. However, the airmass will be marginal, with much of the eastern 2/3rds of N. America remaining above average in the temperature department even after the first wave moves east this weekend.

One area where the two suites still differ, likely resulting in the difference in solutions for this second, potentially more wintry storm, is in how much energy crashes into the NW US this weekend and how much of it ejects east.

The two suites of ensembles above actually are now in much better agreement now in placement and strength of ridging in the Pacific and placement of the polar vortex north of Alaska this weekend...the GFS suite is just in general a bit weaker with the energy diving into the coast on Sunday.

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As we look ahead towards Monday, the ensembles continue to diverge. The GFS ensembles are a bit farther south with the first shortwave, with the -NAO exerting a bit more of an influence on the GFS ensembles than the ECM ensembles. This likely contributes to, along with a weaker initial system diving into the west, the GFS suite in general bringing out a weaker, more strung out piece of energy early next week, with in general a farther south and weaker solution than the European ensembles. However, it should be noted, the GFS suite has trended towards the Euro suite WRT the amount of energy diving into the west this weekend, even if it isn't all the way there yet.

The European suite is a bit more aggressive in building ridging back in towards western Alaska this weekend on a temporary basis than the GFS, which may be resulting in some additional amplification on the west coast within that model suite.

post-525-0-82156900-1355286274_thumb.gif

Either way, a ton of energy will crash in off the Pacific over the next few days, and stormier weather should result. Given festering convection between the dateline and India, I'd favor the Pacific jet breaking behind the current jet streak over the western Pacific, which will favor building heights over the N. Central Pacific until convection becomes re-focused over the Indian Ocean next week.

So, in short:

-First shortwave this weekend, while it may feature some snows for the far northwestern portions of our sub-forum, will be more of a sacrificial wave that will help weaken eastern US ridging some and force any systems following behind it to track farther south.

-West based -NAO will help maintain lower than normal heights over the Great Lakes behind this first system and will help hold in marginal cold air.

-The amount of energy diving into the western US this weekend is paramount. Less energy means a more strung out solution with less snow chances, while more energy diving into the west and ejecting out in one piece means a more amplified solution with more widespread snow chances is more likely.

As always, small scale features that the models will not resolve properly this far in advance such as potential wave interference and truly how strong the Pacific energy is, regardless of the overall pattern, may throw wrenches into this threat, but the pattern supports the first widespread "snowstorm" threat for this sub-forum next Tuesday-Wednesday.

Thanks for that write up, makes things easier to understand. Your last part, about the small scale features the the models don't resolve properly makes quite a bit of sense too. I don't claim to understand everything that goes on here, but write ups like yours make it easier on us "weather noobs".

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One analog that's being tossed around (from the NE forum) is Dec. 10-12, 1992. Looking at the old NARR maps, various parts of that setup for that monster storm are showing up on some model runs. Toronto got about a foot of heavy, wet snow but areas just to the east got almost 2 feet. Temperatures during that storm hovered around freezing or slightly above.

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Of course, the models are trending towards keeping the shortwaves too close together for one to really amplify. That may still change but it's looking quite a bit like another potential, this one more for the southern lakes and OV, is going to be wasted:

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Yeah, I wish we could calm things down. It's hard to be optimistic with how things have gone but this type of setup with fast flow/multiple waves is notorious for model fits so something is telling me to not throw in the towel on snow chances just yet.

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Looks like this storm will miss me here in Toronto. Looks to be a U.S. special.

This december could tie 1877 and 2006 for snow futility in Toronto at the rate we're going. I'd love to see a study on snowfall in Toronto. I'd be willing to hazard a guess that it has declined precipitously in the past few decades.

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