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Midwest Thanksgiving Storm...


Powerball

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Crazy.....the Rgem seems like the nam but faster....

I don't want to ask for your prediction, but this air mass is very very cold...and it's late November not October so this ending as frozen here isn't a total shocker. However i am curious if this is realistic at all...while I can understand what the nam is doing I don't know why. why does the colder stable air slow a trough?

and why does the nam has so much vorticity along the trough compared to the gfs...and what mechanism causes the great lift over the same region(my area) as the cold air punches threw.

sorry for all the questions.

Too many questions to answer at once, but stable cold air has a tendency to slow the progression of a trough across the intermountain west because the various mountain ranges impede the flow of otherwise orderly cold air advection. This is one of the many regions why lee side cyclogenesis has a tendency to be rather rapid with strong shortwaves (geostrophic adjustment...also the conservation of angular momentum/potential vorticity, but that is another matter). Cold air ejecting into the plains has to pass through various mountain ranges extending past 10000-14000 feet, and that is not easy. The NAM has better mountain drag parameterization and less filtering than the GFS and it also has a higher resolution grid, therefore it should do a slightly better job simulating the effects of cold air across the intermountain west.

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Man, very close here. The 0c line is just to the south of here when we are getting some pretty good precip. But on the NAM snow depth simulation, it doesn't show any snow. Cold rain? Ice? What do you guys think?

FLD, right? If we're taking the 0z NAM literally, looking at the sounding, it looks like you start as some snow before going over to a mix or plain rain.

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I spam my blog around here too much, but you guys keep asking me questions that relate to something I have already wrote about. Read this...not too technical at all and quite easy to understand. Has to due with stable air and mountains. http://jasonahsenmacher.wordpress.com/2010/11/16/the-froude-number-and-stable-flow-mountain-blocking/

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I'm growing increasingly interested in the WAA precip lifting out of Missouri and southern Illinois Wednesday morning. Latest NAM, GFS, and RGEM show a shield of freezing rain/sleet as the precip encounters cold air entrenched at the surface over northern Illinois. It doesn't look to last very long at any particular location, but would be quite interesting given most of these areas haven't even received flurries yet. It's going to come down to how quick the precip can move in before the cold air at the surface erodes. The GFS has actually been showing this for a few days, but since the GFS seems to saturate things too quickly sometimes I was a bit skeptical. Now that the RGEM and especially the NAM are on board I'm a little more intrigued.:snowman:

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lol! This literally caused me to laugh for two minutes out loud...I told my wife about it and my eyes were watering. I know you may not have even meant for it to be funny, but it was!

Yes, there is some weenie discussion going on in here, but at least you guys could help by discussing and helping out instead of posting hot dogs everywhere.

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I've completely given up on the secondary wave delivering wintry precip, but it will still prove significant in delivering what looks like some pretty heavy rainfall to downstate Illinois and much of Indiana. Latest GEM pretty much seals the coffin for me as far as any wintry precip from the second wave. Like others have said, the GEM looks like a compromise between the NAM and GFS. It's reasonable because the GFS looks too quick and the NAM looks too slow. On the other hand, even if the NAM ends up being correct, there's still little or no frozen precip on the back side anyway.

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I think those ensemble images are misleading. They show total precip in what I'm guessing is the 6-12hr period before the placement of the surface features/temps on that map. In other words most of the precip will have occurred before the temps shown are that cold.

That's true. It has been doing this frequently since Thursday/Friday.

To add to that the mean is blended between ensemble members. One or two different members could show very cold temperatures (A faster solution) and a few could show a lot of precip (Slower Frontal Passage) and then a few similar to the OP GFS... and with the mean it looks a lot different.

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