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Midwest Thanksgiving Storm...


Powerball

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18z NAM has the low level cold air undercutting the precip.

Like I said in the MOOKARKS discussion, cold air undercutting precip would lead to freezing rain or possibly sleet, not snow. Apparently the NAM is showing more of a "bulldozer" solution with the front instead of an "undercutter" solution. Keep in mind that one bias of the NAM is to show bulldozers too often in the long-range (i.e., not many ice storms show up on hr 84). However, another bias of the NAM is to be too slow with the progression of cold air (the opposite of the GFS).

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Like I said in the MOOKARKS discussion, cold air undercutting precip would lead to freezing rain or possibly sleet, not snow. Apparently the NAM is showing more of a "bulldozer" solution with the front instead of an "undercutter" solution. Keep in mind that one bias of the NAM is to show bulldozers too often in the long-range (i.e., not many ice storms show up on hr 84). However, another bias of the NAM is to be too slow with the progression of cold air (the opposite of the GFS).

Precip algorithm on that counts sleet as snow. And as I showed you, 850 is +5 when the precip is falling in some areas before sharpening up the 850 front. However, 925 is very cold and surface temps marginal. NAM has a superior thermal field due to resolution so it is able to capture the cold air better. However, this is still a long ways out and the 12z was not as cold.

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I suspect the NAM has an idea with the slower progression of the trough (for once) mainly due to the amount of stable, cold air backed up across the intermountain west, GFS seems too fast. NAM is trending ever so slowly faster with each run, but not much. HPC seems to go with CMC/GFS ensemble. Seems reasonable, and probably safe at this time. http://www.hpc.ncep.noaa.gov/h5pref/h5pref.shtml

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I suspect the NAM has an idea with the slower progression of the trough (for once) mainly due to the amount of stable, cold air backed up across the intermountain west, GFS seems too fast. NAM is trending ever so slowly faster with each run, but not much. HPC seems to go with CMC/GFS ensemble. Seems reasonable, and probably safe at this time. http://www.hpc.ncep....ef/h5pref.shtml

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.

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The NAM is way too slow IMO, but the GFS is still correcting from being too progressive. GEM this afternoon was a nice compromise...I like the 0z GFS still slowed down just a bit.

I agree with this. 12z GGEM seems they way to go...at the moment. Let's see what it has up its sleeve tonight.

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