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January 17-18 Winter Storm


Snowstorms
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2 hours ago, Harry Perry said:

I see my local office is getting ready to let the public down again haha. Hard to believe that graphic isn’t from a previous system - because I see no models showing what they’re predicting. 

015200A6-55B4-4358-93BD-3794A1368215.jpeg

I see nothing wrong with this graphic in light of what all models are showing now. They have the potentials highlighted pretty correct right now.

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The GFS and Euro both want to keep the central and northern Chicago metro mostly all snow until flipping to rain late Friday evening or overnight, which explains the jump in amounts the past few runs north of I-80. With the strength of the southerly flow aloft, that would mostly be purely dynamic cooling beating back the WAA after initial evaporative cooling from top down saturation. Still having a tough time believing the WAA won't win out earlier.

 

Interestingly enough, the trend today has been to keep 925 mb temps colder for longer, especially with northward extent. If we assume that the 850 mb temps will verify warmer than modeled, and the 925 mb temps verify closer to correct, then a decent chunk of the metro would more likely flip to sleet before any freezing rain and then rain. Of course, the 925 mb temps might also verify warmer than modeled, which would bring the freezing rain risk farther north quicker.

 

FWIW, the NAM tends to do better with capturing the WAA, but it too keeps 925 mb quite cold deep into the night. I do believe the NAM is overdone with dry air issues given the consensus of the other models.

 

 

 

 

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4 hours ago, Harry Perry said:

I agree.. just an odd graphic. Almost like they are trying to keep it cool in lieu of the last system.

Cannot say I blame them nor APX. Their confidence has to be pretty badly bruised at this point. Still fresh from licking their collective wounds. I still think our office has just terrible intuition or something. Seems like when they go all in, things fall apart but when they call for "nothing to see here..move along" things go well or even over-perform. Idk, it's just really strange. 

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The 00z NAM would indicate a wall of WAA precip during the late afternoon and evening and then dry slot with lighter rates 06z and beyond. Because of the support for heavy rates due to very strong isentropic ascent, dynamic cooling could in that case keep a bulk of the QPF as snow for much of the Chicago metro. The transition over to sleet and freezing rain would happen when precip rates lighten as the mid-level dry slot moves overhead.

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00z NAM is an improvement for LOT at least. Still having some weird dry air issues
Have noticed it's very common with the NAM. It has some threshold for dry air in the sounding that it seemingly automatically bins out those areas from having QPF despite most of the sounding being saturated enough. The fact it has a swath of ~1-2" of snow by 00z from SW WI to northwest IN, a band of 0 snow south of that and then another band of 1-2" south of that illustrates the issue. Given the orientation of the precip shield, that doesn't make much sense.

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