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WxUSAF

December 16-17, 2020 Winter Storm Obs/Nowcasting

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NAM definitely stopped the bleeding wrt to the west shift in track and thermals closer in even near the fall line. Gets borderline warning criteria to a lot of folks in CMD west of 95. The WAA thump is solid on this run. 

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Holy crap this is going to be real close for HoCo MoCo. Taking a looking at the upper levels, whoever stays mostly snow just west of the rain/snow line is going to be a secondary (albeit lower) jackpot of sorts. A few models point to pretty intense lift just west of the fall line. 
 

12k NAM starting off 18z right with a slower progression and slightly deeper dig... let’s keep it going 

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21 minutes ago, 40westwx said:

going from a watch to an advisory is the kiss of death... 

maybe so, but that's how it goes. Watches go up first to get people's attention, then they either go to a warning or advisory depending on what's to come for each local. If you are a southerner expecting a warning, you're just silly. 

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1 minute ago, jayyy said:

Holy crap this is going to be real close for HoCo MoCo. Taking a looking at the upper levels, whoever stays mostly snow just west of the rain/snow line is going to be a secondary (albeit lower) jackpot of sorts. A few models point to pretty intense lift just west of the fall line. 
 

12k NAM starting off 18z right with a slower progression and slightly deeper dig... let’s keep it going 

Unfortunately, the 3K is superior to the 12K and really isn't any better for any of us.

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Yes the 3k is not as good as the 12k depiction. However, take a look at the sounding here in DC which is well into the rain depiction on the frame shown. Column is not far off from being isothermal. Additionally, the low is I'd estimate 50-75 miles east of last run. 

PSU, keep me honest on that (I'm using TTB) 

nam3km_2020121518_fh27_sounding_38.92N_77.09W.png

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2 minutes ago, Kleimax said:

Isn’t the 3k superior to the 12k? 3k not nearly as good for us.

just go with the snowier model. 3k isnt useful till tonight

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1 minute ago, Kleimax said:

Isn’t the 3k superior to the 12k? 3k not nearly as good for us.

Correct me if I'm wrong, but the 3K is higher resolution.  It is razor close along the fall line with surface and 850 temps.  A very small adjustment would make a big difference in snow vs. sleet totals.

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One take away from 12z & 18z... models are still pretty clueless on

a) strength and track of the primary SLP in OV

b) positioning of the high and timing of its retreat 

c) a and b result in 50-60-70 mile differences in track on various models. Which clearly makes or breaks the storm for coasties 
 

Not the worst thing position in the world to be in with a legit airmass in place, but we definitely need to see more support than the 12z NAM before we can get remotely excited

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5 minutes ago, Ji said:

this is so much better. That shows Still snow at Loudoun at 6pm

Yes...track might have been slightly better but it continued the trend of tightening up the mid levels so there is a much tighter thermal gradient NW of the low track.  Verbatim you go to sleet for about an hour in the afternoon...then back to snow for several more hours before you lose the thermals right as the best precip is moving out anyways...you might get a little light snow on the back end but that looks mostly northeats of you.  But this is a pretty good run...80% of the qpf is probably snow.  The warm layer never makes it further west then that ridge you can probably see from your house just west of leesburg.  Adjust 10 miles east and you stay all snow.  Actually...if you take the euro and adjust it to this track we would both get a pretty darn good outcome.  Even closer to the cities would do well.  That is probably what we need here...slight eastward track adjustment and this new idea of a better closed circulation to be correct.  

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1 minute ago, osfan24 said:

Unfortunately, the 3K is superior to the 12K and really isn't any better for any of us.

The low is east of the previous run, that's the most important part.

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A misdiagnosis  that was made at 11am and repeated was that since temp was 40 at 11am we were already in trouble with rising temps. The sun strength is zero right now and rising barometer with fresh nw breeze allows no afternoon warming and that is what happened

Expert similar cold air exertion for the text of the way.  I was 38/20 at 11am and 39/20 now, that’s a snower  for me 

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14 minutes ago, ovechkin said:

Isn’t that bad? Don’t we want the initial stuff to start earlier?

No actually that came sooner but then the storm seemed to slow

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1 minute ago, Kleimax said:

3k is just a sleet bomb for Towson/ Owings mills crew

I agree.  As is, the 3K is snow to sleet back to snow for the Towson area.  I could see a couple of inches of snow followed by 2-3 inches of sleet if that were to play out.

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The 3k definitely took on a double barreled look. Took a 997 due east off OCMD then repopped out of what looked to be a mesolow down to 991 in the DE Bay. Not too sure that would verify as shown, but does certainly help get backside love into CMD and even the metro area this run. 

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1 minute ago, WEATHER53 said:

A misdiagnosis  that was made at 11am and repeated was that since temp was 40 at 11am we were already in trouble with rising temps. The sun strength is zero right now and rising barometer with fresh nw breeze allows no afternoon warming and that is what happened

Expert similar cold air exertion for the text of the way.  I was 38/20 at 11am and 39/20 now, that’s a snower  for me 

I see your point and actually don’t disagree. Yes, that certainly can mean snow- to start. The question is how long does it hold? White rain for an hour or two or a slight dusting to be washed away by 34 degree rain doesn’t get me excited.

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Definitely a trend SE on the NAM - SLP is a bit ESE and as PSUhoffman has been saying,  there appears to be some closed circulation being hinted here. Let’s see if other models see this at all 

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NAM isn't terrible up here, not like 12z. I flip to sleet from hr 26-30 when 850s are +1/+2. But back to 0 and below after hr30. sim radar shows sleet, but not sure how. 850/925 and surface temps are all below freezing.

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Just now, mappy said:

NAM isn't terrible up here, not like 12z. I flip to sleet from hr 26-30 when 850s are +1/+2. But back to 0 and below by hr30. sim radar shows sleet, but not sure how. 850/925 and surface temps are all below freezing.

Yeah I was doing the point and click soundings near us, we'd probably be snowing assuming good precip rates. 

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11 minutes ago, Kleimax said:

Isn’t the 3k superior to the 12k? 3k not nearly as good for us.

 

9 minutes ago, DDweatherman said:

Yes the 3k is not as good as the 12k depiction. However, take a look at the sounding here in DC which is well into the rain depiction on the frame shown. Column is not far off from being isothermal. Additionally, the low is I'd estimate 50-75 miles east of last run. 

PSU, keep me honest on that (I'm using TTB) 

nam3km_2020121518_fh27_sounding_38.92N_77.09W.png

3k is an improvement for dc

positive snow depth
 

18z

image.thumb.png.5e3efc6ecad9472e8cb3151911a3f5ec.png

12z

image.thumb.png.8679953fcaa9813c5ee7006be0605f8b.png

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