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December 11 Frontal Wave Snow Event


WxUSAF
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Something is majorly weird with the GFS and that 850mb dry layer that develops between 0z and 6z Wednesday.  I don't see how that is consistent with 0.2-0.5" of liquid equivalent.  Pivotal shows the same info as TT, so it's not that one is plotting bad data.  Just bizarre.  

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GFS may still be having boundary layer issues, too cold. Solution appears to be overdone. Siding conservative on this one. This type of setup with arctic air timing with precipitation along a front is not a favorable setup for the area. Drier and warmer solutions more likely to verify, though I still hold out hope for a dusting to an inch well N and W of 95. 

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1 minute ago, ers-wxman1 said:

GFS may still be having boundary layer issues, too cold. Solution appears to be overdone. Siding conservative on this one. This type of setup with arctic air timing with precipitation along a front is not a favorable setup for the area. Drier and warmer solutions more likely to verify, though I still hold out hope for a dusting to an inch N and W of 95. 

You do know that if you go to the lower side here, you will be right 95% of the time. 

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7 minutes ago, ers-wxman1 said:

GFS may still be having boundary layer issues, too cold. Solution appears to be overdone. Siding conservative on this one. This type of setup with arctic air timing with precipitation along a front is not a favorable setup for the area. Drier and warmer solutions more likely to verify, though I still hold out hope for a dusting to an inch well N and W of 95. 

18z RGEM is 2"-4"... so I dunno

ETA: Moved the best snows SE compared to its 12z run and did look an inch drier in its accumulations... but still leaned toward the 18z GFS

Also indicated some decent banding around 08z to 11z WED 

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3 minutes ago, yoda said:

18z RGEM is 2"-4"... so I dunno

Moved the best snows SE compared to its 12z run and did look an inch drier in its accumulations... but still leaned toward the 18z GFS

Also indicated some decent banding around 08z to 11z

Boundary layer temps, antecedent warm ground, rates, all dicey. 

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17 minutes ago, WxUSAF said:

Something is majorly weird with the GFS and that 850mb dry layer that develops between 0z and 6z Wednesday.  I don't see how that is consistent with 0.2-0.5" of liquid equivalent.  Pivotal shows the same info as TT, so it's not that one is plotting bad data.  Just bizarre.  

 

 

If you look almost every single model has that dry area around 850. not sure what that means, but im sure its not good

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2 minutes ago, soadforecaster said:

 

 

If you look almost every single model has that dry area around 850. not sure what that means, but im sure its not good

NAM doesn't.  It's drier in terms of precip, but has a moister column below 850mb.  

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Decent discussion from Mount Holly-

Tuesday night the surface cold front will be well off the coast with not much in the way of coverage initially expected. Late Tuesday night into very early Wednesday the 700/600 MB thermal gradient will begin to tighten as the shortwave approaches. Latest GFS and NAM FGEN fields are fairly impressive with lift increasing starting over Maryland and spreading northeast. The NAM and the other high res models are depicting this as precipitation spreading over the area rather rapidly Wednesday morning. The NAM is slightly further south with the strongest forcing for ascent compared to the GFS and ECMWF, but overall agreement is not too bad. The main concern will be precipitation type issues. Over the southern Poconos thermal profiles support all snow for the event, but the strongest FGEN signal remains just southeast of this area. The best forcing for ascent is usually towards the warm side of the FGEN band, which would favor limiting snowfall totals here. Towards the I-95 corridor the best chance of precipitation exists, but thermal profiles initially support rain. Both the NAM and GFS show initially rain changing over to a rain/snow mix and then finally all snow. NAM forecast soundings indicate slightly stronger 700/800 mb winds and implied WAA. A brief transition to sleet is indicated via the NAM, but the most likely scenario appears to be rain/snow to snow as max temperatures aloft on the NAM seem to warm compared to the GFS and ECMWF. The most likely snowfall totals across the area are about 1 to 2" with slightly higher totals over the northwestern zones. The change over to snow and subsequent totals will be highly sensitive to the thermal profile and QPF footprint though.

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HPC has stuck with the higher snow totals well north and west. And what do precip events and cold fronts usually end up being at game time? 
 

Still not sure what to think of this one. Think I’d have to lean toward drier trend continuing. Even the wettest models have trended consistently drier over the past 18 hours. Would be nice to see one come in wetter even if only by a little.

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3 minutes ago, soadforecaster said:

i guess depends on location, but here is that dry level

 

Yeah.  I was looking at 3k NAM soundings around DC/Balt and it's pretty minimal and mostly at 900mb actually.  

The models account for precip evaporating/subliming during descent (I'm 99% sure...) so can't really explain it.

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5 minutes ago, osfan24 said:

Doesn't the GFS have a cold bias now?

Yes, but it’s most severe in the medium and long range. In the short range it’s minimal. But of course that’s an average across lots of space and time, so it could be wrong either way in any particular location and time.

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