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December 16-17, 2020 Winter Storm Obs/Nowcasting


WxUSAF
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Some have mentioned why given the setup all the warm air filtered in. 
best setups for the MA are closed upper low, passing near or just south of us and also deepening. 

The CAD, while in place was marginal. Air being drawn into our region was not all that cold and it was shallow. Advection aloft came right off the mild Atlantic regardless of the low track and caused the changeover. These factors started showing up in the guidance Sunday night/Monday. 
 

At our latitude we need many pieces to come together just right to give a classic heavy snow setup. 

 

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8 minutes ago, ers-wxman1 said:

Some have mentioned why given the setup all the warm air filtered in. 
best setups for the MA are closed upper low, passing near or just south of us and also deepening. 

The CAD, while in place was marginal. Air being drawn into our region was not all that cold and it was shallow. Advection aloft came right off the mild Atlantic regardless of the low track and caused the changeover. These factors started showing up in the guidance Sunday night/Monday. 
 

At our latitude we need many pieces to come together just right to give a classic heavy snow setup. 

 

Is there a reason why this didn't close or it took so long to close off?

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34 minutes ago, Eskimo Joe said:

We never had a closed 850 low to cut off the flood of Altantic air.

And the Bay and Atlantic coastal water SSTs are as warm as they're going to be for the rest of Dec as well as JFM. 

This is quite "climo" for mid December, at least with the absence of antecedent arctic air going in. For me it evokes the Dec 1992 storm, at least for PA (though not as intense).  

Also makes one appreciate Dec18, 2009, in how rare of an event that was early in the season. 11/11/87 was just a freak of nature. 

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It’s important to try not to hug the snowiest model solutions several days out. If you think about it, 120 hours out is 5 days and most pieces of energy that would come together for a MA storm may not even be over the U.S. yet. It’s a simulation of what could happen and with high uncertainty built into it. So many pieces to come together and if timing is off on one of them, a model can go from a 20” snow to a miss or rain just like that. Tempting not to grab on as a snow lover, but helps to avoid disappointment later on.  It’s more important to understand synoptically the “why” vs the “what” and know how weather typically behaves here. A 10-20” snow sampled by the euro, cmc, and gfs at day 4/5 would be in the record books especially for the big cities in December. Records are called records for a reason, they don’t happen too often. Big snows in a model outside the favored climatology should be looked at with caution...biggest one here being the CAD. Yes the signature was there, but the source region of the airmass was modified and not very cold in depth. No closed low was big red flag. 50 degree Atlantic water with easterly flow aloft another flag. The ship began to sink. It’s tough to not latch on. We all feel it, Met or hobbyist, but we can’t overlook the factors that work against us. 

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52 minutes ago, NorthArlington101 said:

I think there was a post-storm model analysis thread that year. If the low doesn't end up shooting up the Bay, it would be really interesting to go back and see what happened/who the winners and losers were. Based on the current track, assuming I'm remembering right, the GEFS would actually get a fair bit of credit. HRRR too. 

Don’t leave out the NAM. It was the first one to sniff out the warmer more inland scenario. It also did great with the start times today. The 3k was awful. From its 50” total to the fiasco it pulled at 6z

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This is why we pretty much don’t ever want to be in the jackpot 4-5 days out with this setup.  9 times out of 10, a Miller A ends up being further north and warmer at 850-925 than progged. DC jackpot 4/5 days out = PHL to NYC special a lot of the time  

Show me a Miller A potential with the jackpot in Charlottesville at day 4-5 and we’ll all be cooking in this forum. 

I really wanted a foot.... but Damn it I saw 2+“ per hour rates in mid December in Maryland and got just shy of 8” to show for it. No complaints here. One happy weenie. 

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4 minutes ago, poolz1 said:

Really surprised to see snow mixing back in with the sleet.  Nothing crazy or accumulating but a change in the upper levels no doubt.

This is good to see. I just hit 30 degrees in Reisterstown and I have mostly ZR with a few pingers mixing in if the precip steps up the intensity. 

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1 minute ago, poolz1 said:

Really surprised to see snow mixing back in with the sleet.  Nothing crazy or accumulating but a change in the upper levels no doubt.

Winds have begun to shif back to the NNE instead of the SE winds we saw this afternoon. Definitely more snow than sleet falling right now here 5 miles N of Union bridge station 

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Final point to make is to not throw out the NAM if it might be outside it’s “wheel house”. At 48, 60, 72 hours we are not to accept its exact QPF or low position but more so it’s giving shorter range clues as to what might be happening with all the pieces sampled with the globals over the medium range, at lower resolution. The NAM outside its wheelhouse picked up on this pronounced warm injection, quick changeover. It’s QPF was way to high over the northern tier but we want features and potential evolution vs exact QPF amounts 2 to 3 days out. No forecaster in the weather business understands how to predict exact QPF, it’s like coastal flooding, smoke and mirrors at times and formulated with a blend of guidance. Focus on features and their evolution vs nailing down amounts and exact track.

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Just now, Eskimo Joe said:

This is good to see. I just hit 30 degrees in Reisterstown and I have mostly ZR with a few pingers mixing in if the precip steps up the intensity. 

Just spitting a flake here and there but something has to be changing around upstairs.  ZR and sleet the past few hours here as well.

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