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December 16-17, 2020 Winter Storm Obs/Nowcasting


WxUSAF
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31 here. Definitely better flakes.  I think we with better rates, we snow up until 12-1230 ish?

Seems to have transitioned fully to better flakes IMBY. Sleet is temporarily gone.

I’m hoping we can push it a little past 12:00-12:30 but that’s because I’m hugging the HRRR. That’s probably a realistic guess, though.
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Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1031 AM EST Wed Dec 16 2020

.SYNOPSIS...
High pressure will reside over Quebec and northern New England,
extending southward across the eastern slopes of the 
Appalachian Mountains. Low pressure over the Southeast will lift
northward today and strengthen as it tracks toward the Virginia
Tidewater region and the Delmarva Peninsula tonight.

&&

.NEAR TERM /THROUGH TONIGHT/...
Mid-morning update: Precipitation is currently overspreading 
the area from southwest to northeast as large scale ascent/warm 
advection aloft overspreads the area in advance of the 
approaching shortwave. A pronounced warm nose at around 800 hPa 
was evident on the 12z IAD sounding, with wetbulb temperatures 
at that level just below freezing. An even stronger warm nose 
was noted on the RNK sounding at the same level (with temps 
there pushing above freezing around 800 hPa). As of 10 AM, this 
warm nose has pushed northward into portions of central 
Virginia. CHO has switched over to sleet, and a drop in CC/ 
increase in Zdr is evident on radar stretching from Greene 
County to southern Stafford County. This depicts the current 
location of the mixing line (drop in CC/increase in Zdr is 
indicative of melting of hydrometeors within the cloud layer). 
The main challenge throughout the day will be forecasting the 
advance of this warm nose aloft toward the north and west, and 
its resultant impact on precipitation types across the area. 
Current guidance suggests that the warm nose will make it 
roughly to the Blue Ridge, making mixing possible at times for 
locations to the east of the Blue Ridge. Mixing with sleet is 
also expected to the west of the Blue Ridge in the central 
Shenandoah Valley. Precipitation is expected to remain all snow 
further north in the I-81 corridor near MRB and points west, 
where the highest totals are expected. Snowfall rates may reach 
1-3 inches per hour in those locations by later this afternoon 
and evening, making for hazardous travel. Surface temperatures 
will need to be watched closely along the I-95 corridor to see 
just how much frozen precip occurs before locations go over to 
plain rain. The rate of advance of the warm nose will also be 
key, as it will determine just how long precip stays snow 
initially. As of now, the only major forecast change planned is 
to cut back snow totals across central Virginia, where they've 
already switched over to mixed precipitation. We will continue 
to monitor trends and adjust the forecast as needed throughout 
the remainder of the day. An 18z balloon launch is planned to 
assess the state of the warm nose aloft. 

 

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2 minutes ago, NorthArlington101 said:


Seems to have transitioned fully to better flakes IMBY. Sleet is temporarily gone.

I’m hoping we can push it a little past 12:00-12:30 but that’s because I’m hugging the HRRR. That’s probably a realistic guess, though.

I'm hugging it like my first born child!  All seriousness living here as long as I have, its a smart move to expect the warm nose earlier than any piece of guidance.

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