Friday 11:45 AM | **Chance continues for a touch of snow Sunday night/early Monday; primarily, north of the PA/MD border…big snow threat at the end of next week which may be followed by bitter cold**

12Z GFS forecast map of 850 mb temperature anomalies for Saturday, December 30th featuring potential brutal cold in much of the eastern US; map courtesy NOAA/EMC, tropicaltidbits.com

Overview (Friday, December 22)
The weather pattern across the nation will become quite wild over the next couple of weeks with widespread bitter cold and multiple snow threats. After a mild and wet day on Saturday in the Mid-Atlantic region, a strong cold front will pass through the region and not only usher in a colder air mass, but begin a pattern change to more sustained cold.  In fact, it looks like once this colder pattern sets up in the Mid-Atlantic region, it’ll stick around going well into the month of January.  In addition to the upcoming cold, there are multiple snow threats going forward including a relatively minor one for the Mid-Atlantic region on Sunday night/early Monday and then a much bigger threat at the end of next week which could be followed by bitter cold air.

12Z GFS forecast map of an impressive upper-level (250 mb) jet streak late Sunday night which should help generate low pressure off the Mid-Atlantic coastline; map courtesy NOAA/EMC, tropicaltidbits.com

Sunday night/early Monday minor snow threat
Two factors argue against anything significant in terms of snowfall for the DC-to-Philly-to-NYC corridor late Sunday night/early Monday: 1) the low pressure system that is likely to form late Sunday will be a rather fast-mover and 2) boundary-layer temperatures will be only marginally cold for snow in the immediate I-95 corridor - at least initially.  Nonetheless, a touch of snow is possible by early Christmas morning in the I-95 corridor; primarily, to the north of the PA/MD border from low pressure that will form off of the Mid-Atlantic coastline.

12Z GFS surface forecast map for late Sunday night with low pressure off the Mid-Atlantic coastline; map courtesy NOAA/EMC, tropicaltidbits.com

A strong cold front will close in on the Mid-Atlantic region on Saturday and low pressure will ride up along the frontal boundary zone producing rain in the area.  Temperatures will turn quite mild ahead of the front with highs well up in the 50’s in most spots along the I-95 corridor.  Colder air will then filter into the I-95 corridor on Saturday night and Sunday following the passage of the cold front and there should be partial sunshine.  Meanwhile, another wave of energy will be advancing to the east on Sunday rotating around the base of an upper-level trough centered over south-central Canada.  Once the jet streak associated with this wave of energy reaches the frontal boundary zone near the Mid-Atlantic coastline late on Sunday, low pressure will likely form.  This low pressure system will then intensify and turn northeast and likely throw some precipitation back into the colder air over the interior part of the Mid-Atlantic and Northeast US.  The precipitation in the I-95 corridor can begin as rain, but is likely to change to snow late Sunday night into early Monday and there can be a small accumulation by early Christmas morning in some spots; primarily, to the north of the PA/MD border.  This low pressure system will then continue to intensify and accelerate past the northern New England coastline on Monday and skies around here should clear for the afternoon hours, but winds will pick up and it’ll be quite cold with temperatures in the 30’s.  

12Z GFS surface forecast map for late Friday night, December 29th with strong low pressure off the Mid-Atlantic coastline and expansive high pressure extending from the Northern Plains to southeastern Canada; map courtesy NOAA/EMC, tropicaltidbits.com

End of week significant snow threat and possible subsequent bitter cold
Later next week, very strong high pressure is likely to trek across the northern US towards southeastern Canada and will likely set up shop to our north and northwest by the end of the week. Strong high pressure to our north is usually an important requirement for snowstorms in the big cities along the I-95 corridor as it is needed to anchor in cold air.  Without anchored cold air, coastal storms in the big cities would quite easily feature "snow-changing-to-rain" scenarios – but, in this case, it looks like a very favorable setup for sustained cold and therefore, potential significant accumulating snow in the Friday/Saturday (12/29-30) time frame. 

With that cold air locked in place late next week, copious amounts of moisture may head our way and this could very well lead to substantial snowfall in the entire DC-to-Philly-to-NYC corridor.  Brutally cold air is likely to plunge into the middle of the country at the end of next week and it this air mass could ultimately push into the Mid-Atlantic region following our potential coastal storm and, given the prospects for fresh snow cover, this type of setup could result in some bitter cold conditions around here for last part of 2017.

Stay tuned - this big threat is still a week or so away.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com

Detailed video discussion on the upcoming snow threats: