11:45 AM | **Accumulating snow on the way...weekend snow threat still on the table**

12Z GFS forecast map for shortly after midnight with snow shown in blue; map courtesy tropicatidbits.com, NOAA

Overview
A significant blast of Arctic air has moved into the region and we’ll stay well below normal in temperatures through Monday of next week.  In addition to the cold, accumulating snow is on the way from one low pressure system and then there will be another system early this weekend that will crush the Southeast US with snow and subsequent brutal cold, and it’ll - at a minimum - come awfully close to the Mid-Atlantic I-95 corridor.

Last night's run of the GFS model for early Saturday afternoon; map courtesy tropicaltidbits.com, NOAA

Snow event tonight/early Friday 
Low pressure will head towards the Mid-Atlantic coastline over the next twelve hours and accumulating snow will move into the big cities of DC, Philly and NYC.  The arrival time of the snow should be as follows: mid-to-late afternoon hours in DC, 8 or 9pm in Philly, and 10 or 11pm in NYC.  The snow will continue through the overnight hours and into early Friday morning with accumulations on the order of a coating to two inches in the DC area, 1-3 inches in the Philly and NYC metro regions.  Some spots closer to the coastline can see as much as 3 or 4 inches of snow by tomorrow morning (e.g., interior central and southern New Jersey).  Roads will become slippery overnight and can stay slippery in many spots right into the Friday morning commute.  The snow winds down around 5 or 6 am on Friday in DC, 7 or 8 am in Philly, and 9 or 10 am near NYC.  
 
Weekend snow threat
The upper-level feature associated with the second (weekend) system will dive deeper into the southern states compared to the initial storm, phase with some northern stream energy, and this will result in a stronger coastal low on Saturday compared to the first system.  It is quite clear that the Southeast US will suffer greatly from this early weekend storm with significant accumulating snow all the way from Alabama to southeastern Virginia and some places will end up with more than a foot of snow (e.g., eastern North Carolina, southeast Virginia).  In addition to the significant snow in the Southeast US, the cold following the storm will be downright brutal and perhaps record-breaking for many spots with zero degrees possible in areas that will become covered by a deep, fresh snowfall.  

Today's run of the GFS model with a noticeable shift from the prior run in its snow field (blue) to the northwest; map courtesy tropicaltidbits.com, NOAA

Farther to the north in the DC-to-Philly-to-NYC corridor, it is still not entirely clear as to how much impact this early weekend system will have on the area.  Signs still suggest the greatest impact will stay just to our southeast; however, there can still be an important north and west shift of this weekend's storm track from current computer model projections or simply an expansion of the precipitation field which could result in significant accumulating snow for the Mid-Atlantic I-95 corridor region of DC, Philly and NYC.  In fact, the shift to the northwest has already begun when comparing last night's run of the GFS computer forecast model to this morning's run - and that is not all that surprising.  This type of late shift in the storm track to the north and west or an expansion of the storm's precipitation field can happen based on the overall unfolding type of upper-level pattern and also as a result of some very warm water in the western Atlantic Ocean.  We'll see if the northwest trend in the model forecasts continues over the next day or two - quite possible in my opinion - and snow is looking increasingly likely in the immediate I-95 corridor for Saturday.

NOAA's forecast map of total precipitation amounts over the next 7 days with mind-boggling numbers in California

California Blitz
Not only is the weather exciting here in the eastern US with the cold wave and potential duo of accumulating snow events, the state of California is going to experience some mind-boggling precipitation amounts over the next week or so.  In fact, there can be more than 25 inches of liquid equivalent precipitation in the Sierra Nevada Mountains region of eastern California and this could translate into 10 feet or more of snow in some of the highest elevations of that region. 
 
Stay tuned.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com 

Extended video discussion:

https://www.youtube.com/watch?v=x3hZvOSxJ6M&feature=em-upload_owner