Today's 12Z GFS model run continues to take a trend towards the Euro which has led the way on this potential major late week coastal storm.  A crucial player in the late week setup is the upper level feature at 500 mb associated with a northern stream disturbance. 

This feature will allow for the rapid intensification of the surface low as a phasing process is undertaken on Friday and the trend on the GFS has been for a deeper and farther southwest upper level feature late Friday.  Take a look at the latest 12Z GFS 500 mb forecast for 7PM Friday night (top below) as compared with yesterday’s 18Z model run (middle below) for the same time. 

The short wave is deeper, farther back, and this should allow for both a rapid strengthening of the surface low on Friday and a “farther south” deformation zone to the NW of the surface low.  This deformation zone (aka comma head) snow now wraps around into SE PA with the latest GFS model run as the dynamic upper level feature is allowing for significant and slow-moving wrap around snows (bottom map below for 1am Sat).

For now this storm continues to look like a massive snowstorm for much of New England including the Boston metro region where the snow may end up being measured in feet.  Farther down the coast in places like NYC and Philly the general scenario looks like the start of the storm late Thursday night/very early Friday will be a mix of snow, ice and/or freezing rain and then there will be a changeover to rain during the day on Friday, and then that will change to accumulating snow Friday night as the storm pulls to the northeast and colder air is drawn in. 

Significant snow is likely in and around the NYC region on the back end of this storm, and a few-to-several inches of snow can even fall Friday night all the way down into the Philly metro region. 

Odds favor mainly rain in the DC metro region, but even there it can start as a mixed bag late Thursday night before changing to rain on Friday and a changeover to snow at the end of the storm cannot be ruled out there. Stay tuned.  Video discussion on this important upper level feature at thesiweather.com .

Paul Dorian/Meteorologist

The SI Organization, Inc.

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