1:15 PM **Coastal storm threat continues for the late weekend…accumulating snow a possibility from DC-to-Philly-to-NYC**

Overview
Today and Thursday will be quite mild in the Mid-Atlantic region with high temperatures well up in the 60’s in most spots and perhaps even 70+ degrees in the DC metro region.  A cold front will arrive on Thursday and its passage will send high temperatures down some ten degrees or so between tomorrow afternoon and Friday afternoon.  An even more important and stronger cold front will push through the region late Friday and this frontal passage will usher in noticeably colder (Arctic) air for the weekend as cold high pressure builds to our north.  By Sunday, attention will shift to the east coast as it is quite likely that a storm will form near the Mid-Atlantic coastline and - given the cold air that will be in place - there is a possibility for accumulating snow in the DC-to-Philly-to-NYC corridor.  By the way, looking far ahead, there are signs for multiple cold air outbreaks as we wind down the month of March and begin April.  

Latest Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) index forecasts (shown in red); courtesy NOAA

Late weekend coastal storm threat
Signs continue to point to a storm forming near the Mid-Atlantic coastline on Sunday and it could be just cold enough for this potential storm to produce accumulating snow in the I-95 corridor.   There are still several days to go before this possible event and many different solutions are still on the table.  One possible solution – the least likely in my opinion - is that a storm develops and it takes an inland track producing only rain in the I-95 corridor.  Another possibility is that any storm that forms on Sunday stays far enough out-to-sea that it produces only a minor impact on the I-95 corridor or perhaps even no impact at all.  Finally, there is the chance that a storm forms just off the Mid-Atlantic coastline on Sunday and it intensifies rapidly as it pulls to the northeast.  Given this particular storm track near the coast, the result could very well be accumulating snow in the immediate I-95 corridor.  

Signals from NAO and AO indices
The solution for an inland type of storm late in the weekend is probably the least likely given the signs coming from the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices. Both of these are forecasted to drop into negative territory (above) over the next couple of days and that generally favors a coastal storm track as opposed to an "inland runner".  Indeed, negative NAO and AO indices this time of year generally favor a colder solution in the I-95 corridor with cold air farther to the south and east (i.e., towards the coastline) and a storm track near or just east of the coastline.  

12Z GFS forecast map for Sunday evening (blue=snow, green=rain); map courtesy tropicaltidbits.com, NOAA

Computer model forecasts
Yesterday's 12Z GFS model forecast for Sunday evening suggested there would be a storm late in the weekend, but it was positioned well off the east coast.  However, there were clues that the GFS could “correct” its "out-to-sea" prediction by ultimately shifting the storm track farther to the north and west as the event time approached.  Indeed, today’s 12Z GFS forecast map for Sunday evening (above) has a "closer to the coast" scenario and snow (blue) falls from DC-to-Philly-to-NYC.  In addition, today’s 12Z Canadian forecast model (below) generally supports this notion as it has a rather intense coastal storm near the Mid-Atlantic coastline on Sunday night which generates snow in the same I-95 corridor region (blue).  Finally, the latest Euro model from 12Z has shifted quite a bit from its model run of last night which featured much more of an "out-to-sea" solution (see below).  

Stay tuned - this will be closely monitored over the next few days.     

12Z Canadian forecast map for Sunday evening (blue=snow, gree=rain); map courtesy tropicaltidbits.com, NOAA

Big changes between last night's 00Z Euro model run (right) and today's 12Z run (left) with respect to the placement of the coastal low and its precipitation shield; map courtesy WSI

Meteorologist Paul Dorian
Vencore, Inc.