12:55 PM | *Thursday snow threat interior Northeast US…longer-term signs for more sustained cold*

12Z GFS surface forecast map for late Thursday night featuring a potential snowstorm for much of New England; map courtesy tropicalttidbits.com, NOAA

Overview
Temperatures today are well above normal in the Mid-Atlantic region, but a cold frontal passage will usher in noticeably colder air for tonight and Wednesday.  Another surface cold front will arrive in the I-95 corridor on Thursday supported by vigorous upper-level energy and this will set the stage for a rapidly intensifying low pressure to form along the Northeast US coastline.  This system is likely to generate some snow in portions of the Mid-Atlantic region (north of the Mason-Dixon Line) and ultimately, lead to a major snowstorm for much of New England.  Following the departure of the Thursday storm, quite cold air will flood the region on Friday riding in on strong NW winds and there can be some snow shower activity in the I-95 corridor from Philly-to-NYC.  In the longer term, there will likely be another warm up in the Mid-Atlantic region early next week; however, signs are growing for widespread cold to develop across most of the US as we progress towards the second week of January.

Deep upper-level low in the Northeast US on Thursday night as forecasted by the 12Z GFS; map courtesy tropicaltidbits.com, NOAA

Short-term forecast
Colder air will push into the region tonight and Wednesday and then moisture should arrive here by early Thursday morning as the next surface cold frontal system slides into the I-95 corridor.  Low pressure will form along the frontal boundary zone and - as impressive upper-level energy arrives - this system is likely to intensify very rapidly as it moves from the New Jersey coastline to the New England coastline.  As a result, it could snow for awhile on Thursday morning across eastern Pennsylvania, interior New Jersey and southern New York with small accumulations even possible.  A brief period of snow in areas south of the PA/MD border is certainly not impossible; however, odds are against it.  

Ultimately, this quickly strengthening storm will produce copious amounts of snow for interior New England.  Following the departure of the storm, winds will get quite strong out of the northwest around here on Friday and temperatures will be held down in the 30’s for highs.  In addition, snow showers are likely to pound areas just downstream of the Great Lakes on Friday, and there can be some snow shower activity all the way into the I-95 corridor in areas from Philly-to-NYC.  The 12Z GFS forecast map of 500 mb height anomalies features strong upper-level low pressure across the Northeast US and the surface forecast map for late Thursday night shows a powerful low pressure near the coastline of Maine. 

00Z Euro ensemble model forecast of 500 mb height anomalies ten days out; map courtesy tropicaltidbits.com

Longer-term outlook
After another quick warm up in the Mid-Atlantic region early next week, colder air will likely return by the end of next week.  In fact, there are signs for widespread cold to encompass much of the US as we progress towards the second week of January.  While we have had numerous cold air outbreaks in the Mid-Atlantic region during the past few weeks, most of these have been relatively short in duration.  Whether the upcoming January cold becomes more sustained than our recent cold air outbreaks remains an open question at this time, but there are signs for that possibility.  The latest run of the European ensemble model features a very strong upper-level ridge near Alaska in ten days (labeled as feature #1; orange, red, purple) and another upper-level ridge near Greenland (labeled as feature #2; orange).  At the same time, deep upper-level troughing takes hold across much of the US (labeled as feature #3; blue).  

Current (arrow) and forecasted (red) NAO and AO index values; courtesy NOAA

If a blocking pattern develops across the northern latitudes at this time, the ridging from Alaska to Greenland can remain fixed in place for awhile and, if so, the upcoming cold can be sustained around here.  One way to track the possibility of such a blocking pattern to form in the upper-levels of the atmosphere is to track the North Atlantic Oscillation and Arctic Oscillation indices (current levels shown by arrows).  If both go negative and stay negative for awhile then this would suggest an upper-level blocking pattern could form across the northern latitudes (e.g., from Greenland to Alaska).  The latest model forecasts (in red) of the NAO and AO indices do strongly suggest a drop into negative territory for both, but it is still a bit too early to say if this will last well into the month of January and with it, our cold weather pattern.  

Stay tuned.   

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com