12Z GFS loop of 850 mb (~5000 foot level) temperature anomalies in 6-hour increments from early today until early Thursday; maps courtesy tropicaltidbits.com, NOAA/EMC
There are three different low pressure systems to monitor for the period extending from tomorrow night into the middle part of next week and accumulating snow is certainly on the table for the I-95 strip from DC-to-Philly-to-NYC, but perhaps the biggest story of all could be the Arctic blast that is coming for the weekend. “Air trajectory” maps show that the air coming into the Mid-Atlantic region for the weekend has its origins in the Canadian Arctic where they have experienced some of the coldest March weather in decades. An Arctic front will slide through the Mid-Atlantic region on Friday – accompanied by strengthening low pressure – and its passage will usher in air that will likely be some 15-20 degrees below normal for this time of year.
The low pressure that forms along this Arctic frontal boundary zone will generate rain at first in the Mid-Atlantic region, but then as colder air filters in from the north, a changeover to snow is likely early Friday and there can be some grassy accumulations in much of the I-95 corridor. After Arctic cold floods the area on Friday night and Saturday, the next storm system will be suppressed well to the south - likely sparing the I-95 corridor of any snow. A third system is likely to develop in the Mid-Atlantic region in the Monday night or Tuesday time frame and it could intensify rapidly near the coastline with lots of cold air to in place.
"Backwards" air trajectory plot showing that the source region of the air reaching central Pennsylvania this weekend is the Canadian Arctic; plot courtesy tweet by Dr. Jon Nese, Penn State University, NOAA
Thursday night/Friday "rain-to-snow" scenario
Low pressure will form along an Arctic frontal boundary zone on Thursday night and rain is likely to break out in the I-95 corridor after midnight. As colder air pushes in from the north, there is likely to be a changeover of rain-to-snow by Friday morning primarily in areas north of the PA/MD border and the changeover will occur in a northwest-to-southeast fashion.
12Z GFS forecast maps for 7am Friday (left) and 7pm Friday (right); maps courtesy tropicaltidbits.com, NOAA/EMC
Even in the DC metro region, the northern suburbs could see a changeover of rain-to-snow during the mid-to-late morning hours on Friday where it is not out of the question that some grassy surfaces pick up a coating of snow. In the Philly suburbs, a coating to an inch or two of grassy snow accumulation is possible early Friday, and 1-3 inches is likely in and around the NYC metro region.
12Z GFS forecast map of total snowfall for the Thursday night/Friday event; map courtesy tropicaltidbits.com, NOAA/EMC
Weekend Arctic blast
By Friday night, winds will intensify out of the north/northwest and Arctic air with its origins in the Canadian Arctic will flood into the Northeast US. As an example as to just how cold this air mass has been at its source region, Mould Bay in northern Canada set a record low temperature last weekend of minus 66.5 degrees (F). In addition, earlier this week and at the beginning of the Iditarod dog sled race, Fairbanks, Alaska saw its temperature plunge to minus 38 degrees (F) – their coldest temperature this late in the season since 1964 [credit to Washington Post/Capital Weather Gang]. High temperatures on Saturday in Philly could struggle to reach the freezing mark at a time of year when normal highs are around 50 degrees. This forceful blast of cold air will contribute to a more suppressed storm track later in the weekend for a system that will drop from the Central Plains and into the Southeast US. This southern route should spare the DC, Philly, NYC corridor from any snowfall during the second half of the weekend, but it’’ll stay very cold for this time of year.
12Z GFS surface forecast map for Tuesday morning; map courtesy tropicaltidbits.com, NOAA/EMC
Monday night/Tuesday storm threat
By early next week, vigorous upper-level energy will likely drop southeastward from central Canada into the Upper Midwest and this disturbance could spawn the formation of a rapidly intensifying low pressure system near the Mid-Atlantic coastline by around Tuesday of next week. There will be plenty of cold air in the Northeast US at this time so the possibility of snow does exist for the Monday night/Tuesday time frame. A reinforcing shot of Arctic air will follow this system and keep it well below normal around here right through the middle of next week. Stay tuned, this is an active and complicated pattern.
Today’s extended video discussion: https://www.youtube.com/watch?v=GGqTu3jNo1E&feature=em-upload_owner