11:45 AM (Tuesday) | ***An Arctic blast for tomorrow night and Thursday in the Mid-Atlantic/NE US***

12Z NAM forecast map of 850 mb temperature anomalies at 1 AM Thursday with an Arctic air mass throughout the Mid-Atlantic/NE US; map courtesy NOAA, tropicaltidbits.com

Overview

The low pressure system that resulted in a wintry mix of precipitation in the Mid-Atlantic/NE US over the past 24 hours or so will reach a position off the New England coastline by later today.  This system will drag a cold front through the I-95 corridor by early tonight and winds will pick up as a moderately cold air mass moves into the area.  Watch for icy spots later tonight as temperatures drop into the 20’s in most areas and there may still be some wet roadways. Another front will arrive in the I-95 corridor tomorrow afternoon and this front will represent the leading edge of an Arctic air mass.  The arrival of the Arctic frontal system on Wednesday afternoon will be accompanied by strong wind gusts, scattered snow showers, and perhaps an isolated snow squall or two.  It’ll stay windy tomorrow night and on Thursday and it’ll turn sharply colder with some of the lowest wind chill values and temperatures of the season so far in the Mid-Atlantic/NE US.

12Z NAM surface forecast map for tomorrow afternoon with an Arctic frontal system pushing towards the I-95 corridor ; map courtesy NOAA, tropicaltidbits.com

Details

The most recent winter storm continues to bring some rain to the Mid-Atlantic region with many areas north of the PA/MD border still hovering near the freezing mark (e.g., Pottstown, PA and Quakertown, PA still at 32 degrees as of 11AM) where ice continues to buildup on trees.  In addition, some dense fog is forming to the south of the PA/MD border as slightly milder air moves on top of still-relatively cold grounds. This departing storm will swing a cold front though the I-95 corridor by early tonight, but this will not be the most important frontal passage of the next 24 hours. Indeed, an Arctic front will push through the I-95 corridor tomorrow afternoon and it will usher in an Arctic air mass featuring some of the coldest air so far this season.  Watch for icy spots later tonight as temperatures drop into the 20’s in most areas and there may still be some wet roadways.

12Z NAM forecast map of low temperatures on Thursday morning; map courtesy NOAA, tropicaltidbits.com

The Arctic frontal passage on Wednesday afternoon will be accompanied by scattered snow showers and there can be a few snow squalls as well. The best chance for snow squalls will be across NE PA and northern NJ, but it is not out of the question that a heavier snow band or two reaches the I-95 corridor from DC-to-Philly-to-NYC.  In addition to the threat for snow bands, this Arctic frontal passage will be accompanied by strong, gusty winds as its parent low pressure system will intensify rapidly tomorrow night as it moves across New England.  As high pressure builds simultaneously in the Ohio Valley, the pressure gradient will tighten around here and the result will likely be wind gusts to 40 mph or so later tomorrow into tomorrow night. 

Gusty winds are likely to continue into the day on Thursday and the combination of abnormally low temperatures for this time of year and the strong winds will produce wind chills in the single digits in many spots of the Mid-Atlantic region during this Arctic blast. Actual temperatures will bottom out in the teens in much of the DC-to-Philly-to-NYC corridor on early Thursday morning and single digits are not out of the question in some spots.  Despite sunshine, high temperatures on Thursday afternoon will likely do no better than the low-to-mid 20’s in the Philly and NYC metro regions and will struggle to make it to 30 degrees in DC.  Some moderation in temperatures will take place on Friday and Saturday, but it’ll remain below-normal for mid-December.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

Video discussion: