What a difference a day makes! Arctic air seeped into the Mid-Atlantic region right on schedule in the overnight hours anchored by strong high pressure currently situated over south-central Canada. As a result, today’s high temperatures will be considerably lower than yesterday’s spring-like readings which reached the 60’s in many areas. Meanwhile, a massive area of moisture [latest radar above courtesy mesonet.org; NOAA GOES water vapor image below] is now heading northeast towards the Mid-Atlantic region. This very impressive area of moisture is well supported by strong upper-level dynamics and there are even numerous severe thunderstorms today across the Deep South with tornado warnings in some isolated locations. As this moist air runs into the cold, dry, dense low-level Arctic air now in place across the Mid-Atlantic region, its progress to the northeast will slowly grind to a halt and eventually it will be forced to move eastward with a sharp precipitation gradient on the northern edge.
The end result of this will be that the highest snowfall accumulations from early tonight into early Monday will tend to be in southern sections of the Mid-Atlantic region such as around the DC metro region, the Delmarva Peninsula, and the southern half of New Jersey. However, moderate amounts of snow accumulation are expected to reach as far north as the city of Philadelphia and its northern and western suburbs. [Note: the Philly National Weather Service does not currently have any of the counties in southeastern Pennsylvania in a "winter weather advisory", but I believe that will (and should) change later in the day]. The least amount of snowfall from this storm along the Mid-Atlantic I-95 corridor should be in the New York City metro region. The precipitation will arrive in the DC metro region early tonight in the form of plain rain, but, it should change over to all snow rather quickly. Virtually all of the precipitation that falls north of the PA/MD border during this storm should be in the form of snow and it should begin during the late evening hours.
Here is the latest thinking on tonight's snow accumulations in the Mid-Atlantic region:
New York City metro region: on the northern edge; anywhere from a coating to an inch or so
Philly metro region: 1-4 inches on average with the higher amounts generally in southern parts of the area [Note - Philly Airport needs only 2.6” of snow to match the 2nd snowiest winter ever of 1995-1996 (65.5”); definitely within reach from this storm]
DC, Delmarva Peninsula and southern New Jersey:: 4-8 inches with isolated higher amounts for this the “southern-most” section of the Mid-Atlantic region
Stay tuned for possible further updates as a small shift can still make a big difference.