11:15 AM | ***A wintry mess in the Mid-Atlantic with an icy buildup later tonight in many areas…an impact for many on the Friday AM commute***

An icy buildup is coming later tonight to many parts of the Mid-Atlantic region and this will have an impact on the Friday AM commute. Map courtesy NOAA, Pivotal Weather

Overview

Temperatures tumbled in the overnight hours in the Mid-Atlantic region following the passage of a cold frontal system and setting the stage for a wintry mess from today into early Friday.  In fact, temperatures dropped nearly 40 degrees in the I-95 corridor from the lower 70’s on Wednesday afternoon to the lower 30’s by early today as high pressure strengthened into southeastern Canada.  From this location, this high pressure system will act as an anchor and a source region for cold air during the next 24 hours – a classic “cold air damming” pattern for the northeastern part of the nation. This kind of an atmospheric setup is setting the stage for mixed precipitation today and an icy buildup later tonight for many in the Mid-Atlantic region and there will likely be an impact on the Friday AM commute.

Significant accumulating snow is on the table for much of upstate New York and New England with cold, high pressure in place across southeastern Canada. Map courtesy NOAA, Pivotal Weather

Details

A “cold air damming” scenario has unfolded in the northeastern states with high pressure playing the key role and it is now getting well established across the southeastern part of Canada. In this kind of a weather pattern, low-level cold, dense air funnels down the east side of the Appalachian Mountains from Canada to the southern Mid-Atlantic and usually becomes quite difficult to dislodge once established. As warmer air tries to push in from the southwest, it is forced to ride up and over the surface-level cold air and this process known as “overrunning” will lead to precipitation in the Mid-Atlantic region in and off and on fashion during the next 24-36 hours.

“Cold air damming” has set up in the northeastern states setting the stage for a wintry mess during the next 24-36 hours. For more on “cold air damming”, check out a detailed video description on our “Meteorology 101” page (https://arcfieldweather.com/meteorology-101 ). Map courtesy NOAA

The precipitation will tend to come in two different waves in the Mid-Atlantic region with today’s initial burst likely lasting from now until mid-afternoon or so and likely staying just to south of the New York City region.  It is now cold enough in the DC and Philly metro regions that some of the precipitation that falls today can be in the frozen variety with, for example, sleet possible in the DC area and snow in the Philly area - any rain that falls can certainly freeze on untreated surfaces as temperatures hover near or just below freezing.  This initial burst of precipitation is likely to slacken off later in the afternoon and this break in the action should last into early tonight. Later this evening, a stronger wave of low pressure will head in this direction and the precipitation shield will re-develop, intensify and expand northeastward. The more intense precipitation that develops later this evening will vary in type depending on specific location along the DC-to-Philly-to-NYC corridor (see below). 

In the DC metro region, freezing rain is possible during the late evening hours and there can be an icy buildup on untreated surface in some of the northern and western suburbs of up to a tenth of an inch later tonight.  In the overnight hours, the precipitation in the DC metro region is likely to be spotty and light, but freezing can continue in some of the northern and western suburbs with an impact possible there for the early morning commute – watch for slippery spots. 

In the Philly metro region, the heavier precipitation should develop later in the evening and it is more than likely going to start as a period of sleet before an ultimate transition to freezing rain later tonight.  The freezing rain later tonight can result in an icy buildup on untreated surfaces of up to a quarter of an inch; especially, across the N/W suburbs – watch for slippery spots. It looks likely that temperatures will hang near the freezing mark early Friday in suburban areas resulting in an impact on the Friday AM commute.  As with any icy buildup of a decent amount as is likely to be the case here, the threat of scattered power outages will be on the table, but the good news is that the trees still do not have leaves which would make the threat even greater as it would increase the weight on tree limbs. By later tomorrow morning, temperatures should climb to above the freezing mark with plain rain possible and overall conditions will improve.

In the NYC metro region, after a likely precipitation-free day on Thursday, snow can break out later in the evening and accumulations of up to a few inches are possible before a transition to sleet and ultimately to a period of freezing rain. There can be an icy buildup of up to a tenth of an inch late tonight on top of the initial snow and the combination can certainly have a negative impact on the Friday AM commute; especially, in the N/W suburbs of the city.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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