12:30 PM (Saturday) | ***Long duration winter weather event...sleet/freezing rain on the front end...accumulating snow on the back end...mesoscale heavy snow bands likely in some spots on Monday***

Vigorous energy in the upper atmosphere (500 mb) will likely lead to localized bands of very heavy snowfall in parts of the Mid-Atlantic region on Monday and Monday night. This forecast map from the 12Z GFS features such a band across NE PA and upstate NJ (dark blue); map courtesy NOAA, tropicaltidbits.com

Overview

The major storm that slammed into the west coast a few days ago will result in a long duration winter weather event for the Mid-Atlantic region from late tonight into Monday night.  This system is now generating blizzard conditions across the Dakotas and heavy snow will extend into the Upper Midwest later today and tonight.  The upper-level feature associated with this storm has made its farthest push to the north into South Dakota and it will now be forced to shift to the east-southeast by an “atmospheric roadblock” in the form of upper-level high pressure ridging over southern Canada. The end result will be a cold air mass stuck in place in the lowest levels of the Mid-Atlantic region when precipitation arrives late tonight/early Sunday and this will lead to a period of sleet and/or freezing rain at the onset of this long duration event in much of the DC-to-Philly-to-NYC corridor.  A change to plain rain is likely in many areas along the immediate I-95 corridor later tomorrow and there even be a slackening off of the precipitation early tomorrow night.  Late tomorrow night and during the day on Monday, colder air will wrap around into intensifying low pressure off the Mid-Atlantic coastline and this will result in a changeover to snow in most areas and significant snow accumulations are possible; primarily, north of the PA/MD border. 

Vigorous energy in the upper atmosphere (500 mb) will likely lead to localized bands of very heavy snowfall in parts of the Mid-Atlantic region on Monday and Monday night; map courtesy NOAA, tropicaltidbits.com

Details

The deep upper-level low associated with the Northern Plains storm will now run into a “atmospheric roadblock” in the atmosphere as high pressure ridging expands across southern Canada and this will force a slide of the system to the east-southeast.  The blocking to the north and the change in path to the east-southeast will be critical to the Mid-Atlantic region in two ways.  First, the change in direction will allow for a cold air mass to maintain its position ahead of the arrival of the precipitation late tonight and second, it will result in an extended winter weather event that will feature intensifying low pressure spinning off the Mid-Atlantic coastline as late as Monday night. 

Winter Storm watches and warnings extend across much of the northern US and blizzard warnings have been issued in parts of this area; map courtesy NOAA

Precipitation is likely to arrive late this evening in the DC metro region and then towards morning in Philly and NYC.  The air is likely to be cold enough for sleet and freezing rain for awhile at the onset; especially, north of the PA/MD border and snow can mix in as well at times.  A changeover to plain rain is likely during the afternoon on Sunday in these areas along the immediate I-95 corridor that experience icing in the morning.  However, this will not be the end of the threat for wintry precipitation. 

As low pressure heads to the Mid-Atlantic coastline, it’ll intensify from late Sunday night into Monday and this will allow for colder air to wrap around and the result will be a changeover to snow in many areas; especially, north of the PA/MD border.  Accumulations are quite likely from the northern and western suburbs of Philly and to points north and east into southeastern New York.  In fact, as much as 3-6 inches of snow is possible later Monday into Monday night from the far northern suburbs of Philly (e.g., Upper Bucks County) to the northern and western suburbs of NYC.  In areas in and around DC, accumulations of snow on Monday will likely be limited to a coating to an inch or two.  One other note, given the strength of the upper-level low, there will likely be some mesoscale banding of heavy snow which can lead to much higher localized amounts; especially, across NE PA and NW NJ where significant accumulations are on the table. One final note, travel conditions are likely to become quite difficult for the first time this season with ice/snow on the front end of this long duration winter weather event and the accumulating snow on the back end.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com