1:00 PM (Sunday) | ***Ice/rain/snow today...accumulating snow tomorrow; primarily, north of the PA/MD border...intense mesoscale snow bands are likely in some spots with significant accumulations***

A vigorous wave of energy aloft will play a key role in the formation of intense mesoscale bands of heavy snowfall on Monday and Monday night in parts of the Mid-Atlantic; map courtesy NOAA, tropicaltidbits.com

Overview

A long duration winter weather event is underway in the Mid-Atlantic region with a cold “plain” rain south of the PA/MD border, freezing rain and sleet across southeast and east-central PA, and snow is a factor in northeastern PA, interior upstate NJ and in the NYC metro region. As colder air wraps around a very slowly moving and intensifying low pressure system late tonight, accumulating snow will become an increasingly important factor for most areas and accumulations are likely; especially, north of the PA/MD border.  In fact, there is the chance that some spots in the Mid-Atlantic region experience significant snowfall amounts on Monday and Monday night as intense mesoscale heavy snow bands are likely to form during this event.

A changeover to snow is likely to occur early in the day in much of the immediate I-95 corridor; map courtesy NOAA, tropicaltidbit.com

Details

Precipitation in the form of “plain” rain continues at mid-day in areas south of the PA/MD border and there is a mixed bag of sleet and freezing rain to the north of the Mason-Dixon Line across southeast and east-central PA. In areas farther to the north, snow is mixed into the picture in places like northeastern PA, upstate interior NJ and the NYC metro region.  As temperatures grudgingly rise this afternoon, much of the sleet and freezing rain in southeast and east-central PA is likely to change to “plain” rain for the mid-to-late afternoon and early evening hours - but watch out for travel difficulties until then on all untreated roadways. The precipitation will actually slacken off this evening with the winding down of “phase 1” of the long duration winter weather event.

Later tonight “phase 2” will begin as low pressure intensifies near the Mid-Atlantic coastline. This low pressure system will run into a blocking pattern in the upper atmosphere forcing it to “rotate” around for much of the day on Monday.  The slow movement of this storm will allow for colder air to wrap into the system and push well to the south and east compared to where it exists today. As a result, any rain or mixed precipitation later tonight in the immediate I-95 corridor will likely change over to snow by early-to-midday on Monday. 

The snow threat will continue into the evening hours on Monday in much of the I-95 corridor from this unfolding slow-moving low pressure system; map courtesy NOAA, tropicaltidbits.com

Snow will continue on Monday at varying intensities – perhaps mixed with rain at times in some spots – and the threat for snow will continue well into the evening.  One of the things we’ll have to watch for during the storm on Monday and Monday night will be the likely formation of intense mesoscale bands of heavy snowfall due to vigorous energy in the upper atmosphere.  These small-scale heavy snow bands can generate much higher localized snowfall accumulation amounts making the overall forecast quite difficult and subject to change.  The storm finally pulls away in the wee hours of Tuesday morning and a much calmer remainder of the week lies ahead for the Mid-Atlantic region.

In terms of snowfall amounts in the Monday/Monday night time frame, here are some preliminary estimates for the DC, Philly, and NYC metro regions:

DC:      minor accumulations of a coating to an inch or so possible; primarily, across the northern MD suburbs

Philly:  there may be quite a sharp gradient of snowfall totals in the metro region with a couple-to-a-few inches possible across much of the area, but as much as six inches in Bucks County.

NYC:   3-6 inches possible in the metro region and 6+ inches possible across the Lower Hudson Valley (NY) and interior parts of northern New Jersey.

Stay tuned…this is a very volatile situation with the potential for lots of last-minute updates due to the expected intense small-scale heavy snow bands.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com