2:30 PM (Tuesday) | *Accumulating snow threat later in the week from southern Virginia to the Carolinas…only adds to the frustration for DC, Philly, NYC snow lovers…may be a bit of hope though*

A significant snow event is depicted by the 12Z NAM for later this week from southern Virginia to upstate South Carolina; map courtesy NOAA, tropicaltidbits.com

Overview

It goes without saying that this winter has been very frustrating for snow lovers in the DC-to-Philly-to-NYC corridor as there just hasn’t been the right combination of cold and moisture to result in significant snowfall.  A strong cold frontal passage at mid-week will indeed usher in a cold air mass fully capable of producing snow in the Mid-Atlantic region as long as moisture becomes available.  In fact, there will be plenty of moisture meandering across the southern US over the next couple of days and it looks like some of this will northeastward into the Carolinas and as far north as southern Virginia at the same time this upcoming cold air mass becomes firmly established. The result may be a significant snow event from southern Virginia to upstate South Carolina with half a foot possible in North Carolina and – as has been the case for much of the winter – snow lovers from DC-to-Philly-to-NYC will be looking with envy from the outside.  Longer term, there are some signs of hope for cold and snow in the Mid-Atlantic’s I-95 corridor, but we have had false promises already this winter and time is beginning to run short. 

A significant snow event is in the cards for later this week from southern Virginia to upstate South Carolina; 500 mb forecast map courtesy NOAA, tropicaltidbits.com

Snow chances later this week in the Carolinas and southern Virginia

Scattered showers today in the eastern US are associated with an approaching cold frontal system and the mild conditions will give way to some cooling on Wednesday and then even an even colder air mass will arrive for Thursday and Friday.  The cold air outbreak for later in the week will be anchored by a cold, strong high pressure system over the middle part of the nation – quite a nice setup for snow in the eastern US if moisture can join the party. In fact, there will be copious amounts of moisture streaming across the southern states over the next couple of days and it looks like a large batch of this moisture will push northeastward on Thursday into the Carolinas and perhaps as far north as southern Virginia. The result could be a significant snow event on Thursday/Thursday night for North Carolina, upstate South Carolina, and perhaps the southern part of Virginia with as much as half a foot possible in some of this region. As it stands right now, the moisture is not likely to make it as far north as Washington, D.C., but it is certainly close enough to monitor this threat over the next 48 hours. 

A significant snow event is in the cards for later this week from southern Virginia to upstate South Carolina; map courtesy NOAA, tropicaltidbits.com

EPO, NAO and the potential for snow in the Mid-Atlantic

In terms of the longer-term outlook for cold and snow in the DC-to-Philly-to-NYC corridor, the bad news is that time is running short on this winter season.  As we progress from late February into March, the seasonal averages for temperatures climb rather sharply from current levels meaning that any air mass that can be supportive of snow has to become more and more abnormally cold for the time of year.  The good news for cold and snow lovers in the eastern US is that there are some signs for pattern changes across North America that could produce more favorable conditions as we progress from late February into March. 

The Eastern Pacific Oscillation (EPO) is forecasted to drop into negative territory in coming days (boxed in region); Map courtesy NOAA

To begin, the pattern that has bottled up cold air over Alaska this winter season may be about to change some – at least temporarily.  A trough of low pressure has been situated for much of the winter season over Alaska along with persistent high pressure to the south of there over the northern Pacific.  This overall pressure pattern has generated a strong upper-level flow of air from west-to-east extending from the northern Pacific to the western part of Canada and into the northwest US.  This type of upper-level pattern has pushed mild air into western Canada and has made it more difficult for cold air masses to drop southeastward from Alaska into the central and eastern US.

One teleconnection index that provides us with information about the pressure pattern across the northern Pacific and Alaska region is known as the Eastern Pacific Oscillation (EPO).  When the EPO is in a positive phase, low pressure tends to dominate over Alaska with high pressure to the south and this generally results in strong winds blowing mild air eastward from the Pacific to western North America – the overall dominate pattern during the past several weeks.  When the EPO index goes negative, high pressure ridging tends to develop over Alaska and it turns warmer-than-normal and cold air outbreaks tend to increase from central Canada into the central and eastern US which makes it more favorable for sustained cold and snow.  The EPO is forecasted by some computer models to go negative in coming days which could change things in the Mid-Atlantic region with respect to the opportunities for sustained cold and snow as we progress to late February and into the early part of March. 

The North Atlantic Oscillation (NAO) index is forecasted to drop to near neutral or even into negative territory in coming days (forecast shown in red). Plot courtesy NOAA

In addition to the EPO, another teleconnection index known as the North Atlantic Oscillation (NAO) is forecasted to drop pretty sharply in coming days from positive territory to near neutral.  This index along with its closely related cousin known as the Arctic Oscillation (AO) provide us with information about the pressure pattern in the North Atlantic Ocean. Similar to the EPO, a negative phase of these teleconnection indices are more favorable for snow in the central and eastern US as cold air masses tend to last for a longer period of time. 

Final thoughts

Stay tuned on the prospects for cold and snow in the eastern US in coming days as there have been many false alarms for pattern changes this winter season.  As for now, the best chance for significant snow in the short-term is across the Carolinas and into the southern part of Virginia as the right combination of cold and moisture will materialize.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

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