December 15, 2020

A Significant Winter Storm Will Plaster The Mid-Atlantic And Northeast This Week

Gird your loaves and invest in milk futures: a snowstorm's comin'. A big one, in fact. Parts of the Mid-Atlantic and Northeast are getting ready for the season's first big-time snowstorm. Some areas will see more snow than they've seen in two years—of course, that's not as impressive as it sounds considering last winter's snows were a dud in the Mid-Atlantic. As we see with so many coastal storms, the precise track of the storm will determine whether you get blanketed by snow or shrouded by bitter disappointment.

The Setup

A trough in the jet stream will drive the formation of the low-pressure system that will become our mid-week winter storm. A low-pressure system will develop in the Carolinas on Tuesday night and move up the coast through Thursday. The system will have plenty of moisture and cold air to work with, so there's not much wishy-washiness about whether or not the system will produce big snowfall totals. There's going to be a sharp cutoff between blockbuster snows and a nuisance, so the ultimate track of the storm will make a big difference on who sees huge snows or a cold rain.

The Timing

Rain will begin over the southeast on Tuesday night and Wednesday morning as the low-pressure system starts to develop. The storm will pick up intensity in a hurry as it approaches the Mid-Atlantic, which is when wintry precipitation will start on the northern side of the storm.

Freezing rain will likely begin in western North Carolina and parts of western Virginia on Wednesday morning and afternoon. Temperatures will eventually climb above freezing across areas expecting freezing rain, so the question will be how long it takes the subfreezing air at the surface to erode. The longer it sticks around, the thicker the crust of ice will grow. It shouldn't be a big ice storm, but any crust of ice is dangerous when there's ice on the roads and if tree limbs or power lines snap under the weight. Temperatures should—should!—climb above freezing by late afternoon across most areas expecting ice.

Farther north, the snow will begin on Wednesday afternoon west of D.C. and spread north through the afternoon and evening hours. Precipitation will taper off on Thursday morning from west to east. 

The Track

Six runs of the GFS model showing how tiny shifts in the storm's position can move around the rain/snow line. SOURCE:

Track track track. Track? Track. Okay, I got that word out of my system. It'll be like Edith and the cling peaches. Won't say that word again.

The structure of coastal storms like this is usually pretty textbook. The rain/snow line follows close to the tra—uh, path—of the center of the low-pressure system, so you wind up with rain to the south/east and snow to the north/west. You get a pretty heavy shield of snow to the northwest of the low that accounts for the greatest snowfall totals. If that line sets up near I-95, you wind up with those memorable storms that immobilize big cities for an entire week.

Since precipitation type and totals are so heavily dependent on the storm's motion, getting it right is crucial to the forecast. If the storm moves 20 miles to the right or to the left of what was forecast, the rain/snow line and shield of heavy snow will follow suit. That could result in surprises and disappointments.

Right now, it's likely that any jog in the storm's motion would be a northerly jog, which would push the heaviest snowfall totals north.

The Snow

Speaking of surprises and disappointments, check out that tremendous cutoff between lots of snow and not much at all. That's why even a tiny shift in the storm's motion could result in a tremendous difference in snowfall amounts for these densely populated communities.

Right now, the National Weather Service's forecast calls for more than a foot of snow from northwestern Virginia to southern Connecticut, which is a vast swath of real estate at risk of seeing a great deal of snow. The greatest totals are likely in central Maryland, eastern Pennsylvania, northern New Jersey, and the Hudson Valley in New York—these areas could see more than a foot-and-a-half of snow by the end of the storm.

Again, it won't take much for that sharp cutoff in totals to nudge a dozen or two miles to the right or to the left, which could have big implications for densely populated communities along I-95. When you're preparing for a storm, it's always best to prepare for the worst so you're ready no matter what happens.

The Weather Prediction Center's new Winter Storm Severity Index (WSSI) shows major to extreme impacts across areas expecting more than a foot of snow. The greatest disruptions are likely in eastern Pennsylvania and northern New Jersey, where a combination of deep snow on the ground and the weight of the snow on trees, power lines, and flat roofs could cause major issues.

Across the hardest hit areas, travel will be all but impossible during and immediately after the storm. It could take several days for crews to reach residential areas. Hopefully the fact that travel is at a minimum and work/school is predominately conducted at home these days will make it easier for crews to safely clear the roads in a reasonable amount of time.

Now that most trees have lost their leaves and the snow isn't going to be particularly wet, there shouldn't be a widespread risk for power outages, but this much snow built up on power lines and tree limbs could lead to some power outages.

It's likely that the snowpack will stick around for a while after the storm—it doesn't look like there are any significant warmups on the way for the eastern states.

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I contribute to The Weather Network as a digital writer, and I've written for Forbes, the Washington Post's Capital Weather Gang, Popular Science, Mental Floss, and Gawker's The Vane. My latest book, The Skies Above, is now available. My first book, The Extreme Weather Survival Manual, arrived in October 2015.