This story was last updated on Saturday. For our latest article on this storm updated Sunday, see: Major winter storm to slam Mid-Atlantic and Northeast midweek, with D.C. on rain-snow dividing line
As so little snow fell last winter, this event has a strong chance to be the most disruptive winter storm since 2019. The storm may also unload heavy snow and/or mixed precipitation into the Northeast, from Philadelphia to Boston, later Wednesday into Thursday.
In the Washington-Baltimore region, confidence is highest in a significant snowstorm for areas in the zone along and west of Warrenton, Leesburg, and Frederick. Heading east, computer model forecast a fairly strong likelihood that a mix of snow, ice and rain will fall. Toward the Chesapeake Bay and Southern Maryland, more rain is likely to fall than frozen precipitation.
Precipitation is forecast to begin between sunrise and early afternoon Wednesday and end late Wednesday night or early Thursday morning.
There’s a high chance most places in the Washington region, except perhaps Southern Maryland, will see a period of snow at the onset of the event. But as the storm cranks up in the eastern Carolinas, it will draw milder ocean air northwestward. The question then becomes where and when a changeover to ice and rain occurs and how much snow falls before the changeover.
At the moment, most models indicate the rain-snow line will push as far west as the I-95 corridor, changing snow to ice and rain in Washington-Baltimore, and perhaps even farther west closer to Leesburg and Frederick on Wednesday afternoon or evening. This would limit the amount of snow that would fall in the immediate Washington area (all locations within about 10 miles of the Beltway). However, some models hold the cold air in enough for precipitation to remain mostly snow, even in the District.
There is also a chance that after flipping from snow to mixed precipitation, some areas could change back to snow before ending when the storm passes north.
As the storm is still four days away, timing, precipitation forecasts and the position of the rain-snow line are all likely to evolve. Right now, we see the most-likely scenario as a period of snow at the onset of the event on Wednesday for the area within about 10 miles of the District, with a light accumulation, before changing to mixed precipitation. However, we cannot rule out snowier or rainier scenarios.
Here’s our current assessment of snow potential:
Chance of at least one inch of snow:
- Front Royal, Winchester, Martinsburg and Hagerstown: 90 percent
- Warrenton, Leesburg and Frederick: 70 percent
- Manassas, Reston and Gaithersburg: 60 percent
- Washington and Baltimore: 45 percent
- Fredericksburg, Waldorf and Annapolis: 30 percent
Chance of at least three inches of snow:
- Front Royal, Winchester, Martinsburg and Hagerstown: 80 percent
- Warrenton, Leesburg and Frederick: 60 percent
- Manassas, Reston and Gaithersburg: 45 percent
- Washington and Baltimore: 30 percent
- Fredericksburg, Waldorf and Annapolis: 15 percent
Chance of at least six inches of snow:
- Front Royal, Winchester, Martinsburg and Hagerstown: 60 percent
- Warrenton, Leesburg and Frederick: 40 percent
- Manassas, Reston and Gaithersburg: 30 percent
- Washington and Baltimore: 20 percent
- Fredericksburg, Waldorf and Annapolis: 10 percent
This looks like a storm with the rain-snow line setting up almost right over D.C. A strong zone of high pressure to our north in southeast Canada is well-positioned to feed chilly air south east of the mountains, facilitating what’s known as cold-air damming. But, as the coastal storm forms, a warm front will develop that will try to push inland. Temperatures in Norfolk will climb into the low 50s and some of that milder ocean air may push toward Washington, depending on the track of the storm.
With cold air damming events, the question is always how much it is eroded by any push of warmer air from the east. The models still don’t have a definitive answer because two critical factors are unknown:
- How quickly the low pressure system will form along the coast.
- Its eventual track.
Those two factors will govern how much snow the storm will produce around Washington. If the coastal low develops slowly, which would allow milder air to invade, or if the low tracks inland, the rain-snow line will eventually push well west of the city. By contrast, if the coastal low develops more quickly and its track stays just offshore, cold air would stay in place and Washington would probably end up with a significant snowstorm.
Fifty simulations from the European model run show a wide variety of storm evolutions and tracks. Their forecasts for the District range from a rainstorm to a major snowstorm (about 20 percent forecast this). The majority suggest something in between.
Forecasts from the European model as well as the American (GFS) model do show a high probability of significant snow along the Interstate 81 corridor from Front Royal and Winchester to Hagerstown.
Another tricky element in this forecast is that the lower-resolution American (GFS) and European models often don’t handle surface temperatures well in cold air damming cases. Their temperature predictions are often too high. This introduces the possibility that there could be a narrow stripe of freezing rain and sleet in the transition zone between the snow in western areas and rain in eastern areas. We will likely have to wait to refine the precipitation-type forecast until we get into the range of the higher-resolution models like the NAM on Monday or Tuesday.
In summary, the I-81 corridor has the best chance of staying all snow and significant accumulations. The farther south and east you go, rain becomes more likely. In between those areas, a messy mix seems most likely at this time.
Outlook for the Northeast
The upcoming storm also has the potential to be a major winter weather event in the big cities of the Northeast, including Philadelphia, New York, Hartford, Providence and Boston. While some of these areas may straddle the transition zone between snow, mixed precipitation and rain, there is the potential for six or more inches of snow in each of these locations.
Looking ahead toward the middle of next week, the potential is increasing for an impactful winter storm to affect the Mid-Atlantic and Northeast. Uncertainty remains highest near major cities along I-95. Here is the latest forecast.❄️ pic.twitter.com/G3Qnuaku4d— NWS Weather Prediction Center (@NWSWPC) December 12, 2020
The National Weather Service forecast office for Philadelphia wrote Saturday morning that the upcoming weather setup is “Possibly the most active winter weather pattern the forecast area has experienced in quite some time,” noting that significant snowfall is possible with the midweek storm into the I-95 corridor.
The precipitation type forecast is not yet clear for New York City, either, but at least some snow is likely from this storm there, along with the potential for gusty winds.
Depending on the storm track and intensity, the snow could be heavy throughout Southern New England, too. The European model brings heavier snow further north than the American model does, and cold air will be more entrenched in this region, raising the odds of an all-snow event.
The big question facing forecasters in Boston is how far north the low pressure area tracks, with some models simulating a more southerly scenario where the heaviest precipitation stays south of the region. Still, the Weather Service noted Saturday morning that six-plus inches of snow are possible across much of Southern New England from the storm on Wednesday into Thursday.