The Washington PostDemocracy Dies in Darkness

Big winter storm Sunday-Monday probably brings snow that changes to rain in D.C. area

There’s a decent chance of snow accumulation and slick roads late Sunday before it changes to mixed precipitation or a cold rain

The European model shows the Washington region near the rain-snow line Sunday night into Monday morning. (WeatherBell)
Placeholder while article actions load

Computer models have come into agreement that a significant winter storm will develop this holiday weekend in the South before charging up the Eastern Seaboard and producing heavy precipitation.

For the Washington region, this most likely means a wintry mix of precipitation, starting as a period of accumulating snow Sunday afternoon or evening before changing to sleet and rain overnight. Precipitation could briefly change back to snow before ending early Monday.

The snow could cause slick roads and challenging travel conditions, especially late Sunday afternoon into the evening. Conditions might improve by Monday morning, depending on how the storm evolves.

While snow amounts in the immediate D.C. area may be modest, heavy totals of at least half a foot become more likely toward Interstate 81 into the mountains.

Significant storm to bring wintry mess, snow to Central, Eastern U.S.

The National Weather Service has declared a Level 3 out of 5 “enhanced” threat of a winter storm for the immediate area and Level 4 out of 5 threat for locations to the west.

Exactly how much snow falls, whether and when it changes to rain and what areas are hardest hit still need to come into focus, as the storm is still five days away. There are a few scenarios for how exactly the storm will play out, but the most likely seems to be up to a few inches of snow in the immediate D.C. area before a changeover to rain.

We think there’s a 50-60 percent chance of at least one inch of snow and about a 30-40 percent chance of at least three inches right around the District. Because of the anticipated switch to rain, the chance of at least six inches is probably only about 20 percent.

Computer model and storm scenario discussion

Computer models are now forecasting enough merging between the storm expected to develop over the South on Saturday and another system dropping southeast from Canada to draw the southern storm up the East Coast.

Now the big question is whether the southern storm comes far enough inland that we end up with more rain than snow for the immediate D.C. area and points east. A storm that tracks inland will draw in mild air off the ocean. Models still show a large spread in possible tracks.

In the first two scenarios described below, precipitation would start Sunday afternoon or evening and end before dawn Monday, falling heavily at times. It now seems very unlikely that the storm will miss to the east, and we’ll probably discard Scenario 3 in future updates.

Scenario 1: Inland track, with some snow accumulation for D.C. area before rain (65 percent)

In this scenario, the storm tracks between the Chesapeake Bay and the District. Such a scenario would offer a front-end thump of snow but would quickly transition to sleet and then rain, as warm air is pulled in from the Atlantic.

Snow would overspread the region Sunday afternoon, but the rain-snow line would quickly shift north and west to the city by early Sunday night. Historically, such a track produces about 1 to 3 inches of snow along Interstate 95, 2 to 4 inches just to the west and at least 4 to 6 inches toward I-81; locations in the mountains would probably see double-digit totals. There is a slight chance that the rain changes back to snow briefly as the storm pulls away early Monday in this scenario.

Scenario 2: Coastal track, with more significant snow for D.C. area (30 percent chance)

In this scenario, the storm tracks near Cape Hatteras in North Carolina and then up the coast but stays offshore. Such a track keeps winds out of the north, helping to hold in the cold air. This scenario would give the city significant snow, though it might still mix with or change to sleet and rain for a time along and east of I-95 before changing back to snow. Such a track would offer the potential for at least 4 to 6 inches along I-95 and 6 to 12 inches to the west, with locally higher amounts.

Scenario 3: Offshore storm track with only light snow (5 percent chance)

In this scenario, the storm would track far enough offshore to miss us or produce only light snow. Even if it snowed around the city, such a track might even leave our western suburbs dry. This scenario seems increasingly unlikely.

Model forecasts

Here’s a quick summary of the models, how much snow they project for the District and which scenario they broadly support:

  • American (GFS): 4 to 6 inches, reflecting a blend of Scenarios 1 and 2. Its modeling system (30 simulations) indicates a 60 percent chance of at least one inch, 50 percent chance of at least 3 to 4 inches and 30 percent chance of at least six inches.
  • European: 6 to 8 inches, reflecting a blend of Scenarios 1 and 2. Its modeling system indicates a 60 percent chance of at least one inch, about a 35 percent chance of at least three inches and 15 percent chance of at least six inches.
  • Canadian: 1 to 2 inches, reflecting Scenario 1. Its modeling system indicates a 70 percent chance of at least an inch of snow, but just a 25 to 30 percent chance of at least three inches and no chance of more than six inches.
  • UKMet: 2 to 4 inches, reflecting Scenario 1.

This storm, while fairly fast-moving, does the have the potential to produce heavy precipitation and significant impacts in the region. We’ll attempt to fine tune the forecast in daily updates.