The chance of a storm along the Mid-Atlantic coast the coming weekend continues to increase. However, the storm’s evolution and its specific effects for our area are less certain.

The most likely timing for the storm would be the second half of Saturday into Sunday. Right now, we favor a mixed bag of precipitation in the D.C. metro area, with an increasing chance of substantial snow towards the mountains, and mostly rain towards the Chesapeake Bay and Delmarva.

Preliminary estimates of the chances of at least one inch of snow:

Washington, D.C. and the I-95 corridor: 25 percent

Columbia to Gaithersburg to Dulles to Warrenton: 35 percent

Frederick to Leesburg to Marshall: 50 percent

Hagerstown to Martinsburg to Winchester: 60 percent

Note that these chances, particularly in the western areas, are conservative because the storm is still 5-6 days away and may well increase in the coming days, depending on what storm scenario plays out. In future days, we’ll add additional categories to include the chances of at least 4 inches and (possibly) at least 8 inches.

Storm scenarios

1) Big storm from the Gulf Coast states coming up the Mid-Atlantic coast (most likely)

In this first and most likely scenario, a low pressure system would track across the Gulf Coast states and Southeast to a location off the Mid-Atlantic coast. This is a classic “Miller A” storm evolution – which typically means the storm is able to tap abundant moisture from the Gulf of Mexico and Atlantic Ocean.



European model forecast of storm Friday through Sunday (WeatherBell.com)

Such a storm track could give parts of the area accumulating snow with the favored north and western suburbs most likely to cash in, especially locations with a little elevation. However, temperatures around the city look like they would be marginal for snow in this scenario, making it a tough call to give much lead time on who might get significant snow or who might end up being wet instead of white. Right now, a mixed bag of precipitation seems most likely for the D.C. area while the I-81 corridor into the mountains may be looking at a heavy snow event if this scenario plays out.

(Favored by the European weather model)

2. A weaker, warmer storm tracking into Ohio Valley and reforming off the Mid-Atlantic coast (less likely)

In this scenario, a storm tracks to the Ohio Valley before reforming along the Atlantic coast. This scenario is still on the books but seems to be slowly slipping away. Such a storm track would allow enough easterly flow (off the ocean) to keep the air mass over the D.C. area sufficiently warm for rain rather than snow even for our western locations. In such a scenario, most of us would get rain rather than snow but some mixed precipitation might occur in colder locations to the north and west of the city.



GFS model simulation of storm at 1 p.m. Saturday with rain over the District and a bit of wintry mix to the north and west (WeatherBell.com)

(Favored by the American weather model)

3) Storm slides by to the south (least likely)

In the third and least likely scenario, the storm track would stay far enough south that the precipitation would fringe us or miss completely.



Canadian Model simulation of weekend storm at 7 p.m. Saturday. (WeatherBell.com)

(Favored by the Canadian weather model)

Right now, the first of the three scenarios is the most likely. However, within this scenario, small changes in the storm track will likely have major implications on the location of the rain-snow line and snow amounts. More of an inland storm track that takes the low pressure system over the eastern shore of Chesapeake Bay would likely lead to more rain except for our far western suburbs and the mountains. A storm track just off the coast, the most favorable track for snow, would give locations just west of I-95 the possibility of a significant snowstorm, and might offer some accumulating snow in the city. There are too many variables that need to be worked out before any definitive forecast on the storm can be made.

The reason we lean towards the first scenario is because it has been shown consistently by the European model in its last four consecutive simulations (run 12 hours apart – since Saturday morning). When a model repeatedly shows a similar storm evolution, it gives us some confidence that it has the right idea. That, along with the fact the European model is the most accurate model on balance and has done the best job forecasting storms in recent weeks, has nudged us towards favoring it.



All above forecasts are for 1 a.m. Sunday morning (or overnight Saturday) except the bottom left panel which is valid at 7 p.m. Saturday evening. (WeatherBell.com)

However, as the storm is still 5-6 days away, there is a lot of time for the forecast to change. Just because the European model has been consistent for the past 48 hours doesn’t mean its forecast may not change in important ways.

Also, even though the European model has been consistent in forecasting a significant storm, its exact storm track has wavered enough for there to be significant uncertainty in the rain-snow line . And, for the Washington, D.C. area, the location of the rain-snow line will ultimately be the deciding factor in determining storm impacts across the region and who sees heavy snow.