* Winter storm watch north and west suburbs late Sunday night through Tuesday morning *

4:10 p.m. update: The National Weather Service has issued a winter storm watch for Montgomery, Loudoun, Fauquier, Fairfax, Prince William, and Stafford counties, and locations to the north and west for the potential for five inches of snow and sleet and then up to 0.25 inches of freezing rain. The watch does not include the District and points east.

Area under winter storm watch shaded in blue. (NWS)

Original post

The North American Model (NAM) shows snow overspreading the D.C. area by 7 a.m. Monday. (WeatherBell)
The North American Model (NAM) shows snow overspreading the D.C. area by 7 a.m. Monday. (WeatherBell)

The Washington area is likely to see a few inches of snow Monday morning into the afternoon, possibly starting prior to the holiday morning commute. For most of the area, the snow is likely to change to an icy mix of sleet and freezing rain by late Monday afternoon or evening, and then to rain by Tuesday morning. But how quickly that transition occurs is still up in the air, as models still disagree on how fast southerly winds will erode the cold Arctic air mass that is delivering our painful punch of polar air this weekend.

Getting around is likely to be difficult Monday into Monday night, since the ground will be very cold and the snow, sleet and freezing rain will stick to all surfaces. How much snow we get in total, and how disruptive the storm might be into Tuesday morning, will be determined by how quickly any changeover occurs. East of the city toward the Bay the changeover to rain is assured by late Monday evening. West and north of the city, especially west and north of the Beltway, is where the biggest forecast uncertainty lies.

We estimate the chance of at least one inch of snow in the immediate metro area at 70%, and the chance of at least 4 inches around 20-40% (lower south and east, higher north and west). But it is the evolution from snow to ice to rain that is perhaps more impactful looking ahead to the Tuesday morning commute after the holiday weekend.

We see two main scenarios as still on the table for this storm…

Scenario 1: Some snow, then brief period of ice, then rain; Lower impact Tuesday morning (60% chance)

Snow changes briefly to ice around the city by Monday evening, and then to heavy rain overnight in most areas. Even this scenario might offer 1 to 2 inches of snow inside the Beltway and might only give folks near the Bay a quick inch. The snow and ice would hang on longer west and north of the Beltway with 2-4″ of snow possible, with around 6 inches possible well west near I-81. However, the snow would change to rain across the entire D.C. metro area by around 1 a.m. Tuesday morning, even in the far western and northern suburbs.

Scenario 2: Some snow, then prolonged ice storm west/north of D.C., then rain; Moderate-to-high impact Tuesday morning for areas west and north of D.C. (40% chance)

Snow changes to an extended period of ice overnight Monday night, especially in western areas, with a likely transition to rain in the immediate D.C. metro area by around 4-7 a.m. Tuesday morning. In such a scenario snow could accumulate around 2-4 inches inside the Beltway and to the south and east, 3-6″ west and north of the Beltway, and 6-8 inches possible along the I-81 corridor. A significant ice storm could play out somewhere west of the city.

Model discussion for scenario 1 (Some snow, then brief period of ice, then rain; Low impact Tuesday morning; 60% chance):

Last night’s European and UKMET models support this scenario, tracking the low to our west (rather than the east like the GFS and Canadian models). A track over or west of the city with a strong high pressure system off the coast would combine to feed warm air from the Atlantic and points south leading to a more rapid changeover than a more easterly track, where lighter north winds would have a hard time scouring out the cold air near the surface.

Today’s European model, which is consistent with last night’s model run, illustrates how quickly it might warm across the area with a storm track to our west. The freezing line at the surface (32F line) is well to our north by 1 a.m. Tuesday. Heavy rain hits us by the Tuesday morning rush, but at least it would be wet rather than icy.

Today’s European model valid at 1 a.m. Tuesday. (StormVistaWxModels.com)

With a wrapped-up low, the heaviest precipitation might skirt us to the north while we are cold enough for snow. The European’s faster changeover from snow to sleet and freezing rain results in only an inch or two for the city and 2-4″ for most of the suburbs to the north and west as shown below.

Snow accumulation forecast by Tuesday morning from today’s European model. (StormVistaWxModels.com)

The European ensembles from last night strongly support the operational European model and its more westerly storm track. The figure below displays the location of each of the individual ensemble members and almost to a member, the track is over or west of the city. The figure also can give you a feel for why such a track would more easily erode the cold air over the region. Each of the lines on the figure represent a line of equal pressure based on the ensemble mean. Note the strong high pressure system toward the right side of the figure. The closer those lines are together, the stronger the surface winds. The winds follow those lines but blow at an angle toward lower pressure, so the winds from the southeast associated with such a pattern would bring relatively warm air from the Atlantic into the region.

Low-pressure positions forecast by the European model ensemble members for 1 a.m. Tuesday. (WeatherBell)
Low-pressure positions forecast by the European model ensemble members for 1 a.m. Tuesday. (WeatherBell)

Model discussion for scenario 2 (Some snow, then prolonged ice storm west/north of D.C., then rain; Moderate-to-high impact Tuesday morning for areas west and northwest of D.C.; 40% chance)

This morning’s NAM supports scenario 2, tracking a weak low pressure center just to our east. The weaker low allows the cold air to hang around long enough to give D.C. and points south and east 2 or 3 inches of snow, and the areas just north and west of D.C. 3 to 6 inches, prior to changing over to sleet and freezing rain. With this scenario there’s a risk that areas west of I-95 could hold on to ice through most or all of Monday night and into Tuesday morning.

Today’s GFS (see below) is also in this camp, forecasting a weaker low just to our east and then allowing it to start intensifying, although it changes the immediate north and west suburbs (e.g., Montgomery and Fairfax counties) to rain more quickly than the NAM. After a good front-end thump of snow and then a period of sleet and freezing rain, the GFS forecast has temperatures in the city rising above freezing by 1 a.m. Tuesday and in the close-in north and west suburbs by 4 a.m. It also brings heavy rain into the city Tuesday morning toward the end of the rush, which would probably help melt the snow such that Tuesday may not be a total loss except for the far northern and western suburbs where the cold lingers longer. The Canadian model has a very similar forecast to the GFS, and like the GFS brings heavy rain into city Tuesday morning.

Total snow accumulation forecast from the GFS model for Monday-Tuesday. (StormVistaWxModels.com)

Wes Junker is the Capital Weather Gang’s Winter Weather Expert.