Snow accumulation forecast. Second call, issued 9:55 p.m. Monday.

(This article was updated based on new computer model runs and winter weather advisories issued Monday afternoon and evening.)

* Winter weather advisory from noon to 7 p.m. Tuesday west and north of the District, including Fairfax and Montgomery counties *

9:55 p.m. – Snow accumulation potential has increased and winter weather advisory is expanded

Computer models this afternoon and evening show an increased likelihood of a period of heavy snow in many parts of the Washington region between about 2 and 6 p.m. on Tuesday (the heavier snow may hold off until 3 p.m. or so east Interstate 95). We have upped our accumulation forecast (see above) to a coating to two inches along the Interstate 95 corridor and one to three inches for our northern and western areas.

During this 2 to 6 p.m. window on Tuesday, snow could fall at rates of up to an inch an hour or so at times, lowering visibility and, especially west of the Interstate 95 corridor, creating the possibility of slick roads. It is in this zone where the National Weather Service has expanded its winter weather advisory.


Area under winter weather advisory on Tuesday.

Along and east of Interstate 95, accumulation is most likely on grassy areas. Snow should mostly melt on pavement but we cannot rule out some slick spots during heavier bursts.

Be aware that schools may decide to dismiss early on Tuesday, especially west of Interstate 95. We anticipate after school activities may be canceled in many areas.

Because heavy snow could impact the early part of the evening commute, we strongly recommend being flexible with your plans. Consider leaving work early - BEFORE 3 p.m., or delaying - AFTER 7 p.m. to avoid the possibility of heavy traffic and possible gridlock.

Snow should rapidly exit the region between 6 and 7 p.m. from west to east.

Original forecast from earlier Monday

This winter so far has been practically snowless, and over the past two weeks, almost every day has exceeded 50 degrees. But a burst of snow Tuesday afternoon into the early evening will offer a reminder of the season.

The snow should not be too disruptive in most areas, given above-freezing air and ground temperatures, but it could come down hard for a time. Between about 3 and 5 p.m., snow could lower visibility, possibly slowing down the early part of the Tuesday evening commute.

Most accumulation should be on grassy areas. However, given a heavy enough burst of snow, temperatures could drop to near-freezing for a time, allowing a slushy buildup on pavement, especially in colder areas north and west of the District.

A few computer models on Tuesday afternoon suggest the possibility of up to a few inches of snow in parts of the region where the heaviest snow falls. If and where such a scenario materializes, this event could prove more disruptive.

Precipitation timing

We expect precipitation to arrive between about midday and 2 p.m., from southwest to northeast. Temperatures initially near and even above 40 mean that precipitation is likely to start as rain or a rain-snow mix, especially along and east of Interstate 95.


High-resolution NAM model forecast radar between 10 a.m. and 7 p.m. Tuesday.

Between about 3 and 5 p.m., everyone should see snow except in far Southern Maryland where more of a rain-snow mix is likely. Then, the precipitation will rapidly exit from west to east between 5 and 7 p.m.

In most areas, precipitation will last about four hours, but it could come down heavily for a time. Although the system never really taps into Gulf of Mexico moisture, it is associated with a fairly vigorous upper-level system providing it with the potential to produce a narrow band of moderate to heavy snow. Snowfall rates of an inch per hour or so cannot be ruled out in this band for a short duration.

Potential snow amounts

Trace amounts of snow (meaning snow in the air melting on the ground) to around an inch is most likely along and east of I-95. In Washington’s western and northwestern suburbs, up to an inch or two is possible.

The accumulation forecast is complicated by uncertainty in where bands of heavy snow might set up. Some models predict that the heaviest precipitation will occur south or southwest of the District, where snow amounts on the high end of forecasts (or a boom scenario) can’t be ruled out. We wouldn’t be shocked to see an inch or two materialize somewhere in the immediate metro area.

Wherever the heaviest precipitation occurs, temperatures are likely to drop the most. If temperatures drop to 32 or 33 at the same time as heavy snow is falling, you could end up with a slushy covering on some roads. But in locations where the precipitation remains light, temperatures will remain safely above freezing, with the possibility of rain mixing in and little or no snow accumulation.


High-resolution NAM model temperature forecast between 1 and 10 p.m. on Tuesday.

After the precipitation exits, temperatures will fall to near-freezing or a little below in our colder areas, meaning a few slick spots could develop after dark as the snow ends, especially west and northwest of the Beltway.

Model forecasts

Computer models project up to several inches of snow in the region from this storm, but they assume that air and ground temperatures are below freezing, which will not be the case. Taking into account above-freezing temperatures and the possibility that some of the precipitation will fall as rain, we would cut the following model forecasts for the District in half:

  • European: 2.2 inches
  • American (GFS): 3.3 inches
  • NAM: 5.7 inches
  • High-resolution NAM: 4.3 inches
  • Canadian: 1.6 inches
  • High-resolution Canadian: 2.0 inches
  • HRRR: 0 inches

If you take the average of the above forecasts and then cut it in half, it comes out to a little more than an inch of snow. But the higher-end projections from these models certainly leave open the possibility of a “boom” scenario in which amounts are more than that.

Schools

The snow will arrive too late for school to be delayed or closed Tuesday morning. However, some systems — especially in the west and southwest part of the region — may decide to dismiss early in the afternoon. We think the best chance of some early dismissals would be in Fauquier, Prince William and Loudoun counties. Elsewhere, with precipitation moving in right around 2 p.m. and the temperature above freezing, we would lean toward on-time dismissals.

In areas where accumulating snow falls, it’s not impossible that a few delays could occur Wednesday morning, since temperatures will drop below freezing Tuesday night. The best chance of this occurring is in the western part of the region.

We’ll evaluate the need for a SchoolCast on Tuesday.