11 p.m. — Evening models so far suggest forecast is mostly on track

Since our update this afternoon, the latest model projections which have come in do not provide a compelling reason to change our forecast. The European model run this afternoon edged a little south with its projection for the most snowfall (toward central Virginia), but the American and NAM models edged slightly north (into Maryland).

Looking at the universe of models, there’s some sense that their projected snowfall totals averaged together have come down slightly, but not enough for us to change our forecast (we had anticipated this might happen). However, if the models reduce their amounts any further, we may have to consider some adjustments on Wednesday.

Original article from the afternoon

The Washington region faces two waves of snowfall in the second half of the workweek that, put together, may blanket the area in several inches to a half-foot of snowfall.

The first wave is set to arrive Wednesday afternoon and evening and taper off Thursday morning. The second wave, expected Thursday evening into Friday morning, may generate the heaviest precipitation south of the District.

In total, most locations should see two to three inches of snow on the low end and up to six to seven inches on the high end. However, locally, somewhat lesser or greater amounts cannot be ruled out, depending on the exact track of the two waves, surface temperatures and where any heavier bursts of snow develop.

Potential snow amounts

Temperatures hovering near or even a little above freezing may limit the potential for snow accumulation during the first wave, especially inside the Beltway and to the east and south, where snow could mix with rain or sleet at times. However, some models show colder temperatures and the potential for a heavier burst of snow, allowing amounts from the first wave to exceed current expectations even in the immediate area.

Cold air should become better established by the time the second wave arrives, but by then the heaviest snow may focus south of Washington. Models differ in just how far south, and some show the possibility of up to several inches as far north as Washington’s northern suburbs.

Taking both waves together, there is considerable uncertainty and boom and bust potential for snowfall amounts, but a few inches to around half a foot is the most likely range. We expect we’ll need to refine this forecast into Wednesday and, for the second wave, possibly Thursday.

Storm timeline

Here’s roughly how we expect this event to evolve. Fine-tuning will be necessary in future forecasts as it draws closer. The given temperature ranges generally indicate the coldest readings in our northwestern areas and mildest in our southeastern suburbs.

3 to 7 p.m. Wednesday: Wet snow develops, possibly mixed with rain or sleet in downtown Washington and points south and east. Temperatures: 34 to 39.

7 p.m. Wednesday to 4 a.m. Thursday: Periods of snow, except possibly a wintry mix in downtown Washington and points south and east. Temperatures: 29 to 34.

4 to 10 a.m. Thursday: Snow, gradually tapering off by midmorning. Temperatures: 27 to 32.

10 a.m. to 4 p.m. Thursday: Pause in precipitation. Some melting may occur in milder areas. Temperatures: 29 to 34.

4 to 10 p.m. Thursday: Snow redevelops and is steadiest from the District south. Temperatures: 27 to 32.

10 p.m. Thursday to 4 a.m. Friday: Snow, steadiest from the District south. Temperatures: 26 to 31.

4 to 8 a.m. Friday: Snow tapers off. Temperatures: 24 to 29.

What we know

  • An Arctic front will be located to our south, providing a supply of cold air at low and mid-levels. Aloft, winds blowing from southwest to northeast will blow over the top of the cold dome, generating a prolonged period of winter weather between late Wednesday afternoon and Friday morning, much of it in the form of snow.
  • There will probably be a break in the precipitation during the day Thursday.
  • Temperatures at the ground will probably be above freezing when the snow first starts Wednesday afternoon and evening. Above-freezing ground temperatures could cut down on snow accumulations unless the snow falls heavily. Snow will more readily stick north and west of the city.
  • Models sometimes overestimate precipitation totals with these kinds of events, which are known as “overrunning” situations, because precipitation can diminish as it passes over the mountains from the west.
  • The best chance of accumulating snow from the District south will occur Thursday night into Friday morning with the second wave, as temperatures drop into the 20s.
  • Snow will end Friday morning.

What we’re less confident about

  • How quickly the surface temperatures will cool to below freezing in the city and points to the south Wednesday night. Models vary widely in their temperature projections. Above-freezing temperatures and the possibility of sleet Wednesday night near and to the south of the city may limit the accumulation potential unless a heavier burst of snow develops or the colder model projections are correct.
  • Will one of the two pulses of snow underperform model projections? In the past few storms, some models have grossly overestimated snowfall totals, and that may occur again (though our predictions take this possibility into account to some extent).
  • Exactly where the axis of the heaviest bands of snowfall will set up during both waves of winter weather. The models have differed on this issue. The first wave could miss the immediate area Wednesday night to the north or south, while the second could miss to the south. That’s the prescription for a busted forecast. Alternatively, both waves could hit head on, resulting in the season’s biggest snow.
  • Will a narrow band of heavy snow set up during either of the two weather systems? Narrow bands of enhanced snowfall rates can sometimes be located north and west of where the models simulate them and could result in “boom” snowfall scenarios.

Model discussion

For the second wave, models are in general agreement that the heaviest totals will focus south of Washington and that about two to three inches will fall in the immediate area. (Some models suggest more is possible.)

But there is more model disagreement, oddly, for the first wave, even though it’s not as far into the future. The American (Global Forecast System, or GFS) and high-resolution NAM (North American Mesoscale) models favor more snow north and northwest of the District, while the European, Canadian and UKMet models lean toward the heaviest snow in the immediate area or just to the south. This latter set of international models also has lower temperatures, facilitating more snow accumulation

First wave
Second wave
American (GFS)
High-resolution Canadian
High-resolution NAM

Our forecast reflects a blend of these two camps but may need to be adjusted depending how they shift. We’ve also shaved a bit off their average projections given their tendency to overestimate amounts in these overrunning events.