* Winter weather advisory for Loudoun, Frederick, northern Fauquier northwest Howard and northwest Montgomery counties from 2 a.m. to 8 a.m. Wednesday *

SchoolCast and FedCast | Announced school delays

Tuesday evening update (9:00 p.m.) — Very minor snow event mostly on track

Temperatures this evening are on their way down, ranging from about 40 to 45 degrees just before 9 p.m. (except upper 30s toward Interstate 81 and upper 40s close to the Bay). It does seem like we’ll be dipping into the 30s after midnight which should allow the rain to change to sleet and then wet snow (this could happen even before midnight in our colder suburbs north and west).


Simulated radar at 1 a.m. Wednesday from high-resolution NAM model.

But the window for snow will be pretty short. Based on radar and short-term models, here are the most likely periods of snowfall by region:

  • Midnight to 3 a.m. for our western areas
  • 1 a.m. to 4 a.m. in the immediate area (may be mixed with rain and/or sleet initially)
  • 2 a.m. to 6 a.m. in our eastern areas (may be mixed with rain and/or sleet initially)

As we note below, accumulation, if any, should be mostly on grassy areas. We can’t rule out a bit of slush on roads, mainly in our colder areas north and west. Considering most snow should be over by sunrise, we don’t expect too many issues for the morning commute, but check our latest forecast and outside conditions before venturing out.

From the afternoon...

Temperatures surged to near 60 degrees Tuesday morning, but a big cold front sweeping through the region will drop temperatures into the 40s by dark. By around midnight Wednesday, they’ll be dipping into the 30s, cold enough to allow rain showers to change to sleet and then snow, ending before sunrise in most locations.

However, compared with Monday, forecast models have generally lessened the intensity and duration of frozen precipitation moving through as temperatures fall overnight, meaning the potential for accumulating snowfall is pretty limited.

It is likely a large part of the region may see just brief periods of either non-accumulating snow or snow that just coats grassy areas (and other cold surfaces like mulch and car tops) rather than roads.


Capital Weather Gang snowfall forecast for Wednesday. Trace amounts indicate snow falling but not accumulating.

The best chance for snow accumulation, up to an inch or so, is in colder areas north and west of the Beltway, especially in the zone from central Loudoun County to western Montgomery County and to the north and west. This area is under a winter weather advisory from the National Weather Service for one to two inches of snow (and up to 3 inches in the mountains). Here, rain will change to snow fastest, temperatures will be lowest (slipping to 31 degrees to 33 degrees), and a few slick spots could develop, especially on untreated roads, bridges, ramps and overpasses.

Everywhere else, the window for snowfall will be rather short, falling when temperatures are mostly above freezing (33 degrees to 37 degrees). However, there’s an outside chance the ground is quickly coated and there’s a little slush buildup on the roads if a band of heavier snowfall develops. Some models actually favor this happening east of Interstate 95 during the predawn hours, but confidence is very low regarding if and where a heavier band of snow develops.


Because of the timing of snow, this map mostly applies to the window between 4 and 7 a.m. Conditions should improve after that.

In all of these areas, precipitation will begin as rain showers Tuesday afternoon and evening, before sleet and snow start to mix between 10 p.m. Tuesday and 3 a.m. Wednesday from northwest to southeast. Here is the most likely timing for possible snow showers in the region:

  • North and west of the Beltway: Changeover from rain to snow between 10 p.m. Tuesday and 1 a.m. Wednesday. Snow ends between 3 and 5 a.m. Main period of concern: Midnight to 4 a.m.
  • Immediate area: Changeover from rain to snow between 12 a.m. and 3 a.m. Wednesday. Snow ends between 4 and 6 a.m. Main period of concern: 1 to 5 a.m.
  • East of the Beltway: Changeover from rain to snow between 1 a.m. and 4 a.m. Wednesday. Snow ends between 5 and 8 a.m. Main period of concern: 3 to 7 a.m.

High-resolution NAM model forecast of rain and snow between Tuesday evening and Wednesday morning.

Because the snow will exit the region between about 5 and 8 a.m., any slick spots from snow would probably focus in the early part of the commute. Conditions should improve after sunrise with skies gradually clearing and temperatures rising above freezing.

Out of an abundance of caution, the National Weather Service issued a special “potential winter commuting hazard” statement for the region Wednesday morning for “up to an inch” of snow, mainly north and west of Interstate 95, but indicated this scenario has just a 30 percent chance.

“If this threat does materialize during the Wednesday morning rush-hour, many roads could quickly turn slippery and visibility may be reduced,” the statement said. “This could lead to dangerous traveling conditions, multiple accidents, and extensive delays.”


Temperature forecast from high-resolution NAM model between 7 p.m. Tuesday and 10 a.m. Wednesday.

Discussion

Cold-front passages similar to this one in the past have failed to produce much in the way of accumulation. The one way to garner a boom scenario, which is enough snow to accumulate and create slick spots on roads in this case, would be for a moderate to heavy band to develop just before dawn when temperatures reach their lowest point of the night.

The latest model runs do generally feature a band of snow passing through the region during the predawn hours. But the warm ground and road surfaces will make it tough for the snow to accumulate unless it turns out to be intense — which is unlikely.

The other negative factor for much snow is that this band of snow is expected to be fast-moving. This morning’s high-resolution NAM model forecast suggests the band will sweep through the city by 5 a.m. and through most of the eastern suburbs except southern Calvert and St. Mary’s counties by 7 a.m.

Overall, this looks mostly like a coating-type snow event for most of the area except our far north and west suburbs, where an inch or so is possible and which could lead to a little slush on the roads if the snow falls hard enough. If the snow stays light, then roads are likely to remain wet.