* Winter weather advisory 10 p.m. Thursday to 10 a.m. Friday | SchoolCast | Announced school closings and delays *

11:30 p.m. - Periods of snow and sleet overnight, check conditions in the morning

Areas of snow and sleet should move in from the west or develop overhead during the next couple of hours. Otherwise there are no late changes to our forecast. We encourage you to check back for the latest conditions before you head out in the morning. For those areas that do see roads turn slick--best chance being north of the Beltway--we expect conditions to quickly improve after the precipitation moves out by 8 a.m. or so.

9:30 p.m. - Precipitation arriving later this evening, coldest in northern suburbs

Looking at radar and short-term modeling, we don’t expect snow and sleet to start locally until around midnight to 1 a.m. Current temperatures tell the story why the best chance of snow or sleet sticking to pavement is north of the Beltway, where readings are down to the mid-30s, and poised to fall to around 30-32 when precipitation starts to fall.


Temperatures as of about 9:30 p.m. (Weather Underground)

Temperatures are still in the mid-to-upper 30s around and south of the Beltway, where snow or sleet will have a harder time sticking. Although even here, a heavier burst of snow or sleet could make roads slick and reduce visibility for a short period of time.

Consistent with our detailed forecast below, the latest model data suggests the precipitation should taper from west to east by around 6-8 a.m

5 p.m. - Forecast holding steady

Based on this afternoon’s model data, we see no reason to stray from our forecast and accumulation projections below. Most models continue to simulate very light amounts of snow and sleet for locations near the District and points south (trace amounts of snow to an inch or so), with the potential for around 1 to 3 inches as you head into northern Maryland.

There are a couple models which show the potential for 2 or 3 inches right around Washington. We can’t rule out a localized heavier area of snow setting up over the immediate area but this has just about a 25 percent chance of happening. It is indicated in our “boom” scenario on our snow accumulation map (below).

After new model data comes in, we’ll post an update in the 9 o’clock hour this evening.

Main forecast briefing (from earlier this afternoon)

A rather modest weather system will sweep across the Washington region late Thursday into Friday morning. But there may be just enough moisture and cold air for some snow accumulation, especially in colder areas north and west of the city.

This snow could lead to travel delays and school disruptions Friday morning.

A coating to a couple of inches of snow, possibly mixed with sleet, is possible just north of downtown Washington, including areas such as Vienna, Bethesda, Olney and Columbia. Projected amounts increase to about one to three inches in northern Maryland and northwest Virginia.


Capital Weather Gang snowfall forecast for March 1.

In both of these two northern zones, untreated roads and sidewalks could become slick, especially during any bursts of moderate to heavy snowfall.

From downtown Washington and points south, more of a mix of snow, sleet and rain is likely. Here, most snowfall, if it materializes, would most likely be limited to grassy areas, mulch and car tops as air and pavement temperatures are likely to remain near or above freezing. That said, if a band of heavier snow or sleet develops over this zone, we cannot rule out a period of slick roads and reduced visibility.

“The greatest threat for impact from this snowfall during the Friday morning rush hour will be north of the Capital Beltway Interstate 495,” the National Weather Service wrote in a special statement, and we concur. “If you plan on commuting Friday morning, be aware of the potential for travel disruptions. Plan ahead by allowing for extra travel time, and consider using public transportation and telework options.”

Storm timeline


High-resolution Canadian model simulation of precipitation Thursday night into Friday.

10 p.m. Thursday to 2 a.m. Friday: Snow develops in our northern areas, with a wintry mix developing elsewhere — from west to east. Temperatures 33 to 39, northwest to southeast. Few problems expected.

2 a.m. to 6 a.m. Friday: The heart of the event. Snow, possibly moderate to heavy in our northern areas. Snow and/or sleet in our immediate area. A wintry mix of snow, sleet and rain in our southern areas. Temperatures 30 to 34 degrees, northwest to southeast. Slick spots possible mainly north and northwest of the Beltway.

6 a.m. to 10 a.m. Friday: Precipitation tapers off to drizzle from west to east. Temperatures 31 to 34 degrees, northwest to southeast. Slick spots possible, mainly north and northwest of the Beltway.

Storm impacts

On Capital Weather Gang’s winter storm impact scale, this event rates as a Category 2 “disruptive” event in our northernmost areas because of the combination of one to three inches of snow, temperatures near and below freezing, and the snowfall affecting the morning commute. Slick spots and delays are likely.

In the zone where we expect a coating to two inches, this event rates as a Category 1 “nuisance” event, which could cause some slick spots but that shouldn’t cause big problems unless there is some surprise.


(CWG)

To the south, where we expect no accumulation to an inch, this is a borderline Category 1 and barely registers on our scale. Its impact will depend on whether steady enough snow can fall to lower air and ground temperatures to near freezing and lead to some slick spots.

Model forecasts and discussion

This forecast, like many for snow events in the Washington region, is tricky as model forecasts vary considerably. Here’s the spread for forecasts in downtown Washington:

  • NAM: 0 inches
  • SREF: 0.5 inches
  • European: 1 inch
  • HRRR: 1.1 inches
  • Canadian: 1.3 inches
  • High-resolution NAM: 1.6 inches
  • High-resolution Canadian: 3.1 inches
  • GFS model: 3.2 inches
  • HREF: 2 to 4 inches

Note that these models assume every flake of snow will stick, which is a bad assumption for this event considering that temperatures, especially outside of our colder areas north and west of Washington, will be above freezing for a portion of the event.

“Two related questions remain that make this forecast a tough one,” explained Wes Junker, Capital Weather Gang’s winter-weather expert. “Where will the band of heaviest precipitation set up and how quickly will the surface temperatures fall below freezing?”

He continued, “If band sets up north of us like the NAM models forecast, the city might only receive a coating of snow or even mostly sleet as there would not be enough cooling from the precipitation to quash a mid-level warm layer. Plus the surface temperatures might struggle to fall to freezing.

“But if you shove the heavier precipitation south, temperatures in the immediate area would be colder and D.C. might experience mostly snow with the potential for a couple of inches.

"A small change to the precipitation axis might be the difference between having no significant snowfall accumulation and having a rough morning commute in and around the city.”