Accumulating snow overnight with slick spots into the morning commute

Travel will be difficult during and just after the heaviest snow. Slick spots are likely into the morning commute with temperatures in the 20s, but increasing sunshine should help treated main roads improve as the morning continues.

This is our last update on the blog for tonight. In the meantime, keep an eye on our Twitter feed @capitalweather, and check the blog for our latest forecasts and updates at 5 a.m.

10 p.m. — Temperatures to drop as snow gradually increases from west to east the next few hours

Snow is just now moving into our far western and northwest suburbs, into far western Loudoun, Fauquier and Frederick counties, and will gradually spread east across the area over the next few hours. As the snow starts to pick up, temperatures currently in the mid-30s should drop to near freezing by around midnight, when the snow should start to accumulate on pavement.

The overall forecast remains stable based on evening model runs, which continue to support accumulations of 1 to 3 inches across the area, with some spots seeing a little more or a little less (the new NAM model shows most of the immediate area getting 2 to 3 inches). We expect the heaviest snow to fall around 1 to 4 a.m. Even treated roads may become treacherous during this time.

7:30 p.m. — Dry air shielding D.C. area from precipitation for now

As expected, radar above shows the precipitation continuing to stall out as it crosses the mountains (Frostburg, Md. has reported 1 inch of accumulation) and runs into drier air. As a result the D.C. metro area has remained dry thus far. Some flurries, light snow, or a light rain/snow mix could develop after 10 p.m. or so, but we still don’t expect meaningful accumulation to begin until around midnight.

The latest European model (pictured below) is out and it agrees nicely with our forecast (see map under “amounts” section below) of 1 to 3 inches in the immediate area and up to 4 inches as you head north and west.

Our next update will be around 10 p.m., unless conditions warrant earlier

5:45 p.m. — Light snow arrives in western Maryland and eastern panhandle of West Virginia

At 5 p.m., both Cumberland, Md. and Petersburg, W. Va. were reporting light snow as precipitation exited the Tennessee Valley toward the Central Appalachians. Those two locations both reported temperatures around freezing. The snow will take its time crossing the mountains and the onset near the Interstate 95 corridor may still hold off until around midnight.

Even with the snow less than a two-hour drive from Washington, models continue to have a considerable spread in their projections. The latest HRRR model run out in the last hour only predicts 1 to 2 inches for the immediate D.C. area but the American (GFS) model calls for closer to 4 inches.

Our next update will be posted around 7:30 p.m.

4:10 p.m. — Forecast is on track as snow spreads into West Virginia

Radar shows snow falling over most of Kentucky and West Virginia as the storm system sweeps eastward. The snow is still set to arrive in our area between around 11 p.m. and 1 a.m., or maybe a little later in our eastern areas.

In Nashville, where 4 inches has fallen from this system, it’s the snowiest day since Jan. 22, 2016, according to AccuWeather.

The latest models continue to show 1 to 3 inches falling over the immediate D.C. area; here are some of the latest projections for the District based on simulations run in the past hour:

• High-resolution NAM: Around 3 inches
• NAM: 2-2.5 inches
• HRRR: 1.5-2 inches

We should note that the National Weather Service did lower its projections slightly, from 2 to 3 inches from 3 to 4 inches, and they are now pretty consistent with ours.

Our next update will be around 5:45 p.m.

Original forecast from 12:30 p.m.

While most of us sleep overnight, a high-altitude weather disturbance will zip across the Mid-Atlantic, coating the Washington and Baltimore area in snow.

The snow should develop about midnight and end by about 5 a.m. Friday.

Amounts may vary between about one and four inches over the region, but around two inches should be most common. Because temperatures will fall into the 20s as the flakes fall, many roads will become slick and treacherous.

There’s the possibility of a brief burst of heavy snow between about 1 and 3 a.m., falling at the clip of about an inch an hour, limiting visibility and making travel difficult.

Mid-Atlantic, Northeast brace for burst of snow Friday morning

Although the snow will end well before sunrise, untreated roads will probably be slippery for the Friday morning commute, so we expect school delays and cancellations.

Amounts

Computer model snowfall projections have edged slightly lower since Wednesday night, so we have shaved an inch off the most likely amounts in the immediate area, from two to four inches to one to three inches.

While we don’t expect a lot of regional variability in accumulation, we would generally lean toward the higher amounts north and west of Washington and somewhat lower totals to the south and southeast.

The track of this disturbance is farther north than Monday’s storm, so the jackpot totals will also probably be farther north.

Here’s how much snow is forecast for Washington by the different models:

• American (GFS): 4 inches
• European: 2 inches
• High-resolution NAM: 2 inches
• NAM: 2 inches
• High-resolution Canadian: 2 inches
• Canadian: 2 inches
• UKMet: 1 to 1.5 inches
• HRRR: 1 to 1.5 inches

Timeline and temperatures

The snow should move in between 11 p.m. and 1 a.m. from west to east and end between 4 and 6 a.m., lasting only four to five hours in any one location.

The window for a possible burst of heavier snow is between about 1 and 3 a.m.

10 p.m. to midnight: Snow develops in our western areas. Temperatures: 30 to 35.

Midnight to 2 a.m.: Snow begins in our eastern areas, while increasing in intensity to the west. Temperatures: 30 to 34 degrees.

2 a.m. to 4 a.m.: Snow lightens in western areas, while picking up to the east. Temperatures 26 to 31.

4 a.m. to 6 a.m.: Snow decreases in eastern areas and ends everywhere from west to east. Temperatures 25 to 29.

Temperature ranges shown above are lowest in our northwest areas (Loudoun and Frederick counties) and highest to the southeast (Stafford County, Fredericksburg and southern Maryland).

Impacts

On the Capital Weather Gang’s winter storm impact scale, this system rates right at the intersection of a Category 1 “nuisance” event and Category 2 “disruptive” event. While the system may produce a brief burst of heavy snow and create slick conditions as temperatures fall, its impact will be mitigated by the fact that the snow is falling overnight and ending well before the morning commute.

But as many neighborhood roads may well be untreated through rush hour, we do expect widespread school delays and some cancellations. Also, with temperatures holding at or below freezing on Friday, little will melt.

We don’t expect wind to be a big issue with this system Thursday night. However, as the system pulls away Friday morning, winds will become gusty, blowing the snow around a bit and creating bitter wind chills in the teens and 20s.

The consistency of the snow may start off as a bit heavy and wet, but it should turn more powdery as temperatures fall. The risk of power outages is low.

Could the ‘D.C. snow hole’ return with this event?

While one to three inches of snow seem most likely, and locally heavier amounts are possible, there’s also the possibility of just a dusting.

Some models are hinting that the system will be partially stripped of its moisture as it crosses the mountains before drawing in Atlantic moisture for areas close to the coast. But the area caught in the middle could see a gap in meaningful snow accumulation.

We sometimes see this circumstance in the D.C. area with storm systems coming in from the west, and it has been given the endearing name of the “D.C. snow hole.”

Around downtown Washington, the Potomac River and the Chesapeake Bay, some snow accumulation may also be lost because of melting, as temperatures may be a little above freezing as the snow develops before slowly falling.

How might we see a snow ‘boom’?

While the snow hole scenario is possible, there’s also a way for snowfall to exceed expectations.

There is some possibility that a narrow corridor of heavy snow will form in the Washington region as the result of a phenomenon known as “frontogenesis,” which is essentially the formation of a localized front due to temperature contrasts over the area. Frontogenesis can intensify snowfall, and some models have simulated some pretty heavy snow bands developing over the region.

Some models have even shown an elevated unstable layer that could briefly enhance snowfall rates.

The big forecast problem is whether and where any bands of heavier snow occur.