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Burst of wet snow Thursday morning could slow commute in D.C. area

Winter weather advisory issued for region

Totals are dependent on whether a burst of snow materializes Thursday morning. Most accumulation is expected on grassy areas, although some slushy or slick spots are possible on roads during any heavy snow, especially northwest of downtown Washington. (CWG)
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10 p.m. — Forecast mostly on track for a burst of snow Thursday morning

Afternoon and evening computer models continue to advertise rain changing to snow Thursday morning, probably between about 6 and 9 a.m. from northwest to southeast. A burst of heavy snow, possibly falling at the rate of up to an inch per hour or so, could occur between about 8 and 10 a.m. which is when it will be best to stay off the roads if possible. The snow should end in most spots between about 10 a.m. and 1 p.m. from northwest to southeast.

Some area schools canceling in-person classes Thursday because of expected morning snow

Snow amounts of 1 to 2 inches still seem most probable in our colder areas north and west of downtown; elsewhere, a coating to 2 inches is most likely. Because of the rain prior to the anticipated changeover to snow, roads will not be pretreated so if the heavy burst materializes, roads could indeed become quite slick.

A couple models (including the European) have backed off their forecasts for snowfall intensity which would also mean a slower decrease in temperature and less snow accumulation. If these models are right, the impact of this event would be rather modest with perhaps just a coating on a grassy areas. However, there is enough support for this burst of snow among other models that we don’t think adjustments to our accumulation forecast are necessary.

You can of course see how conditions are evolving in the morning to help plan your day and we’ll have a fresh forecast up at 5 a.m. and will post updates hourly through midday.

2:15 p.m. — Winter weather advisory issued Thursday from 6 a.m. to 1 p.m.

Anticipating the burst of snow Thursday morning, the National Weather Service has issued a winter weather advisory for the region.

“Snow accumulations of up to two inches with locally higher amounts around three inches possible,” the advisory says. “Plan on slippery road conditions. The hazardous conditions will impact the morning commute.”

The advisory includes the entire area within a two county radius of the District with the exception of Calvert and St. Mary’s counties in southern Maryland, where little accumulation is anticipated.

Original article from noon

It won’t last long, but a poorly timed burst of snow Thursday, coinciding with the morning commute, could cause a period of challenging travel in the D.C. area.

As a strong cold front pushes south, rain will change to snow, which could be heavy for a time between about 7 and 10 a.m. It will probably too warm for the snow to stick at first. But, as temperatures fall, slick spots could develop, especially in our colder areas north and west of the Beltway.

Some areas could pick up a quick inch or two, and the snow could come down heavily for a couple of hours, lowering visibility below one-half mile. We expect some schools may delay or close, and we would advise people to stay off the roads if possible from 7 to 10 a.m.

Meanwhile, the chance for snow Friday night into the weekend is barely hanging on and will probably have much greater impacts in southern Virginia and the Carolinas. But we need to keep the door a crack open for some effects in our region.

Thursday snow timeline and temperatures

Precipitation is likely to begin as rain after 11 p.m. Wednesday night, with the transition to snow occurring from northwest to southeast between about 6 and 9 a.m. Thursday

Given above-freezing temperatures as the rain transitions to snow, it may take until between 8 and 10 a.m. or so before flakes start accumulating. Especially from the District south and east, a lot of the accumulation may be on grassy areas, but we can’t rule out some slush on roads during heavier bursts.

Temperatures shown in the timeline below are lowest in our northwest areas (toward Frederick, Md.) and highest to the southeast (toward Fredericksburg and southern Maryland).

11 p.m. Wednesday to 1 a.m. Thursday: Light rain develops. Temperatures: 39 to 45.

1 a.m. to 6 a.m. Thursday: Periods of light rain. Temperatures: 34 to 42.

6 a.m. to 8 a.m.: Rain changes to snow from northwest to southeast. Temperatures: 32 to 40.

8 a.m. to 10 a.m.: Snow, heavy at times. Slushy accumulation possible. Temperatures: 30 to 34.

10 a.m. to noon: Snow tapers off northwest to southeast. Temperatures: 29 to 33.

Accumulation potential Thursday

The computer model consensus is for between about 1 and 3 inches in the D.C. area, but these models do not take into account that temperatures will be above freezing when the snow begins, meaning some accumulation will be lost because of melting. We would shave an inch or so from these model projections, and that’s reflected in our map at the top of this article.

Here’s how much different models project for the District: High-resolution NAM and NAM: about 3 inches | GFS: 3 inches | SREF: 2.5 to 3 inches | HRRR: 2 to 2.5 inches | European: 1.5 to 2 inches | UKMet: 1 to 2 inches | ICON: 1-inch | Canadian and high-resolution Canadian: coating

Remarkably, a lot of the snow is projected to fall in a two-hour window between about 8 and 10 a.m. in the immediate area when snowfall rates could top an inch per hour. This is why we’re concerned about travel difficulties during that period; snowfall of this intensity can overcome air and ground temperatures that may be a little above freezing and still create slick roads.

If the majority of models are wrong and we don’t see a burst of heavier snow, we would expect just a coating or so to accumulate, mainly on grassy areas, with minimal travel impacts. This is our bust scenario.

The boom scenario allows for the possibility that localized very heavy bands of snow form, pushing totals above 2 or 3 inches in some areas.

The snowfall forecast from the National Weather Service, shown above, is more or less consistent with our forecast of 1 to 2 inches, but shows a somewhat different pattern.

Thursday’s storm impact rating and SchoolCast

On Capital Weather Gang’s winter storm impact scale, this rates at the intersection between a Category 1 “nuisance” event and Category 2 “disruptive” event. What would tip the scales toward a Category 2 would be if the heavier burst of snow does indeed coincide with the Thursday morning rush.

This will be a wet snow, good for snowballs and packing.

What will also potentially increase the impact of this event is low temperatures in its wake. Temperatures Thursday afternoon will hold steady near freezing before tumbling during the evening, with lows in the teens (single-digit wind chills) by Friday morning. Any wet areas will freeze over solidly. We would advise shoveling any snow shortly after it falls before it becomes cement-like.

This is going to be a challenging event for schools. Unless they decide to close or delay preemptively Wednesday evening, not much will be happening at decision time in the predawn hours Thursday morning. A good call might be to delay two hours and reassess conditions as the event unfolds, but that might not leave enough time to fully take stock of how the morning burst of snow is playing out.

For our SchoolCast, we’d give this our 2-out-of-4-apple rating, meaning there’s a good (75 percent) chance of delays for most systems and probably a 50-50 chance of a snow day.

Friday night and Saturday snow potential

The models have trended away from a significant snowstorm for this weekend. The European model, which had been the most aggressive model in forecasting snow, now has only a period of light snow late Friday night into early Saturday morning, as it now tracks the coastal storm well to our south and east. If it’s right, the District could see a coating or so and our southeast suburbs toward Southern Maryland up to an inch or two.

Other models such as the NAM, GFS and Canadian keep any snow to our south. Probabilities based on numerous model forecasts with slight differences in their initial conditions suggest that the probability of the District getting an inch of snow is about 30 percent, and that may be too high.

The vast majority of the latest American modeling system simulations keep us dry. The most likely scenario is for the storm to miss us to the south or provide us with a few mood flakes. There is still time for shifts in the forecast, so we’ll update this outlook on Thursday.