Update at 2:05 p.m.: The National Weather Service has issued a winter weather advisory for parts of northern Virginia and Maryland, including Washington, D.C. until 1 p.m. on Tuesday.

Read on for the full Capital Weather Gang forecast, including snow totals and timing.



A winter weather advisory has been issued for areas in purple. (NWS)

Original post:

It’s looking more likely that the D.C. area will see accumulating snow during Tuesday morning’s rush hour.

A low pressure system with the fast moving clipper is forecast to track just to our south, which historically has been a favorable track for giving us an inch or two of snow. We also cannot rule out some locations topping 3 inches if an intense snow band develops.

Temperatures are expected to be in the 20s when the snow starts falling in the pre-dawn hours, which should allow for accumulations on road surfaces. The roads, especially secondary roads, could become slick. The band of snow that falls in these clipper storms is usually quite narrow, which makes it hard to pinpoint exactly who will get the most snow. The most snow usually falls to the north of the surface low pressure track.

In the heaviest snow band, 1 to 2 inches is likely with isolated reports of 3 inches possible. Outside of the band, a dusting to 1 inch is probable. The heaviest band is expected to develop somewhere between southern Pennsylvania and the District of Columbia.



Capital Weather Gang forecast of snow accumulation for Tuesday morning around the D.C. area.

The snow will likely start in the western areas (Loudoun County and points west) by 4 or 5 a.m., with the snow progressing from west to east. The District will probably see snow by 6 a.m., and the entire metro area should see flakes by 7 a.m. The storm should be a quick hitter with most of the snow ending by 1 p.m.

The cold temperatures will likely allow the snow to stick to roads. Within the heavier band, slick roads may develop which could prompt some counties in the area to delay school openings or even cancel classes. While this storm is not expected to produce a lot of snow, any snow that sticks on the roads in our area usually bring on commuting woes.

The bottom line

Snow is a good bet across the Washington area and roads may become slick so you might want to give yourself extra time for your commute. There is a chance that some school districts will elect to cancel school while other districts carry on as usual. The best bet for accumulations is an inch or two with the heaviest accumulations probably occurring north of the city.

SchoolCast

The District, Stafford, Charles, Calvert, and St. Mary’s counties:



Everywhere else:



FedCast:

(Slight chance of a delay or unscheduled leave-telework option)

Technical discussion

Over the past 48 hours, the models have trended southward with the clipper track and its associated snow band. In terms of liquid equivalent, the operational model forecasts vary from 0.1 inches (the GFS, and European model) to 0.24 inches (the 06Z NAM). Today’s NAM pretty much held serve. Our typical snow to liquid ratio is around 11 or 12 liquid to 1 inch of snow. So 0.24 inches liquid equivalent would give us over 2.5 inches of snow, but the ratios with clippers can sometimes be higher. Both of last night’s runs of the GFS and the European model predicted 1 to 1.5 inches of snow across the Washington D.C.-Baltimore area. The 06Z and 12Z NAM predicted 2 to 3 inches of snow, but the model tends to run too wet.

Our forecast leans toward the European and GFS models, but went a little higher for our maxes fearing that there could be a narrow band of heavier snow that could bring totals to 3 inches in a few spots.

Unfortunately, the models also differ somewhat on where the stripe of heaviest snow might fall. Last night’s European model predicted that the heaviest would fall in the mountains of western Maryland and with another strip just north of the Mason-Dixon Line while the NAM has it right through the D.C.-Baltimore area. We’ve opted towards the more northward location of the band based on climatology and because the terrain up towards the Mason-Dixon line is a little higher than over the District.

The various ensemble members from last night’s SREF guidance package are pretty consistent in having the snow start in the city before 7 a.m. Remember each ensemble member is an atmospheric simulation (model run) with a slightly different initial condition or with its physics tweaked a bit. In this case, these differences did not have much impact on when the snow might develop as all the members started the snow in the early morning hours before 7 a.m., but did impact how much snow might fall.

Each line below represents the snow that is being forecast by that member. The dark black line is the mean of all the members. Essentially, the various forecasts vary from a dusting to 3 inches with average of all members at around 1.7”. The SREF guidance tends to run a little wet at times which is one reason we have opted for a dusting to 2 inches across most of the area, except those to the south of the city where we leaned towards a dusting to an inch.

Why have you trended heavier with your snow forecast over the past couple of days?

First, the models have trended south with the low and associated snow band. The second reason is that the upper wind pattern is one that is favorable for the development of precipitation. The jet stream not only steers weather systems but also modulates them. The winds in the jet stream are not constant but vary and areas where they are faster than the surrounding air within the jet stream are called jet streaks.

Note that the winds are strongest over eastern Pennsylvania and that the winds are lighter to the west where the jet dips in association with the approach of the upper trough. Such a configuration is favorable for tracking the surface low to our south but also is favorable for supplying upper level divergence and lifting within the black oval (see below). The bottom line that the jet stream is supplying dynamics to support us getting some snow.

More timing details

The high resolution NAM forecast from last night (see below) provides a decent first guess of the timing of the onset of precipitation and a first approximation of where the band of snow might be heaviest. The snow is expected to begin across the western suburbs (Leesburg, Sterling) between 4 and 5Am and with the snow streaking eastward into the city by 6 AM. The heaviest snow will probably fall west of the mountains in West Virginia or western Maryland. East of the mountains, the heaviest now probably will fall from the city northward to Mason-Dixon line.

Jason Samenow and Dan Stillman contributed to this post.