The Washington Post

Another borderline chance of snow Friday night

Another tricky winter weather event is on the horizon for Friday night into early Saturday morning. Like so many of the events recently, there are still questions about how much cold air and moisture will be available to the system.

Unlike the last two chances for snow, this one will occur mostly at night which might help the chances of accumulations north and west of the city.

However, models generally have the surface temperature in the low 40s after dark on Friday night before the precipitation arrives. The temperatures then slowly drop to around 34 towards the end of the event. Where have we heard that before? Pretty much every storm this year.

For the areas inside the Beltway, the precipitation is likely to start as rain with the rain continuing through much of the event before maybe ending as snow. In the far western suburbs towards Leesburg and Frederick, the models suggest a more winter like scenario with much of the precipitation probably falling as snow giving that area some potential for accumulations. For areas between the beltway and Leesburg and Frederick, you probably see some sort of mix.


This morning’s GFS and NAM have both trended wetter with their simulations as they now take a southern stream upper level impulse and try to phase it with the northern stream trough that will be delivering our weekend cold shot. That gives this system a little more potential to tap moisture than the last one. However, the southern stream impulse is a weak one and the surface low development associated with it does not look vigorous enough to really tap into Gulf or Atlantic moisture to make this more than a nuisance type event.

NAM model simulations for 1 a.m. and 4 a.m. Saturday. Note the dark blue 32 degree line is well north and west of the District (NOAA)

Both the GFS and NAM now forecast over 0.20” liquid equivalent precipitation. If we had cold air in place, that might be enough to cause some excitement but with the temperatures forecast to stay above freezing, this look more like another tease than a bonafide threat except for the far western suburbs.

SREF probability of snow at 1 am. and 4 a.m. Saturday morning (see scale on right). Precipitation amounts indicated by black contours

SREF plume diagram for Washington, D.C. The green lines indicate when rain is falling, the blue represents snow. Each line represents a different “member” or simulation of the ensemble.

The high snow probabilities in SREF guidance indicate the best snow potentialy as you north and west of the city towards higher terrain. However, even as far west as Dulles airport, the precipitation is likely to start as rain as the NAM temperatures at 1 a.m. are still forecast to be around 35 degrees.

In these types of weather systems, precipitation rates often are the deciding factor for determining whether snow sticks or not. If we get a period of moderate precipitation, it might help lower the temperatures enough for snow to stick at least on grass west of D.C. as far east as Sterling and Gaithersburg. From the city south and east, temperatures again look to be too warm for accumulation.

The latest European model is even less impressive than the NAM, GFS, and SREF mean, generating very light precipitation amounts mostly in the form of rain except in the far west and north suburbs.

The best chance of snow flakes around the city may come Saturday afternoon in the wake of the cold front as a weak upper level impulse zips by.


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