Snowfall Update:
This clipper has turned out to be a fairly significant snow producer. Most locations north and west of the District will end up receiving 2-5 inches with locally higher amounts west of the Blue Ridge.

It appears as if at least light snow will continue around the metro region for another hour or two before everything shuts off from northwest to southeast.

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The low that passed a strong cold front through the area yesterday is currently deepening off the coast of Maine, where up to two feet of snow is expected by tomorrow morning. The tight pressure gradient behind this storm is driving cold northwesterly winds down from Canada. Cold air continues to filter into the region, and temperatures tomorrow will struggle to get into the low 40s. Winds will also turn gusty in the afternoon with 30-40 mph gusts likely in most locations.

This new pool of cold air will set the stage for the Washington metro areas first snowfall of the winter season:

By late tomorrow night, a piece of energy from a strong Pacific Northwest storm system will have been ejected southeastward into the Midwest (image at right). Clear, to partly cloudy skies Tuesday night will allow much of the daytime heating to radiate out, allowing temperatures to drop into the mid to upper 20s.

At this point, timing the onset of precipitation is very difficult due to a dry surface (or boundary layer) that will be in place early Wednesday morning. Any precipitation that develops over the area will evaporate before reaching the ground. The current thinking is light snow will start falling between 5 and 7AM on Wednesday.

As the snow falls into the dry boundary layer, surface temperatures will likely fall a few more degrees towards the wet bulb, which is simply the temperature to which the atmosphere will cool at 100% saturation (i.e. when the snow falls).

Snow should become a little heavier during the early afternoon hours as the clipper begins to slide south of us. In the image above, snow growth levels have fallen to near 12,000 feet and have begun to intersect areas of moderate to high omega (lifting) values. This often indicates a period of heavier snowfall, depending on the magnitude of the forcing. In this case, forcing is decent, but not great, which would support a period of moderate snowfall rates around 12 to 5 or 6PM.

Afternoon temperatures on Wednesday may pose a problem for snowfall accumulations, as they may rise slightly above freezing. My best guess on snowfall totals at this point is between 1-3 inches on elevated or grassy surfaces, and a Trace-2 inches on untreated roadways and sidewalks.

This will only be a "nuisance" snow event, but since it's the first of the season, and snow will likely begin before rush hour, it may cause some problems for Wednesday morning commuters.