Most of the precipitation with this winter storm (at least before Sunday morning) will fall in the form of sleet or freezing rain. Areas in extreme Northwestern Maryland have the best chances of seeing accumulating snow, but even there, warm surface temperatures may turn things over to rain briefly during the afternoon.

As for the District and Baltimore, this now looks like a primarily rain event. We may see a few flakes or sleet pellets before the storm ends tomorrow afternoon as cold air filters down from the northwest.

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A Winter Weather Advisory is in effect from 3AM to Noon for areas north and west of the District including Montgomery, Loudon and Howard Counties. The National Weather Service anticipates icy conditions tomorrow morning.

Icy situation tomorrow morning:
Models over the past few days have trended cooler with a developing low pressure system in the southern United States. Temperatures are expected to cool into the low to mid 30s north and west of the District. As the upper levels warm above freezing, sleet and freezing rain are slated to develop around the aforementioned areas sometime tomorrow morning.

I don't really see this storm being much of a wintry event for the immediate metro regions, but northern sections of Montgomery and Loudon counties and points north and west could be in for a slippery ride tomorrow. Anywhere from a trace to one tenth of an inch of ice accumulation is expected before everything turns over the rain by midday.

After this storm exits the region tomorrow evening, our full attention turns to a potential large coastal storm.

To Snow or not to Snow:
Numerical model runs on Tuesday afternoon turned most Washington snow-lovers euphoric. The normally trustworthy GFS pumped out an eye-popping 18-24"+ of snow for most I-95 cities on Saturday through Sunday. Recent runs, however, have brought the storm farther inland, and instead of a monster pure snow event, forecasts snow changing over to plain rain.

But, there are some very promising signs that things are about to change. The newest computer model runs hint at the possibility that the low scoots a bit farther east, bringing a prolonged period of snow back into the equation. It's still too early, and I'm too uncertain, to make any calls at this point, but the potential for a large winter storm is still a very real possibility.

Current Weather:
Presently, a developing area of low pressure around the Ohio River valley is pushing rain showers into northern Maryland from west to east.

The current thinking is that rain showers will stay generally north of Washington until tomorrow around noon as bands of light precipitation move southward as a small cell of high pressure pushes in from the northwest. Even with the high pressure in place for Tuesday, skies are expected to remain generally cloudy with high temperatures climbing into the upper 50s to near 60.

While our temperatures remain warm enough for a pure rain event, folks up in New York and northern Pennsylvania are in for a significant icing event. Freezing rain is expected to coat the region in over a quarter inch of ice.

Forecast through the rest of the week:
Another developing low during the middle of the week is expected to bring the area yet another shot at some light rainfall. Our chances for rain will slowly increase through the day on Wednesday as a warm front lifts north of the region before the low moves east by Thursday evening.

Even with all this rain in the forecast (looking at the National Weather Service forecasts, you'll see rain symbols every single period up until Thursday), just know it will not be raining every single minute for the next 4 days. We will have sporadic, elevated chances of rainfall through the week, with the best chances coming during the day tomorrow and Wednesday night into Thursday.

Rainfall accumulations are quite high through the week given all of these stubborn low pressure systems. The graphic to the right show total precipitation accumulations through Friday night as forecast by the GFS (Global Forecast System) model. Granted, I think the totals are too high for most locations, but it just gives you an idea of how much moisture there is in the atmosphere this week.

Image above: GFS Total precipitation accumulations through 00Z Saturday. Courtesy of Accuweather.

I thought Hurricane Season was over?
Hurricane season officially ended on November 30, but the National Hurricane Center is monitoring a disturbance in the Caribbean that may develop into yet another tropical system:

WATER TEMPERATURES ARE STILL WARM ENOUGH IN THIS AREA TO
SUPPORT TROPICAL CYCLONE FORMATION...AND UPPER-LEVEL WINDS ARE NOT ESPECIALLY PROHIBITIVE FOR DEVELOPMENT. THIS SYSTEM HAS THE POTENTIAL TO BECOME A TROPICAL OR SUBTROPICAL STORM DURING THE NEXT 24 HOURS

Hurricane Season hasn't given up yet.

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.

--Previous Post--

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.