Precip won't last long, amounts to be light
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After all of the big-time snow events this season which have met or exceeded initial snow total projections, it looks as if today's event is more likely to fall short. As the storm center approaches and and passes over us or just to our west, we are likely to see a period of precipitation mainly between mid-afternoon and mid-evening. After that, it will quickly cutoff.
Why will snow totals probably be on the low side?
Timing and temperatures: The arrival of the precipitation during the warmest part of the day (mid-afternoon) will make it difficult for snow to stick, particularly in the District and to the southeast.
Mixing: Although temperatures will be cold enough aloft for snow, temperatures near the surface especially from the District and to the southeast may be sufficiently warm for snowflakes to melt to rain before reaching the ground (especially when precipitation is light).
Precipitation intensity: Although there may be brief periods of moderate precipitation, a lot of the snow (or rain) that falls will be light as this storm doesn't have a ton of moisture to work with and some of the moisture will be snowed out as the the storm crosses the mountain.
Short duration and dry slot: Because the storm center is passing to our west or just directly over us, a dry slot is likely to move into the region once the center moves north of our latitude. That will quickly cutoff the precipitation this evening (between 6 and 10 p.m.), although some wrap-around light snow or flurries could impact the northern suburbs thereafter (but odds are just about 30%).
For all of these reasons, I now think snow totals will be on the low end of the ranges in the chart shown earlier. Is there some chance the higher end totals could be achieved? Yes...about 1 in 3. Because this disturbance/clipper swinging through is energetic (notice the nice, well-defined swirl to the system on satellite and radar), it's not out of the question the precipitation could fall heavily for a short period...producing a quick couple inches of snow. The most likely time for this would be between 4 and 7 p.m. if it were to happen. Having said that, I'm favoring a pretty modest event with enough to whiten the ground from downtown and to the east and southeast, and around an inch to the north and west (though Frederick and Loudoun counties have a decent chance of 2-3 inches or so).
Our next update will be around 3:30 p.m.