Thursday, December 6, 2007

Storm Update: First Snow of the Season

* Snow Advisory in effect until 11pm tonight*

9:30pm update: Snow is diminishing to flurries across the area. Generally 2-4" of snow has fallen (about 4-4.5" on untreated surfaces where I live in NW DC with some higher amounts to the north and west), with accumulation on untreated roadways. To snow lover's delight, this storm over-performed as the scenario we described (25% probability of 2"+) highlighting the potential for the development of localized heavy bands materialized.


4pm update: The early afternoon weather map showed the main center of the clipper system located in southwestern Virginia, while a new center was beginning to develop off the Outer Banks. Radar indicates light to moderate snow from central Maryland southward through the District. Temperatures have risen a couple of degrees to freezing or above during the afternoon at most locations in the immediate metro area. After reaching 34° in the previous hour, National has now slipped back to 33° and Dulles is at 31°.

Some updated unofficial snow totals reported in the last hour: Cumberland 2.5", Hunt Valley 2.4", Baltimore City 1.5", Frederick 3.0", Columbia 2.3", Germantown 2.3", Clarksburg 1.5" (TV met, probably Howard B), Bowie 1.5", National 0.7", Vienna 2.5", Reston 2.1", Leesburg 2.0". (Note that the 1" reported for National in the Comments was actually rounded up, so technically the official amount is not yet 1".)

Light to possibly locally moderate snow will continue through the rest of the afternoon, tapering off this evening as the new low pressure area in the Atlantic develops and moves away to the northeast. At this point, the majority of the accumulation has already occurred in most places. The NWS is planning to let the Snow Advisory expire on schedule at 8pm.

Stay tuned to for any further updates.

1pm update: The fast-moving clipper system hit the Washington metro area right where it hurts this morning: in the middle of rush hour. The combination of temperatures just a little below freezing and the timing of the arrival of the first flakes (5:47 at Dulles, 6:23 at National, 7:32 at BWI) made for a very nasty trip for many commuters and caused some school systems to throw in the towel by closing early.

Although the snow intensity increased to heavy at National for a short time around 12:40 with visibility only a quarter of a mile, actual precipitation amounts so far are quite light: National didn't pick up its first 0.01" until 11:00, and the total so far is just 0.08" of water. With temperatures still at or slightly below freezing, however, some measurable snow has accumulated, especially on grassy and elevated surfaces. Unofficial observations shortly after noon included (some observations made earlier): Cumberland 2", Frederick 2.5", Columbia 1.1", Great Falls 0.9", Leesburg 1.4", Sterling 0.5".

Current radar shows the back edge of the current precipitation moving eastward from western Frederick Co. MD and Loudoun Co. VA. There's another batch of snow moving eastward from the Ohio Valley which should bring a resumption of light to possibly moderate snow in the area later this afternoon before it all tapers off this evening.

We'll be back by around 4:00 with a further update, earlier if conditions warrant.

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