11 PM Update: Colder temperatures and higher snowfall rates have picked up the accumulations significantly since early this evening. National now has 8" on the ground, Dulles has 10" of new snow, for a total of 11" on the ground. Baltimore/BWI has 6".
Wall-to-wall coverage of the checkout lines at Giant supermarket, snow shovel buyers at Strosnider's Hardware and Home Depot. Don't forget the live shot from the salt domes.
Do they have your attention yet?
The much anticipated Blizzard of 2010 is making itself felt in the Washington metro area. In what must be an unprecedented move, the normally-cautious National Weather Service at 12:34 issued a prediction that puts the all-time Washington snowfall record in jeopardy:
...RECORD SNOWFALL FORECAST IN THE BALTIMORE-WASHINGTON DC REGION...Snow began falling in the mid to late morning; here in Montgomery County MD it started with some very tiny flakes and a temperature of 37° at 10:40. The pace picked up into early afternoon, but even after several hours of precipitation, the temperature was only down to 34.5°. The above-freezing temperatures kept paved surfaces wet for several hours. Although residential streets are now snow-covered, conditions were not treacherous enough to prevent the UPS man from delivering on time a reinforcing supply of warm Gold Toe Socks.
EXTREMELY DANGEROUS WINTER WEATHER CONDITIONS DEVELOPING TONIGHT
GUSTY NORTHEAST WINDS 20 TO 30 MPH WITH VISIBILITIES FREQUENTLY
FALLING BELOW ONE-QUARTER MILE DUE TO HEAVY SNOW WILL DEVELOP
TONIGHT TO PRODUCE NEAR-BLIZZARD AND EXTREMELY HAZARDOUS WINTER WEATHER
CONDITIONS TONIGHT THROUGH SATURDAY MORNING. TRAVEL IS HIGHLY
DISCOURAGED TONIGHT AND WILL BE VERY DANGEROUS.
LOOKING BACK AT THE BIGGEST STORM OF RECORD FOR WASHINGTON DC... THE
JANUARY 1922 KNICKERBOCKER STORM...28.0 INCHES OF SNOW WAS
PRODUCED FROM 3.02 INCHES OF LIQUID WATER. CURRENT FORECASTS FOR
THIS EVENT HAVE TOTAL LIQUID FALLING FROM THIS STORM APPROACHING 3
INCHES...WHICH ACCORDINGLY WOULD CREATE A SNOWFALL THAT WILL RIVAL
THE KNICKERBOCKER STORM TOTAL. GENERALLY ACROSS THE REGION...20 TO
30 INCHES OF SNOW WILL FALL BY SATURDAY EVENING.
BALTIMORES RECORD OF 26.8 INCHES FROM THE PRESIDENTS DAY FEBRUARY 2003
STORM WILL ALSO BE THREATENED.
It's still early into what is shaping up as a major storm, but in much of the metro area, it would have to be rated as underperforming its advertised target of 2-4" this afternoon. As of 4 pm, only Dulles was in the range with 3.4", which was a record for the date, since the old one was a puny 1" in 2001. National was at 1.1", and BWI had only a trace.
We noted here earlier that the major risk to the forecast for a record-breaking event was the liquid vs. frozen precipitation issue, and that is certainly still the case. In the past hour or so, radar indicates that the rain/snow line has advanced northward from Richmond to near Fredericksburg.
The chart shows the accumulated frozen and liquid precipitation at Washington National through 6 pm. The scales are in the ratio of 10:1 of frozen to liquid, so if the green is ahead of the blue, the water content is relatively higher (lower frozen to liquid ratio). Check back for updates as conditions warrant through the storm.
Image (click to enlarge): Washington National accumulated frozen vs. liquid precipitation, CapitalClimate chart from National Weather Service data