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Posted at 11:00 PM ET, 02/21/2008

Forecast: A Bit of Snow, Then an Icy Mess

By Capital Weather Gang

*Winter Storm Warning in effect until 10pm Friday night*

Snow has already moved into the Maryland suburbs north and west of the beltway. It is these areas that will likely receive enough snow to shovel by morning. In the immediate metro area (inside the beltway) and further south, perhaps just an inch or so will fall before changing to ice. Ice is likely this storm's big story as it produces a glaze across the region -- with the worst conditions during the first half of the day Friday.

ACCUMULATION MAP AND TIMELINE

11 p.m. THU to 6 a.m. FRI
Flurries or light snow possibly mixing with sleet. Dusting to 1 inch accumulation immediate metro area, 1-2" north. Temps 25-31.
6 a.m. FRI to 10 a.m. FRI
Light snow changing to sleet and freezing rain from south to north. Some slick spots. Temps 26-30.
10 a.m. FRI to 4 p.m. FRI
Mostly freezing rain. Significant ice accumulations possible, especially N & W. Temps 26-32.
4 p.m. FRI to 7 p.m. FRI
Freezing rain north and west. Freezing rain possibly changing to rain in DC and south and east. Temps 29-34
7 p.m. FRI to 10 p.m. FRI
Freezing rain and rain tapering to freezing drizzle and drizzle from southwest to northeast. Temps 30-34.

*Black stripe in accumulation map indicates where we estimate surface temperatures will be at or below freezing (above the line) and at or above freezing (below the line) at about 6 p.m. tomorrow.

IMPACT FORECASTS

STORM FAQ

How confident are you in your forecast? We're fairly confident that precipitation will start as snow over much of the area, and that the precipitation will eventually change to sleet and freezing rain during Friday morning. We are less confident about how much snow we'll be able to squeeze out tonight and how much ice will be able to accumulate on Friday. Will temperatures stay below freezing ALL day?

Keep reading for answers to more FAQs about the storm ...

When/where will the heaviest frozen precipitation fall? In general, the heaviest precipitation will likely fall north and west of DC. Areas south and east will have lighter precipitation, higher temperatures and lower accumulations. These areas will also likely soonest have snow change over to a mix and then possibly rain. The heaviest snow amounts will probably be in spots like Frederick where 2-3" is possible followed by ice accumulations of perhaps over 1/4" since surface temperatures have the greatest chances in those spots of staying below freezing.

Will the frozen precipitation stick? Any steady snow that falls overnight will stick pretty much everywhere. Ice will accumulate on untreated surfaces in the morning, including roads that have not been treated, trees and power lines. During the mid-day hours, freezing rain will may have a harder time sticking to roads due to the effect of the sun's energy through the clouds and warming temperatures -- especially in town. However, ramps, overpasses, bridges and elevated roadways that are untreated may remain slick throughout the day. As with most winter storms, the further north and west you get from DC, the colder the temperatures will be and the more likely frozen precipitation will accumulate throughout this storm.

When will travel be most difficult? Most likely during the morning rush hour. Precipitation during the day Friday should fall at a pace slow enough for salt crews to keep up with and the sun's energy will also help slow ice accumulation. Drive cautiously, however and be careful walking on sidewalks. A second particularly hazardous period may occur after about 4pm, as the sun lowers and conditions likely again deteriorate... especially north and west of town.

When will conditions improve? By mid to late evening, when the last of the freezing rain or plain rain ends throughout the region.

Could it fizzle out? The storm itself is unlikely to fizzle out since it has already developed and there is some model agreement on the potential effects for our area. Snow may be less than forecast in DC, Virginia due to a dry slot forecast to develop south of the Maryland/DC line. Ice could be less than forecast, again especially in DC and points south, if temperatures rise above freezing faster than anticipated.

Could the storm bring more ice/snow than expected? Possibly. Areas that could receive more snow than forecast are mainly north of town in places like northern Montgomery County, Frederick County, and Howard County.

What are the different possible outcomes? Snowfall probabilities inside the Beltway: 15% no accumulation, 50% chance dusting to an inch, 25% chance 1-2", 10% 2"+

What will conditions be like after the storm? We should have mostly cloudy and breezy conditions Saturday with highs near 40 . Outside a few lingering showers (maybe snow showers N & W) in the morning, most of the precipitation should be done. After a chilly start in the low to mid 20s (watch out for refreezing) sunshine returns Sunday, with highs in the mid 40s.

What are other forecast outlets calling for? (from web sites)

National Weather Service: 1-2" snow then 1/4" ice.
NBC4: Up to 1" snow then 1/4" ice.
WUSA-9: 1" snow then 1/4" ice.
WJLA-7: 1-2" snow and then 0.1" ice.
WTTG-5: 1-2", up to 1/4" ice.


By Capital Weather Gang  | February 21, 2008; 11:00 PM ET
Categories:  Forecasts, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   Del.icio.us   StumbleUpon   Technorati   Google Buzz   Previous: Update: Snow Sliding North, Ice Still on Track
Next: Update: Icy Mix Overtakes Region

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