Tuesday, February 13, 2007
Evening Update: Mixed Precipitation Continues
Significant ice possible overnight, especially north and west
Team Forecast @ 9:15 PM
Click here to jump straight to the accumulation map and storm timeline.
Nowcast
9:00pm -- Mixed precipitation (mostly sleet and freezing rain) is beginning to increase in coverage to the southwest as temperatures hold in the mid to upper 20s throughout the region. The coverage and intensity of precipitation will gradually increase over the next several hours. Icy conditions will be widespread on untreated roads and may even spread to major roads during heavier bursts of precipitation towards midnight. Please exercise caution.
By popular demand: SchoolCast 
Overnight
A new low pressure area is developing in South Carolina and headed towards the coast. The developing coastal storm will start to take over overnight and strengthen rapidly as it moves up the coast. This will bring heavy precipitation to the region late this evening and continuing into the first half of tomorrow.
The most difficult question to answer is: Are going temperatures going to rise and when? Some forecast outlets (like the National Weather Service, which recently increased its ice accumulation forecast) believe it will mostly stay subfreezing for the duration of the storm due to a persistent cold northeast wind -- even in the immediate metro area. Other outlets believe strong southerly winds about a mile up will mix down to the surface with temperatures rapidly rising to well above freezing (e.g. Bob Ryan) in DC and points south and east. We believe the truth lies somewhere in the middle but will re-evaluate this situation when we get new data in the next couple hours. Our current thinking is:
- Temperatures in the District and south and east are likely to rise to or above freezing for several hours towards dawn before falling back down late morning as the storm heads to our northeast.
- Some areas inside the beltway, north and west of town (such as McLean and Bethesda), may remain below freezing for a longer period and briefly change to rain.
- Outside the beltway, locations such as Reston and Gaithersburg to our north and west are areas most likely to remain below freezing for the entire event -- but temperatures above 32 for a period cannot even be ruled out there.
Bottom Line
Light to moderate ice accumulations are likely inside the Beltway, and major ice accumulation is likely to the north and west of DC outside the Beltway. Any ice accretion on power lines and trees will be exacerbated by very windy conditions overnight and tomorrow. The precipitation will likely end as a period of all snow Wednesday morning before it shuts off.
We will continue to monitor the situation through the evening. Stay tuned for further updates.
Transcript of Live Chat with CapitalWeather.com's Dan Stillman this afternoon at washingtonpost.com
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