Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 160119 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 919 PM EDT Tue Aug 15 2017 .SYNOPSIS...
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An area of high pressure will build into the region tonight through Wednesday night. A warm front will move northward Thursday into Thursday night followed by a cold front that should pass through the region early Saturday.
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&& .NEAR TERM /UNTIL 9 AM WEDNESDAY MORNING/...
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Mid level dry air behind a departing shortwave trough is bringing an end to the showers except a few in the Potomac Highlands along a theta-e gradient. These should gradually dissipate as ribbon of mid level moisture lifts to the northeast and nocturnal stability increases. The actual front is still located in northern PA, and thus low level moisture still remains in place. With light winds, fog and/or low clouds may become an issue, and already have a few locations reporting 3-5 SM visibility. One limiting factor may be mid and high level clouds advancing from the southwest. Have upped wording in forecast to "areas of fog" but have not included dense wording yet. Guidance, in general, has the lowest forecast visibilities east of the Blue Ridge, but can`t rule out fog anywhere. Slightly drier air advancing from the north should bring a quick end to any poor visibility after sunrise, though any low clouds may take some time to scatter/lift due to a subsidence inversion. Current dew points in the region suggest overnight lows will be in the mid 60s to lower 70s.
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&& .SHORT TERM /9 AM WEDNESDAY MORNING THROUGH THURSDAY/...
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High pressure will build into the region from the northwest through Wednesday night with dry weather expected. As the high moves east Thursday, a warm front will develop across southwest Virginia and propagate northward. A chance of showers and a few thunderstorms is expected along and just north of the warm front, mainly Thursday afternoon. Temperatures are expected to climb into the upper 80s with dewpoint temperatures climbing into the lower to middle 70s.
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&& .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... A cold front will be approaching our area on Friday with humid and warmer conditions ahead of it, which could lead to the development of showers and thunderstorms. The front will move across Friday night and stall near/south of our area Saturday into Sunday. This will continue to enhance the development of showers and thunderstorms through the weekend. There is uncertainty on the timing and as to how far south the front will stall. Uncertainty continues Monday into Tuesday as guidance suggests that upper level energy could enhance showers and thunderstorms, but other guidance is drier. High temperatures will be in the mid to upper 80s... 70s at higher elevations. && .AVIATION /01Z WEDNESDAY THROUGH SUNDAY/...
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The main concern for tonight will be the development of low clouds and fog. Most guidance hits this scenario pretty hard, especially in the metros. Advancing mid/high clouds could be a limiting factor, although I thought there was enough evidence to include a period of IFR (except DCA - for climatological reasons). However, dense fog and/or ceilings less than 500 ft are within the realm of possibility. Poor visibility should improve after sunrise, but any low clouds may take some time to scatter/lift due to a subsidence inversion. VFR conditions expected Wednesday through Thursday. Exceptions: patchy fog Wednesday night and a chance for showers and thunderstorms Thursday afternoon. VFR conditions expected with periods of sub-VFR possible as showers and thunderstorms could develop Friday into Sunday with frontal boundary near by.
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&& .MARINE...
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The threat for thunderstorms has ended for the night. No marine hazards expected through Thursday night. Winds becoming northwest 5 to 10 knots tonight and Wednesday, then southeast 5 to 10 knots Wednesday night into Thursday. There will be a chance of showers and thunderstorms by Thursday afternoon. Mainly dry conditions expected with periods of showers and thunderstorms developing Friday into Sunday with frontal boundary near by. Wind gusts are expected to stay below the small craft advisory threshold.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...KLW NEAR TERM...ADS SHORT TERM...KLW LONG TERM...IMR AVIATION...ADS/IMR/KLW MARINE...ADS/IMR/KLW

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