Area Forecast Discussion
Issued by NWS Sterling, VA

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368 FXUS61 KLWX 211910 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 310 PM EDT Thu Jul 21 2016 .SYNOPSIS... High pressure over the area will drift offshore this afternoon. A system will push north of the region on Friday, followed by a front moving into the area Friday night into Saturday. High pressure will be in control of the area`s weather Sunday before another front enters the area Monday night. && .NEAR TERM /THROUGH TONIGHT/...
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As of 3pm, 1021mb mslp is across the central Mid-Atlantic, down a millibar since this morning as the high pressure center shifts east offshore. Flow is generally 10 mph or less and Sly. Dewpoints have been slow to rise, mid 50s north to mid 60s south. Dewpoints increase to the upper 60s to 70F over the next 24 hrs. MCS has made it to Nrn IN and SErn MI. All guidance, including CAMs have struggled with this feature all day. 17Z HRRR kills the activity over OH this evening. 500mb flow would keep it west of the area. The lack of moisture advection thus far into the area has limited thunderstorm development and would limit potential of any activity advecting in. Mostly clear tonight with min temps in the 60s inland to low to mid 70s urban/nearshore.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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Interaction of heat dome spreading east from the central CONUS and low pressure tracking across northern Quebec will determine weather for Mid-Atlantic Friday and Friday night. Heat is expected with heat indices approaching 100F. Trough from the low will approach from the north Friday evening and should provide enough forcing for scattered thunderstorms into the overnight. However, given mesoscale activity, the timing and location cannot be certain until the activity develops. A remnant frontal boundary from overnight activity should set up near of over the area Saturday. This will be more of a moisture/dew point boundary with areas south having dewpoints over 70F, perhaps mid 60s north. Temperatures expected to be a degree or so higher, highs mid to upper 90s. Good chance for 105F heat indices. With temperature and moisture this high, any thunderstorms could easily become severe.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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Hot and often humid weather continues through the long term, with little "true" relief from the heat through Thursday (>90F forecast each day). Hottest weather expected Sunday through early next week, with H85 temperatures exceeding 20C as upper ridge noses into the area. Guidance is suggesting that dewpoints could drop some Sunday, as slightly drier air filters into the area, which would keep heat indices near 100F. However, any respite from the humidity would be short-lived, as dewpoints rise back well into the 70s by Monday...with heat headlines possible as heat indices approach 105F. Only low thunderstorm chances Sunday, as aside from terrain circulations forcing remains weak. Approaching cold front increases thunderstorm chances Monday, as storms develop to our N/NW and propagate toward the area. H5 heights begin to sag south Tuesday in response to wave moving along with the northern stream. At the same time, aforementioned cold front will approach the area from the NW, which could offer some relief from the oppressive humidity as drier air possibly filters into the area. However, as is always the case this time of year, forward progress of the front remains unclear, which reduces forecast skill with remainder of forecast.
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&& .AVIATION /19Z THURSDAY THROUGH TUESDAY/... VFR under high pressure into Friday. Isolated to scattered showers/thunderstorms around both Friday and Saturday. Light and variable winds this morning turn light southerly this afternoon, southwest Friday, and then westerly Saturday. Primarily VFR Sunday. May have some early morning fog, with a low chance at late day thunderstorms. Thunderstorm chances improve Monday, but coverage still appears to be scattered. && .MARINE...
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Sub-SCA conditions expected into Friday. Light southerly through tonight. Winds turn southwest Friday, and a SCA may be needed for Srn MD waters Friday night as southwest winds increase. Winds then turn west on Saturday. High pressure remains in control Sunday, with southerly channeling possible overnight leading to elevated winds near SCA criteria. Gradient winds will increase Monday ahead of cold front.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BAJ NEAR TERM...BAJ SHORT TERM...BAJ LONG TERM...MSE AVIATION...BAJ/MSE MARINE...BAJ/MSE

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