Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 191853 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 253 PM EDT Tue Jul 19 2016 .SYNOPSIS... A cold front will stall out overhead before passing through the entire area tonight. High pressure will build overhead for Wednesday and Thursday. High pressure over the Atlantic will control the weather pattern for Friday through Sunday. A cold front may impact the area early next week. && .NEAR TERM /THROUGH TONIGHT/...
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Severe thunderstorms are currently present across the Central Foothills this afternoon. The Mid-Atlantic region is between a surface trough to the south and a dewpoint gradient near the MD/PA border. This leaves the region in a humid airmass with dewpts in the mid to upper 60s mainly across the southern half of the outlook area. Instability is greater than 2k j/kg with 20-25 kt 0-6 km shear this afternoon and storms have been intensifying rapidly. Large hail and damaging winds are the main threats with these severe storms. Numerous showers and thunderstorms are expected across the Central Foothills...Piedmont...far southern MD and surrounding waters the rest of the afternoon. Further north...dewpts in the 50s will advect into the region and only isolated showers are possible. The advection of lower dewpts from the north will continue this afternoon and evening as the gradient or frontal boundary sags southward. A shortwave trough will cross the region early evening and may initiate showers mainly across the Chesapeake Bay region. Confidence is low at this time as dewpts will be dropping. Clearing will occur from N to S tonight and into Wednesday. Patchy fog is possible especially for places receiving rain this afternoon.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... High pressure will remain over the region for Wednesday through Wednesday night...bringing dry and seasonably warm conditions along with lower humidity. The high will shift offshore Thursday and a return flow will develop. Humidity will begin to increase during this time. A popup shower or thunderstorm cannot be ruled out over the Potomac Highlands into central Virginia during the afternoon and evening hours...especially over the higher terrain. Thursday night will turn out warm and humid as high pressure settles off to the south and east. && .LONG TERM /FRIDAY THROUGH TUESDAY/... The return and amplification of heat and humidity will be the main story for the end of the week and into the weekend. Much of the time period will feature northwest flow aloft with warming temperatures and a gradual return in humidity. Core of large upper ridge will be centered over the central US with the Northeastern US/Mid Atlantic on the periphery with largely west/northwest flow. With this type of pattern would expect a risk of some showers/thunderstorms with daytime instability and potential MCS`s moving along the periphery of the ridge. Expecting widespread 90s for high temperatures Friday through Monday. Humidity should be initially tempered in the westerly flow, but as northwest flow weakens Sunday and possibly turns southwest ahead of an approaching front, will see humidity values rise. Thus the combination of heat/humidity will likely lead to heat index values in excess of 100F, and heat headlines may be necessary. The most oppressive days currently appear to be Saturday and Sunday. As frontal system approaches Monday, will see the chances for synoptically driven showers/thunderstorms increase. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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TSRA/SHRA expected at cho this afternoon. Gusty winds and large hail are possible in thunderstorm activity. Elsewhere...vcts expected for the rest of the afternoon at IAD-DCA but coverage is expected to be isolated. VFR expected tonight and continue through Thursday. Generally VFR expected from Friday and through the weekend. A few showers/thunderstorms are possible which may bring a brief period of sub-IFR conditions. Patchy fog also possible during overnights, but coverage not likely to be widespread.
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&& .MARINE...
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A cold front will slowly drop through the waters today into tonight. A few showers and thunderstorms are possible mainly across the Tidal Potomac and lower Chesapeake Bay. Thunderstorms will be capable of producing locally gusty winds. A pressure surge will develop behind the cold front late tonight into early Wednesday. Winds may approach SCA criteria for a brief period during this time. However...the pressure surge will be weak so confidence was too low for an SCA headline at this point. High pressure will remain over the waters Wednesday through Wednesday night before moving offshore Thursday. A return southerly flow will develop for Thursday into Thursday night. Mostly sub-SCA conditions expected for the end of the week and weekend, however brief intervals of gusts up to around 20 knots are possible.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...HAS NEAR TERM...HAS SHORT TERM...BJL LONG TERM...MM AVIATION...HAS/MM MARINE...HAS/MM

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