Area Forecast Discussion
Issued by NWS Sterling, VA

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233 FXUS61 KLWX 191342 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 942 AM 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/... A surface trough extends from coastal New England to southern VA this morning. Further north...a cold front or dewpt gradient is across southern PA. Winds are northerly across the Mid- Atlantic however lower dewpts have yet to advect into the region as they are still north of Harrisburg PA. This is keeping it warm and humid this morning. It won`t be until late this afternoon when we start to see dewpts dropping into the 50s from N to S and will continue into the evening. Scattered mid-high level clouds will continue this morning with temps rising into the upper 80s by afternoon. Due to the higher dewpts today...marginal instability is expected with higher amts across the southern half of the outlook area where dewpts are higher including CHO-CJR-EZF-NHK. Showers and scattered thunderstorms are expected to form around noon as convective temps are reached. The coverage will likely be isolated across the MD/PA border to scattered/numerous across Central VA... where the dewpts are greatest. A shortwave trough will also dive into the Mid- Atlantic mid afternoon and into the evening which may cause additional thunderstorms to form. This disturbance and the lower dewpts coming in from the north will compete. If the dewpts are slower to advect in towards evening then thunderstorm coverage will likely be greater across the region...if lower dewpts are faster then the thunderstorms will be limited to the southern portion of the area. Prev discussion... The cold front will stall out just to the south of the area tonight and a northerly flow will allow for drier and more comfortable conditions overnight. && .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 MONDAY/... 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 /14Z TUESDAY THROUGH SATURDAY/... VFR condtions are expected most of the time today through Wednesday night. A cold front will slowly drop south through the area this afternoon into tonight. A few showers and thunderstorms are possible this afternoon and evening. Coverage will be isolated to widely scattered. The best chance for a thunderstorm will be across KCHO...and any thunderstorms that do develop may contain gusty winds or hail. High pressure will build overhead for Wednesday through Wednesday night. The high will move off the coast for Thursday and Thursday night...and a return southerly flow will develop. However...VFR conditions will likely persist. 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. && .MARINE... A cold front will slowly drop through the waters today into tonight. A few showers and thunderstorms will develop along and ahead of the boundary. Coverage of convection will be isolated to scattered...but 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. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BJL NEAR TERM...BJL/HAS SHORT TERM...BJL LONG TERM...MM AVIATION...BJL/MM/HAS MARINE...BJL/MM is the U.S. government's official web portal to all federal, state and local government web resources and services.