Area Forecast Discussion
Issued by NWS Sterling, VA

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056 FXUS61 KLWX 191842 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 242 PM EDT Sat Aug 19 2017 .SYNOPSIS... Weak high pressure will build in today...but an upper-level disturbance and reinforcing cold front will pass through the area this evening. High pressure will build overhead late tonight through Sunday before moving offshore Monday. High pressure will remain offshore Tuesday and a stronger cold front will pass through Wednesday. High pressure returns for the second half of the week. && .NEAR TERM /TONIGHT/... An upper level trough is crossing the Great Lakes this afternoon which is leading to a thick cu field across OH and W PA. Sfc high pressure will continue to move into the Mid-Atlantic region today and tonight. As the trough approaches, a cu field will begin to develop later today across the Mid-Atlantic region. Cooler temps in the mid-levels will lead to steepening lapse rates across OH/PA/W WV this afternoon. Showers and thunderstorms are expected to develop in this region where marginal instability and shear is coupled with the trough passage. There is a slight/marginal risk of severe thunderstorms just to the north of the forecast area today. This activity will approach the Appalachians late this afternoon into early evening and a few showers and an isolated thunderstorm may drop south into the Potomac Highlands. Some hi-res guidance has some showers making it as far east as I-95 tonight. The loss of daytime heating and weaker lapse rates will inhibit further downstream thunderstorm development. The trough axis will cross the Mid-Atlantic late tonight and clearing is expected tonight. Clear and calm conditions may lead to fog development in low-lying areas such as the Shenandoah Valley and central Foothills. High pressure will be across the region Sunday and dry weather is expected. Temps will be cooler Sunday with high temperatures reaching the mid to upper 80s. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... High pressure will move off the coast by Monday morning. Southerly flow will increase resulting in the return of warm and humid conditions. Westerly flow aloft will lead to the potential upper level disturbances to pass across the Mid-Atlantic region. A CU field will likely develop by afternoon and showers and thunderstorms are possible. At this time...disorganized activity is expected through the afternoon and early evening. Any convection will likely wane Monday night resulting in mild and dry conditions. && .LONG TERM /TUESDAY THROUGH SATURDAY/... A cold front is expected to move into the Ohio Valley Tuesday afternoon. This will present a chance of thunderstorms across teh northern part of our forecast area by mid/late afternoon, and across the entire region Tuesday night as the front moves into PA. The chance should persist through at least midday Wednesday. Ahead of the front east of the mountains highs Tuesday will reach the 90 degree mark, with lows in the lower to mid 70s. Wednesday highs should drop back into the 80s. High pressure will decend out of the upper Great Lakes and bring some very pleasant weather to the region for the second half of the week. Lows Thursday and Friday night will drop into the 50s west of the mountains, and lower 60s east. Highs Thursday through Saturday around 80. && .AVIATION /17Z SATURDAY THROUGH WEDNESDAY/... VFR conditions are expected most of the time through Wednesday. An isolated shower or thunderstorm is possible late this afternoon and evening across the northern terminals...but most areas will be dry. Patchy fog cannot be ruled out early Sunday morning. Scattered thunderstorms are possible later Monday into Monday night as a southerly flow allow for more atmospheric moisture to return. A cold front is expected to move into PA Tuesday night...and cross the Bay midday Wednesday. Thunderstorms/sub-VFR weather will be possible during this time. VFR conditions to end the day Wednesday. && .MARINE... A reinforcing cold front will pass through the waters this evening. A pop up shower cannot be ruled out but most areas will be dry. High pressure will build overhead for Sunday before moving offshore Monday. A return southerly flow will develop Monday and persist through Tuesday night ahead of a cold front. A cold front is expected to push through the waters by midday Wednesday. Showers and thunderstorms will be possible beginning late Tuesday afternoon. Wind gusts may reach SCA levels Tuesday and Wednesday. && .TIDES/COASTAL FLOODING... Waters may rise to minor levels at Annapolis Monday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ .PRODUCTS...HSK/WOODY! is the U.S. government's official web portal to all federal, state and local government web resources and services.