Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201812 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 212 PM EDT Sun Aug 20 2017 .SYNOPSIS...
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High pressure over the area will move offshore tonight through Monday. A cold front will approach the area Tuesday before passing through Wednesday. High pressure will build in behind the boundary for Thursday through Saturday.
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&& .NEAR TERM /THROUGH MONDAY/...
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High pressure is overhead this afternoon. A cu field is developing along the ridge lines out west. High pressure will slowly move eastward today and a light southerly flow will begin mainly across the VA Highlands, central Shenandoah Valley and central foothills. This will increase dewpoints some and iso showers and thunderstorms are possible in this area this afternoon/evening. High pressure will continue to move eastward and off the coast tonight, and southerly flow will increase moisture into the region overnight. Although any shower/thunderstorm activity will likely be diurnally driven, waning early this evening, some activity may continue to move eastward possibly reaching I-95 by early Monday morning. Many NCAR ensemble members show this. Moisture advection will continue Monday and low stratus will likely be across much of the region Monday morning. Daytime mixing will help break up low status by late morning and breaks of sunshine are expected early Monday afternoon. We are concerned with the activity across IA/MO this afternoon as it will track eastward in the westerly flow aloft and be near our outlook area Monday afternoon. Increased instability and shear will lead to an environment conducive for showers and thunderstorms to form across the region. There should be a lee trough in place and coupled with a crossing disturbance some storms may become strong to severe.
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&& .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
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Convection will likely be ongoing Monday night mainly near I-95 and east. Little change to the environment overnight into Tuesday so leftover boundaries may cause a pop-up shower and thunderstorm into Tuesday morning. Warm and humid conditions will continue Tuesday as a cold front crosses the Ohio Valley. Showers and thunderstorms are possible...especially late Tuesday into Tuesday night across northern and central portions of the CWA closer to the cold front. Some storms may be strong to severe given the instability and strengthening shear profiles.
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&& .LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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Most of the guidance is in decent agreement that the cold front won`t clear southern parts of the CWA (CHO to southern MD) by Wednesday afternoon, which means another round of scattered showers and thunderstorms will be possible for these areas. With the upper trough diving down/0-6 km shear still modestly high (especially for August), can`t rule out a few strong to isolated severe storms for this corridor Wednesday afternoon and early evening, as well. A rather strong area of high pressure for this time of year (~1025 mb) will drop across the Great Lakes Wednesday night into Thursday which should push the front well south. The high pressure center will gradually weaken as it drifts just north of the area Friday into the weekend, which should result in below average temperatures and humidity with mainly clear and dry weather conditions.
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&& .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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Light flow AOB 10 kts today gradually veers from W to SE by evening. LAMP/MET guidance wants to bring in widespread IFR vsbys but given climo/synoptic setup believe this is over done. MVFR vsbys/cigs psbl CHO/MRB included in TAF, maybe IAD/BWI/MTN by daybreak but not confident enough to put in TAF at these sites. May be VCSH near CHO this aftn 20-23z but again confidence too low/coverage too sparse for TAF. Brief break in clouds (from bkn to sct) between 14-18z Tue, then back to bkn E of Blue Ridge (all terminals except MRB) w/ pop up shower or tstm seeming plausible 18-00z timeframe. Too sparse this far out for mention in TAF just yet. Brief vsby restrictions psbl. Tstm psbl Wed PM near CHO, should stay S of DCA/IAD unless front slows its southward progress. Otherwise mainly VFR. Generally NW flow AOB 10 kts.
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&& .MARINE...
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High pressure will build over the waters today before shifting toward the Mid-Atlantic Coast tonight. The high will move offshore Monday through Monday night and a cold front will approach the waters Tuesday through Tuesday night. The boundary will pass through Wednesday and high pressure will build over the waters for late next week. A Small Craft Advisory may be needed for portions of the waters Monday night through Wednesday night. Isolated thunderstorms are possible over the waters late tonight through Monday morning. Scattered thunderstorms are possible Monday afternoon and evening. Some storms may produce gusty winds...especially later Monday into Monday evening. A few showers and thunderstorms are possible later Tuesday into Wednesday ahead of a cold front as well...and some of these storms have the potential to be strong to locally severe. Northerly pressure surge/channeling behind cold frontal passage Wednesday into Wednesday night will likely require the issuance of Small Craft Advisories.
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&& .TIDES/COASTAL FLOODING...
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Minor coastal flooding is possible at Annapolis Tuesday and Wednesday morning.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ PRODUCTS...HSK/DFH/WOODY!

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