Area Forecast Discussion
Issued by NWS Sterling, VA

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375 FXUS61 KLWX 200742 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 342 AM EDT Sun Aug 20 2017 .SYNOPSIS...
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High pressure will build overhead today before moving 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 TONIGHT/...
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The upper-level disturbance and weak reinforcing cold front that caused the convection Saturday night has moved away from the area. High pressure is building in from the north. A northerly flow ahead of the high has allowed for drier conditions. Patchy fog is still possible since the northerly flow is light...especially across central Virginia early this morning. Weak high pressure will continue to build overhead through midday before moving toward the Mid-Atlantic coast late in the day. Most areas will have dry conditions along with sunshine and lower humidity. However...a light return flow will develop late today over the Shenandoah Valley and portions of the Potomac Highlands. A surface trough will also develop from daytime heating...and this may be enough to trigger an isolated shower or thunderstorm toward evening. Most places should be dry though since moisture will remain somewhat limited and forcing will be weak. The high will will remain along the Mid-Atlantic Coast tonight and a return flow will develop for most other areas. Increased moisture and slight isentropic upglide will result in more clouds and perhaps even an isolated shower or thunderstorm. Forcing should still be weak so any convection appears that it would be isolated.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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High pressure will continue to shift offshore Monday and a southerly flow will strengthen...allowing for more humid conditions to return. The increased humidity will cause an unstable atmosphere and another surface trough is expected to develop...acting as the lifting mechanism for isolated to scattered thunderstorms. High amounts of instability are possible with dewpoints likely climbing back into the 70s for many locations. NAM Bufkit is showing around 2000-3000 J/KG of MLCAPE developing in the afternoon. Other guidance has less instability and it will depend on how quickly dewpoints increase. For now...the latest forecast leans toward the NAM thinking that a return southerly flow will transport more moisture from the warm waters nearby this time of year. Despite weaker shear profiles and weak mid-level lapse rates and weak forcing...if this instability is realized then strong to locally severe storms are possible. For the eclipse...the isentropic upglide that is likely to bring more clouds Sunday night should gradually weaken Monday morning. This will allow for breaks of sunshine during the late morning through the middle afternoon hours. A pop shower or thunderstorm cannot be ruled out from daytime heating. However...the most likely scenario for the strong to perhaps even locally severe storms along with more cloud cover is that they would hold off until late in the afternoon into Monday evening. Will have to keep a close eye on this because when timing convection this far out confidence is low. High pressure will remain offshore Monday night. Any convection that does develop should gradually decrease in coverage later Monday night. More warm and humid conditions are expected with patch fog. A cold front will move from the Great Lakes toward the Ohio Valley Tuesday before eventually moving toward our area Tuesday night. Shear profiles will increase later Tuesday through Tuesday night as an upper-level trough digs over the Great Lakes toward New England. Warm and humid conditions ahead of the boundary will continue to a southwest flow ahead of the approaching cold front. A few 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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A cold front will move across the region Wednesday before moving offshore Wednesday night. There is a chance for showers and thunderstorms ahead of and along the front. Temperatures will be seasonably warm with highs in the middle 80s along and east of the Blue Ridge, while highs manage to reach the middle to upper 70s west in the Potomac Highlands. A broad area of high pressure will build in from the northwest behind the front Wednesday night through Thursday night, ushering in cooler and drier air. Low temperatures Wednesday night will be about 10 degrees cooler than previous nights. Thursday`s high temperatures may only top the 80 degree mark in most locations, while we can expect Thursday night`s low temperatures to be about the same as Wednesday night`s low temperatures...in the lower to middle 60s. Although high pressure is expected to keep most of our region dry and comfortable Friday, we can`t rule out a pop-up shower or thunderstorm over the higher elevations along the Appalachian Front during the afternoon or early evening. High pressure should regain control of the entire region overnight Friday night and Saturday. Dry conditions and seasonably cool temperatures should continue across the region.
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&& .AVIATION /07Z SUNDAY THROUGH THURSDAY/...
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VFR conditions are most likely through Tuesday...but patchy fog is possible during the early morning hours each day. Isolated showers and thunderstorms are possible tonight through Tuesday. Some storms may produce gusty winds...mainly during the afternoon and evening hours each day. A cold front will approach the terminals Tuesday night. More showers and thunderstorms are possible. A few storms may be strong to severe. VFR conditions Wednesday through Thursday night. There could be brief periods of SubVFR conditions at any one of the terminals due to shower and thunderstorm activity ahead of and along the cold front Wednesday. Winds southwest 5 to 10 knots Wednesday morning, becoming northwest around 10 knots with gusts 15 to 20 knots Wednesday afternoon through Wednesday night. Winds becoming north 5 to 10 knots Thursday and Thursday night.
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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.
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&& .TIDES/COASTAL FLOODING...
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Elevated water levels continue early this morning. The flow has turned offshore and it will remain offshore through midday...but it will be light. A light onshore flow is expected to develop late today and tonight. A southerly flow will strengthen a bit for later Monday through early Wednesday ahead of a cold front. Minor tidal flooding is possible for sensitive areas near high tide during the overnight and morning hours both days. A stronger offshore flow is expected behind the cold front later Wednesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...KLW AVIATION...BJL/KLW MARINE...BJL/KLW TIDES/COASTAL FLOODING...BJL

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