Area Forecast Discussion
Issued by NWS Sterling, VA

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495 FXUS61 KLWX 260752 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 352 AM EDT Wed Apr 26 2017 .SYNOPSIS...
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Low pressure off of the DelMarVa will move northeastward away from the Mid-Atlantic coast today. A cold front will approach the region and weaken over the area Thursday night. A backdoor front may slide into the area late Saturday into Sunday. A cold front will pass through the region early next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Vertically-stacked low that has been plaguing our weather the past several days is currently analyzed over the DelMarVa coast. Low clouds and some patchy drizzle/fog will remain through daybreak today...with improvement expected from south-to-north through early afternoon as low finally shifts off to the northeast. Primary forecast challenge today will be timing when clouds break...and subsequent high temperature forecast. Most confident in highs across central VA...where clouds break early and insolation is strongest. Expect highs in the U70s to near 80F across this region. Slightly less confidence from the DC metro northward to the Mason-Dixon line...where clearing is expected near midday (DC) and then into the afternoon (Mason-Dixon line). If clearing is slower (faster) then highs would be lower (higher) across these areas.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...
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Ridging develops over the area tonight...leading to light winds and relatively clear skies. With dewpoints in the U50s/L60s...expect lows near 60F...with the possibility of fog due to radiational cooling. Very warm with increasing humidity across the area Thursday as southerly return flow strengthens ahead of a weak cold front. Front will begin to washout as it approaches our area...bringing the possibility of showers/elevated thunderstorms to the higher terrain late Thursday. Only a low chance that a shower or storm pushes east of the Blue Ridge...as forecast thermodynamic profiles suggest capping inversion would generally suppress convection. Warm and humid weather continues Friday. Warm air aloft will lead to a capping inversion near 800 mb that will likely suppress convection across the much of the area...especially with a lack of a focusing mechanism. One exception could be across the higher terrain where an isolated shower/thunderstorm could develop by late afternoon. Though, poor mid-level lapse rates across this area would likely limit parcel accelerations and prevent necessary ingredients for lightning.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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There is (unfortunately) tremendous spread in the weekend forecast. The synoptic setup by that time will be for a warm (or backdoor cool) front stretched east-west across the Mid Atlantic and the Ohio Valley, to low pressure which will be in the southern Plains. The quandry is how far south will the front make it. The GFS, supported by the GEFS mean and a number of its ensemble members, suggests somewhere around southern Pennsylvania or northern Maryland...which would obviously keep the forecast area on the warm side of the boundary. Some GEFS members are still further north than the mean. The ECMWF, on the other hand, represents the furthest south of the ensemble spread, although some GEFS members support it, too. Based on this solution, the forecast area will be impacted by a backdoor front, which would drop temperatures by around 10 degrees, most notably across the northern portion of the forecast area. Will seek some sort of middle ground, although will still favor warmer solutions until forecast mean comes into some sort of consensus. By Sunday, it seems apparent that we will be under the influence of a backdoor front. The only question here will be how cool will it get. Errors from the Saturday forecast will only multiply, so will not deviate from the model blend at this time. There are precip implications to the above, as warmer weather will yield greater instability, and the presence of a surface boundary potentially could yield thunderstorm development. Will be keeping the Saturday forecast as chance showers/thunderstorms. Am thinking it may be dubious for storms to have a chance on Sunday (they`d be shunted to the south instead). Will utilize showers instead. Either way, by Monday the front would be dragged north of the area again as low pressure intensifies and heads toward the Great Lakes. That would pave the way for a late Monday or Monday night cold frontal passage.
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&& .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
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Low cigs/vis will cause IFR (to at times MVFR) conditions across much of the area through daybreak...with clearing (and a return to VFR) expected from south to north through the morning into midday. VFR expected by afternoon at all TAF sites. Sub-VFR could return tonight as fog develops over the area (especially across the NE). VFR returns during the day Thursday. Cold front approaches the area late Thursday with the chance for isolated showers/thunderstorms. Dry Friday with VFR conditions. There is a large degree of uncertainty in the weekend forecast, as a frontal boundary will be meandering around the area. Showers and thunderstorms will be likely, but areal coverage will be a question. If the front does make it south of the terminals, then there will be the opportunity for low clouds to work inland. Lots of opportunities for flight restrictions, but details unclear at this time.
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&& .MARINE...
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Light winds expected today/tonight as low pressure continues to move away from the area. Strengthening southerly flow Thursday ahead of a cold front could lead to low-end SCA wind gusts. Low confidence precludes issuance of SCA at this time range. Light winds and dry weather expected Friday. A front likely will be meandering across the waters. As a result, the pressure gradient likely to not be strong. Direction of winds unclear, dependent upon what happens with the front. If the front does sink south, then a northeast wind may pick up Saturday night.
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&& .TIDES/COASTAL FLOODING...
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Elevated water levels continue, but departures are not as high as last evening. Have cancelled the Coastal Flood Advisory for Calvert County, as tides have come in an inch or two under minor threshold. Straits Point definitively above minor, so that Advisory in good shape. The theme of water levels hovering around the minor threshold will be working north, which does reduce confidence. Have opted to issue a Coastal FLood Advisory for Anne Arundel County nonetheless. No action north of there. On the Potomac, the Advisory for Alexandria and Southwest Washington DC seems to be on track. However, base on what`s happening downriver, am not completely sold on this outcome. The overnight tide cycle is the higher of the two astronomically. Since these forecasts are right at threshold, that will be the only cycle of concern. Will need to monitor trends for tomorrow night/Thursday morning, and perhaps again Friday morning.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until noon EDT today for DCZ001. MD...Coastal Flood Advisory until 8 AM EDT this morning for MDZ014. Coastal Flood Advisory until 6 AM EDT early this morning for MDZ017. VA...Coastal Flood Advisory until noon EDT today for VAZ054. WV...None. MARINE...None.
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&& $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...HTS AVIATION...MSE/HTS MARINE...MSE/HTS TIDES/COASTAL FLOODING...HTS

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