Area Forecast Discussion
Issued by NWS Sterling, VA

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312 FXUS61 KLWX 281252 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 852 AM EDT Fri Apr 28 2017 .SYNOPSIS...
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High pressure builds over the area today, then slides off the southeast coast Saturday. A cold front will approach the region from the north Saturday night into Sunday before lifting back northward late Sunday. A cold front will move through the region from the west Monday night. Canadian high pressure will then build over the area through the middle of next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure builds into the area today. Dewpoints fall into the U50s. Despite frontal passage...highs only a few degrees cooler than yesterday (U70s/L80s across the area). Front lifts back north during the late afternoon/early evening. This will allow moisture to surge back into the area...with dewpoints rising into the U60s by 12z Saturday. Increasing moisture will lead to very warm nighttime temperatures...with high minimum records in jeopardy (see climate section below for more details).
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Mid-Atlantic remains sandwiched between Bermuda high and low pressure over the plains through the short term. This will create a constant fetch of moisture rich low-level air with source regions across the Gulf of Mexico and Tropical Atlantic. At the same time, a frontal boundary will be located nearby...with embedded shortwaves riding the periphery of ridge aloft. These features will provide the necessary ingredients for deep moist convection across the Mid- Atlantic. Where exactly storms form will be largely dependent on frontal location. Guidance continues to struggle with handling the spatiotemporal evolution of front...which makes sense...as front will likely be reinforced by convection. Thus, Sunday`s forecast remains very uncertain. Additionally, it remains unclear if convection can develop in the warm sector...as capping inversion due to nearby ridging may stunt convective initiation. Though...terrain circulations may provide enough additional lift to allow parcels to reach their LFC. If storms can develop over the area the favorable juxtaposition of CAPE/SHEAR parameters imply locally strong storms would be possible. Best chances for storms generally from DC northward and across the higher terrain. This fits well with current Day 2 SPC Marginal Outlook. If it appears convection is more likely across our area would expect threat level to increase somewhat. Additionally, unseasonably warm weather will overspread the region south of frontal boundary. Record highs are possible Saturday (highs near 90F) and record high minimums Saturday night and Sunday night (see climate section below for more details). To the north of the front temperatures/dewpoints will be ~10 F cooler than to the south. && .LONG TERM /MONDAY THROUGH THURSDAY/...
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By Monday morning, the forecast area will be in the warm sector, with the warm front approaching the Canadian border and the attendant cold front crossing the Midwest. Fairly well stacked low pressure will be emerging from the Plains. GFS/ECMWF in decent agreement on synoptic features, with a Monday evening cold frontal passage. The air mass prior to fropa will contain plenty of shear, but instability isn`t jumping off the charts. GFS holding onto a line of QPF across area, while ECMWF weakening system as energy crosses Great Lakes. Am fairly comfortable forecasting thunderstorms, especially during the late afternoon and evening hours, but the strength and areal coverage both up for debate. Zonal flow aloft will dominate Tuesday and Wednesday, as the upper low migrates across the Great Lakes to the St Lawrence Valley. Several pieces of vorticity potentially could eject east in this fast flow. Thus, it won`t be purely sunny. Deep westerly flow suggests that clouds/precip may get caught up in the mountains. By Wednesday night and Thursday, guidance is hinting at a pattern change, as a trough axis cuts across the central CONUS. Low level moisture in advance of the trough advects off of the Gulf of Mexico, with a plume of precipitation approaching the Mid Atlantic. Interactions potentially could even induce cyclogenesis by the end of the forecast period. Monday`s temperatures will be relatively similar to Sunday. Weak cold advection will have in impact for midweek, as highs slowly ease closer to normal. That subtlely will end by Wednesday night and Thursday in what possibly could be a wet and cool period.
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&& .AVIATION /13Z FRIDAY THROUGH TUESDAY/...
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VFR will continue through at least Saturday morning...as high pressure influences the weather. Cold front will approach the area from the north Saturday. If it makes it into the area thunderstorms would be possible (mainly at MRB/BWI/MTN). Front may sag further southward into the area late Saturday into Sunday with another possibility of thunderstorms...best chance along/north DCA/IAD. If a storm impacts a terminal it could be locally strong with brief sub- VFR conditions. A cold frontal passage Monday PM will likely result in thunderstorm development. The areal coverage and strength both up for debate. However, local/brief IFR possible within thunderstorms. VFR expected behind the front on Tuesday.
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&& .MARINE... Relatively light winds expected over the waters Friday into Sunday. If a cold front can slide into our area from the north...shower and thunderstorms would be possible Saturday afternoon/evening (mainly for northern Chesapeake Bay). Another round of storms possible Sunday as front remains nearby...again best chance on the northern waters. Any storms that develop during this period could be locally strong. Gradient winds (from the south) will increase ahead of a cold front Monday. Its unclear how well vertical transport will overcome the cooler waters, but Small Craft conditions at least possible. Then, with the cold frontal passage, some thunderstorms with strong gusts possible. West flow behind the front Tuesday will provide a high confidence Small Craft Advisory day. && .TIDES/COASTAL FLOODING... Tidal anomalies remain elevated across the area as southerly flow has kept water from evacuating the estuary. Coastal Flood Advisory continues for Straits Point...as it is currently exceeding minor flood stage. Issued a Coastal Flood Advisory for this tide cycle at Annapolis since forecast of minor tidal flooding is within the margin of error (a tenth of a foot or so). Minor flooding is also possible this morning at DC. Though, confidence is too low to act on it now...will continue to monitor. Tidal anamolies are expected to decrease slightly over the next several days as wind flow decreases. However, return of southerly flow later today into the weekend will keep anamolies elevated. && .CLIMATE...
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Record highs/warm lows through the weekend: Washington DC... Friday 28 April...92 (in 1957)/66 (in 1990) Saturday 29 April...91 (in 1974)/68 (in 1956) Sunday 30 April...92 (in 1942)/67 (in 1983) BWI Airport... Friday 28 April...90 (in 1957)/67 (in 2009) Saturday 29 April...91 (in 1974)/67 (in 1956) Sunday 30 April...92 (in 1910)/63 (in 1983) Dulles Airport.. Friday 28 April...88 (in 1990)/62 (in 2009) Saturday 29 April...87 (in 1996)/62 (in 1996) Sunday 30 April...86 (in 2007)/64 (in 1983) The only day when record high temperatures will be challenged appears to be Saturday. Record warm low temperatures, on the other hand, may be broken (or come close) each day.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Coastal Flood Advisory until 9 AM EDT this morning for MDZ014. VA...None. WV...None. MARINE...None.
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&& $$ UPDATE...WOODY! PREVIOUS...MSE/HTS

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