Area Forecast Discussion
Issued by NWS Sterling, VA

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734 FXUS61 KLWX 210810 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 310 AM EST Tue Nov 21 2017 .SYNOPSIS... High pressure will move offshore today and a cold front will pass through tonight into early Wednesday. High pressure will build back overhead later Wednesday through Thanksgiving Day. The high will build to our south for Friday and Saturday before a cold front moves through Saturday night. Canadian high pressure will build overhead early next week. && .NEAR TERM /THROUGH TONIGHT/... High pressure to our south early this morning will move offshore later today. A southwest flow around the high will usher in milder conditions after a chilly start early this morning. Max temps will range from the upper 40s and lower 50s in the mountains to the upper 50s and lower 60s across most other locations. There will be a thin deck of high clouds due to a southwest flow aloft...but the cloud deck should be thin enough for a partly to mostly sunny sky. A cold front over the Ohio Valley this evening will move into the area late tonight. A southerly flow ahead of the boundary will cause clouds to increase early this evening and a few showers are possible with the frontal passage later this evening into the overnight hours. Moisture will be limited and forcing will be weak with most of the dynamics off to the north. Therefore...any showers that do occur will be light. The best chance for showers will be across eastern areas where moisture will have a little more time to advect in from the south before the frontal passage. Min temps will range from the upper 20s to around 30 in the Allegheny Highlands to the mid and upper 40s in downtown Washington and Baltimore into southern Maryland. A period of upslope snow showers are expected late tonight behind the cold front for locations along/west of the Allegheny Highlands. Accumulations should be around one inch or less since moisture will be limited...but this may cause slippery conditions by the early morning hours Wednesday. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... The cold front will move off to the south and east early Wednesday morning. Canadian high pressure will approach from the north and west and a northwest flow ahead of the high will usher in drier and chillier conditions. Max temps will hold in the 40s for most locations north and west of Interstate 95 and in the lower to middle 50s farther south and east. High pressure will build overhead Wednesday night...bringing dry and cold conditions. Min temps will drop into the 20s for most areas. High pressure will remain overhead Thanksgiving Day through Thanksgiving night...bringing more dry and chilly conditions. && .LONG TERM /FRIDAY THROUGH MONDAY/... High pressure will be in control of the region`s weather on Friday leading to fair skies and dry weather. By Friday night, a potent northern stream system and its associated cold fronts will move into the upper Midwest, before barrelling towards the Mid-Atlantic states Saturday and Saturday night. The series of cold fronts will cross the region during this time period with chances of some showers. Southern stream energy will be present as well near the southeastern states but at this time, the fast flow aloft and orientation of the upper level trough suggests that any surface low that can develop will favor a track well offshore with little to no impact to the region. On Sunday, behind the fronts and with the upper trough moving overhead, blustery and colder conditions are likely, along with the potential for snow showers over the higher terrain. As the upper trough does move overhead, some scattered flurries/snow showers are even possible in lower terrain. High pressure will build back overhead by Monday with fair skies and diminishing winds. Temperatures will start out near normal or even a bit above for Friday and Saturday (50s), before falling back below for Sunday and Monday (40s). && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... Southwest winds will gust around 20 knots late this morning and afternoon. A cold front will pass through the terminals tonight. A few showers cannot be ruled out...but any precipitation amounts will be light. Brief MVFR conditions are possible in any showers that do occur...but VFR conditions are most likely for most of the time. Northwest winds will develop behind the cold front early Wednesday morning and gusts around 20 to 25 knots are expected for most of Wednesday behind the cold front. High pressure will settle overhead for Thursday through Thursday night with VFR conditions likely. VFR and light winds expected Friday with surface high pressure. Chances for some showers exist on Saturday with a frontal system, but widespread restrictions are not currently expected. && .MARINE... A southwest flow will increase today ahead of a cold front. A Small Craft Advisory is in effect for the waters. The SCA continues through this evening for most of the Bay and lower Tidal Potomac. Winds should decrease overnight as the gradient relaxes. A cold front will pass through early Wednesday morning and northwest winds will increase behind the boundary with gusts 20 to 30 knots most likely. An SCA is in effect for the waters. High pressure will build over the waters for Wednesday night through Thursday night. An SCA may be needed for portions of the waters Wednesday night. Sub-SCA conditions expected Friday with surface high pressure. SCA possible Friday night and Saturday ahead of next frontal system. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until midnight EST tonight for ANZ531>534- 537-539>543. Small Craft Advisory from 6 AM to 6 PM EST Wednesday for ANZ530>543. Small Craft Advisory from 6 AM this morning to 6 PM EST this evening for ANZ535-536-538. Small Craft Advisory from 6 AM this morning to midnight EST tonight for ANZ530. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...MM AVIATION...BJL/MM MARINE...BJL/MM

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