Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 212000 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 300 PM EST Tue Nov 21 2017 .SYNOPSIS...
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High pressure will move offshore today. A cold front will pass through tonight into early Wednesday, while weak low pressure moves up the coast. High pressure will build back overhead later Wednesday through Thanksgiving Day. High pressure will be overhead Friday. A front approaches from the west on Saturday, High pressure builds later on Sunday and will remain in control through Tuesday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure ebbs away tonight as we get caught in between a cold front moving in from the west, and weak low pressure moving up the coast. While the front is not expected to bring much if any precip to our forecast area, the coastal low will bring rain up the coast and into lower southern MD and the southern MD Ches Bay. Precip PoPs fall back to chance levels over the major metro areas, and then to slight chance and less for the Shenandoah Valley. Looking at the latest convective allowing meso models and LAMP, the timing looks to be a few hours slower...keeping most of the precip in our CWA after midnight and possibly lingering into Wed morning closer to the Bay. Amounts for those who get rain are expected to be a quarter inch or less. A period of upslope snow showers are expected late tonight into Wed morning behind the cold front for locations along/west of the Allegheny Highlands. Accumulations should be an inch or less since moisture will be limited...but this may cause slippery conditions by the early morning hours Wednesday.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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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, except lower 30s downtown DC/Balt. High pressure will remain overhead Thanksgiving Day through Thanksgiving night...bringing more dry and chilly conditions.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/...
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High pressure will be overhead Friday, keeping dry conditions over our region, before it moves offshore late on Friday. A front approaches from the west on Saturday, as upper trough deepens over the eastern CONUS. PoPs increases with this boundary on Saturday, but deterministic guidance and GEFS are not too aggressive with precipitation over our CWA. At the same time a low pressure system will develop off of the southeast Atlantic coast but it is expected to stay offshore with little to no impacts to our area. Upslope showers expected on Sunday with the northwest flow and upper trough axis moving through our area. Surface high pressure builds later on Sunday and will remain in control through Tuesday.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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Southwest winds will gust around 20 knots this afternoon over the metro airports. A cold front will cross late tonight. A few showers cannot be ruled out (especially for DCA/BWI/MTN)...but any precipitation amounts will be light. MVFR conditions are expected late tonight into early Wed, most prolonged for DCA/BWI/MTN. 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. VFR conditions expected Wednesday into early Saturday. Precipitation possible later on Saturday as a cold front moves trough our area. Dry/VFR conditions return on Sunday.
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&& .MARINE...
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With the southwest flow ahead of a cold front today, 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 25 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 was extended through Wed night for most of the Bay and lower Potomac. Winds are expected to stay less than 20 kts Friday into Saturday night. Winds will increase on Sunday as low pressure moves NE offshore the Delmarva Peninsula, therefore, a small craft advisory is anticipated on Sunday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until midnight EST tonight for ANZ530>534- 537-539>543. Small Craft Advisory from 6 AM to 6 PM EST Wednesday for ANZ530>543. Small Craft Advisory until 6 PM EST this evening for ANZ535- 536-538. && $$ STRONG/FLING/IMR

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