Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 040838 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 338 AM EST Sun Dec 4 2016 .SYNOPSIS...
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High pressure will remain overhead today. An upper-level disturbance will pass through the area tonight and high pressure will briefly return for Monday. Coastal low pressure will impact the area Tuesday before high pressure briefly returns for Wednesday. A potent cold front will move through late next week.
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&& .NEAR TERM /THROUGH TODAY/...
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Southwest flow aloft will increase throughout the day today ahead of a northern stream trough moving out of the central plains. This will help to usher in warm air advection and moisture aloft, leading to thickening and gradually lowering clouds through today. As system pushes eastward, some light precipitation will begin to approach the region late in the day, but most places remain dry through the afternoon. Temperatures will be on the cool side with highs in the 40s for most locations, except some 30s across the higher terrain.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM MONDAY/...
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Northern stream trough will push across the region later this evening and overnight, picking up some energy/moisture from the southern stream, but largely remaining separate and unphased. This will lead to a period of precipitation this evening and overnight, but keeping amounts on the light side. With southwest flow and marginally cool atmosphere in place to start, the majority of the area will see rain as the predominant precipitation type. As wet- bulb cooling occurs aloft when precipitation overspreads the area, there will likely be a period when sleet mixes in with the rain, but with little to no sleet accumulation. However across the higher elevations of the Alleghenies and possibly the Blue Ridge that reach into that colder layer, some freezing rain is expected. Thus have issued a Winter Weather Advisory from 5pm this evening to 6am Monday morning for a light ice accumulation less than a tenth of an inch. In addition, there may be some snow across portions of northern/western Maryland and into northeastern West Virginia, but any accumulations will be light and less than one inch, and mostly confined to the higher elevations. As warming occurs late at night will see most locations warm above freezing and precipitation should end as some light rain by morning. Lows in the 30s to around 40F. High pressure will build in behind the system on Monday, leading to a drying trend. Highs back up into the upper 40s to mid 50s.
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&& .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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Low pressure will track through the Gulf Coast States into the Tennessee Valley Monday night. The surface ridge from high pressure over New England will continue to extend southwest into our area. Clouds will lower and thicken as the low approaches from the southwest. Latest guidance has slowed the onset of precipitation...so most areas will be dry. However...rain/mixed precip may approach central Virginia toward daybreak. The low will slowly move northeast into the Ohio Valley Tuesday while it transfers its energy to a coastal low along the North Carolina Coast. Warm and moist air from the Gulf of Mexico and Atlantic Ocean will overrun the surface cooler air in place...likely resulting in a soaking rain for most areas. There will be plenty of dry air ahead of the system and as precipitation into the dry air...it will lead to insitu cold air damming from evaporative cooling. The cooling effect may be enough for a cold layer cold air to remain trapped near the surface. This may cause freezing rain...especially for the higher elevations in the Allegheny and Potomac Highlands. Elsewhere...evaporative cooling may lead to some sleet at the onset but surface temps should be above freezing. Any ice amounts from freezing rain should be light with temperatures near 32...but that can still cause slippery conditions and it will have to be monitored. Coastal low pressure will slowly move off to our north and east Tuesday night. Rain in the evening should give way to drying conditions overnight. Exact timing of precipitation ending is a bit uncertain at this time. High pressure will briefly build overhead for Wednesday. A potent cold front will pass through Thursday and much colder conditions are expected behind the boundary for the end of the week.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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VFR conditions will continue through today with a gradually thickening and lowering cirrus deck ahead of an approaching low pressure system. Precipitation in the form of rain and possibly some sleet will overspread the TAF sites this evening and tonight bringing widespread sub-VFR conditions overnight and into early Monday morning. Conditions will then improve back to VFR during the day Monday. Winds light northerly this morning become southerly by this afternoon before turning back to the northwest on Monday. VFR conditions expected Monday night. Rain will overspread the terminals Tuesday morning and it will continue through Tuesday afternoon before gradually ending Tuesday night. Subvfr conditions are expected during this time. Rain may start off as a period of sleet Tuesday morning. Rain may also freeze on some surfaces for a brief period Tuesday morning around KMRB...but confidence is low at this time.
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&& .MARINE...
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Small Craft Advisory remains in effect for portions of the central Chesapeake and lower Tidal Potomac through 6am with continued northwest winds gusting up to about 20 knots. Winds then finally drop below SCA later this morning and remain below through Monday. Coastal low pressure will develop to the south Monday night and it will pass by to the east later Tuesday into Tuesday night. High pressure will briefly build toward the waters Wednesday...but a potent cold front is expected to pass through Thursday. A Small Craft Advisory may be needed for the waters Tuesday into Wednesday. A Gale Warning may be needed for the waters Thursday night and Friday behind the cold front.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Winter Weather Advisory from 5 PM this afternoon to 6 AM EST Monday for MDZ501. VA...Winter Weather Advisory from 5 PM this afternoon to 6 AM EST Monday for VAZ503-504. WV...Winter Weather Advisory from 5 PM this afternoon to 6 AM EST Monday for WVZ501-503-505-506. MARINE...Small Craft Advisory until 6 AM EST early this morning for ANZ533-534-537-541-543.
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&& $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL/MM AVIATION...BJL/MM MARINE...BJL/MM

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