Area Forecast Discussion
Issued by NWS Sterling, VA

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206 FXUS61 KLWX 220202 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 902 PM EST Sat Jan 21 2017 .SYNOPSIS...
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Low pressure will impact the area tonight through Monday before moving away Tuesday. A cold front will pass through the area Wednesday into Wednesday night and an upper-level trough may build overhead late next week.
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&& .NEAR TERM /THROUGH SUNDAY/...
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Areas of dense fog have been developing across north central MD, along the western shore of the Chesapeake Bay and portions of the eastern WV panhandle. Every piece of model guidance shows visibilites lowering overnight across these areas and expanding southward in coverage. A dense fog advisory has been issued until 10Z and may need to be expanded southward and extended in time. Rain is now moving into the cntrl Shenandoah valley and the VA foothills. This is associated with a developing sfc wave across North Carolina. Brought a chance of rain farther north into ncntrl MD to the PA border based on 12Z ECMWF ensembles and NSSL WRF-ARW. Low clouds and areas of dense fog are expected again this evening as low-level mixing shuts off. The next wave will cross the region tonight and rain will move into the region. Model guidance is depicting most of the rain to be across the southern half of the outlook area however some hi-res guidance is showing the rain may move further north. The best chance for rain overnight will be across the southern half of the region including DC due to the vicinity of a forming sfc low. Areas across the northern half have the potential to get widespread dense fog due to the absence of rain and an advisory may be needed. Rain will move east of the Mid-Atlantic region by Sunday morning. Another dreary day is expected Sunday as northeast winds keep low level moisture in place. Mostly dry conditions are expected Sunday morning before more rain arrives Sunday afternoon. Moisture advection will increase Sunday afternoon and periods of rain are expected into Sunday night. Temperatures will be the 50s Sunday.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... As low pressure takes shape across the TN valley/Appalachians Sunday night into Monday a low level jet is expected to strengthen over the Mid-Atlantic region. Moderate to heavy rain is expected Sunday night into Monday. Rainfall totals between 1-2 inches are expected across the region. Widespread rain will end as the dry slot moves into the Mid-Atlantic region Monday afternoon. Can not rule out convective showers along the leading edge of the dry slot Monday afternoon. A rumble of thunder is possible mainly across Central VA to Southern MD. Occational rain showers are expected through Monday night as an upper level low continues to spin over the region. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Surface low pressure continues to moves northeast away from the Mid- Atlantic coast on Tuesday as showers start tapering off, but breezy conditions pick up. Upper level ridge builds Tuesday night into Wednesday... and a cold front moves across on Wednesday night with limited precipitation associated to it. Upper level trough behind it could bring some precipitation Thursday through Saturday... mainly over upslope areas, but uncertain at this moment. Breezy conditions anticipated during parts of this period... especially over coastal areas and higher elevations. High temperatures will be in the 40s and 50s Tuesday and Wednesday, decreasing to the 30s and 40s. Low temperatures will be in the 20s and 30s.. with some 40s Tuesday and Wednesday night. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/...
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-RA is likely across DCA/IAD/BWI tonight and may improve vsbys. -RA is possible further north however confidence is low. Dense fog is also possible across the terminals overnight. Low clouds and fog will likely linger into Sunday. Another system will approach the region Sunday and periods of RA are expected Sunday afternoon into Monday night. RA may be heavy at times reducing vsbys. IFR conditions expected during this time. Surface low pressure moves away from the Mid-Atlantic region on Tuesday. Some showers possible over the terminals. Dry conditions expected Tuesday night into Wednesday night, before a cold front moves across. Upper level disturbance will affect us on Thursday... with possible showers. Breezy conditions expected for parts of this period, gusting up to 20 kt. Sub-VFR conditions possible mainly early on Tuesday and Wednesday night.
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&& .MARINE... An onshore flow will continue through tonight...but wind speeds should remain light. Low pressure will track through the Gulf Coast States Sunday. An easterly flow will strengthen during this time. A Small Craft Advisory is in effect for the Bay and lower Tidal Potomac River Sunday afternoon. Low pressure will approach the waters from the south and west Sunday night before moving overhead Monday. The pressure gradient will continue to strengthen ahead of the low. A Small Craft Advisory is in effect for all the waters Sunday evening and for portions of the waters into Monday afternoon. A Gale Warning is in effect for the northern waters from the Patapsco River north. A Gale warning may be needed further south when confidence is higher. Surface low pressure moves away from the Mid-Atlantic region on Tuesday. Some showers possible over the terminals. Dry conditions expected Tuesday night into Wednesday night, before a cold front moves across. Upper level disturbance will affect us on Thursday... with possible showers. Breezy conditions expected for parts of this period, gusting up to 20 kt. Small craft advisory conditions expected for most of this period. && .TIDES/COASTAL FLOODING... An onshore flow will continue through tonight...but it should be light. Therefore...elevated water levels are expected but tidal flooding is not expected. An easterly flow will strengthen Sunday into Monday. This may cause anomalies to increase during this time. Minor tidal flooding is possible near night tide...especially late Sunday through Monday. An offshore flow will develop behind the departing low Monday night into Tuesday. Minor flooding is still possible Monday night since exact timing of the offshore flow is still uncertain as of now. && .TIDES/COASTAL FLOODING... && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Dense Fog Advisory until 5 AM EST Sunday for MDZ003>006-011- 014-507-508. VA...None. WV...Dense Fog Advisory until 5 AM EST Sunday for WVZ051-052. MARINE...Small Craft Advisory from 6 PM Sunday to 1 PM EST Monday for ANZ535-536. Small Craft Advisory from noon Sunday to 1 PM EST Monday for ANZ532>534-537-540>543. Small Craft Advisory from noon Sunday to 1 AM EST Monday for ANZ530-531-538-539. Gale Warning from 1 AM to 1 PM EST Monday for ANZ530-531-538- 539.
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&& $$ SYNOPSIS...HSK NEAR TERM...LFR SHORT TERM...HSK LONG TERM...IMR AVIATION...IMR/LFR MARINE...IMR/LFR TIDES/COASTAL FLOODING...RM

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