Area Forecast Discussion
Issued by NWS Sterling, VA

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736 FXUS61 KLWX 210739 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 339 AM EDT Fri Apr 21 2017 .SYNOPSIS...
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A cold front will pass through the area late this afternoon through this evening. The front will stall to the south Friday night and low pressure will develop along the boundary...impacting our area for the weekend through early next week.
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&& .NEAR TERM /THROUGH TODAY/...
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A cold front will approach and cross the region through today. There are some showers out ahead of the front that have been moving through portions of western Maryland/Virginia and eastern West Virginia, and some of these will persist through he morning hours, although coverage will be waning. As we head into the afternoon, despite clouds, temperatures will rise into the upper 70s and and mid 80s. This combined with dew points in the upper 50s and lower 60s will lead to the development of 1000+ J/KG of MLCAPE across much of the region, especially east of the Blue Ridge Mountains. As the front presses eastward into this air mass, additional scattered showers and thunderstorms are expected to develop in the afternoon ours out ahead of the front. With the increased CAPE values and 0-6 KM shear near 40 knots, some of the thunderstorms may be strong to severe, especially from I-95 south and east. Westerly surface flow may tend to inhibit coverage west of there. Because of this, the Storm Prediction Center has placed part of the area in a Marginal Risk for severe thunderstorms. Most widespread thunderstorm coverage expected from 18z-00z, with marginally severe hail/wind the main threats.
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&& .SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...
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The frontal boundary will settle south of the area tonight and remain relatively stationary through the weekend. Multiple waves of moisture/energy will ride along this boundary through the period ahead of a main low pressure system that will move along the boundary from the southern plains tonight and to the east coast early next week. There are still relatively significant discrepancies as to northern extent and timing of these individual waves, but several periods of rain and showers are likely from late tonight through at least Sunday night, with the greatest coverage and intensity across central and western Virginia, where model consensus is growing for potentially several inches of rain during this period. For areas to the north, have concentrated the highest likelihood of rain from Saturday afternoon into Sunday morning, although this is uncertain. Temperatures will be quite cool during this timeframe given the abundance of clouds and showers and northerly flow on the northern side of the boundary. Temperatures tonight will drop into the 50s, with not much variation expected once this happens, with highs Saturday and Sunday in the 50s to near 60F, and lows Saturday and Sunday nights in the 40s to around 50F.
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&& .LONG TERM /MONDAY THROUGH THURSDAY/...
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A cutoff upper-level low will swing through the Southeastern CONUS Monday before moving off the Mid-Atlantic Coast Tuesday. This system will cause coastal low pressure along the North Carolina Coast Monday that will slowly move out to sea on Tuesday. High pressure will remain over southern Canada into New England during this time. An onshore flow will cause unusually cool conditions for this time of year at the same time warm and moist air is overrunning the cool air...providing a soaking rain Monday. Rain will continue through Tuesday...but rain rates and coverage may diminish some Tuesday afternoon as the low begins to pull away. The low will continue to move out to sea Tuesday night and high pressure will develop over the central Atlantic. A south to southwest flow around this high will likely bring a return to warm and more humid conditions for the middle portion of next week. Drier conditions are expected during this time...but convection cannot be ruled out due to the warm and humid air mass.
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&& .AVIATION /07Z FRIDAY THROUGH TUESDAY/...
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Low clouds and areas of fog have developed overnight and early this morning across portions of northeastern Maryland with IFR/LIFR being observed at BWI/MTN. Elsewhere mainly VFR has been maintained, although some patchy fog is around. As the morning progresses, expect ceilings/visibilities to improve to VFR at all sites. A cold front will then cross the region this afternoon and evening, with scattered showers and thunderstorms likely, some of which may contain strong gusty winds and hail. Greatest chance for TSRA will be between 18z and 00z. Otherwise, VFR expected for most sites much of tonight. Low clouds and periods of rain will approach from the southwest early Saturday morning through Sunday with the greatest chances for significant rainfall and associated prolonged sub-VFR conditions at CHO. However at least intermittent periods of reductions expected just about every TAF site Saturday through Sunday night. Westerly winds today turn northerly tonight and Saturday then northeasterly by Sunday. Coastal low pressure will impact the terminals Monday through Tuesday with Sub-VFR conditions and rain.
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&& .MARINE...
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Sub-SCA west/southwesterly winds expected for much of today. Scattered showers and thunderstorms this afternoon and evening may produce gusty winds and small hail, and Special Marine Warnings may be necessary. Winds then turn northerly overnight following passage of the cold front and increase on Saturday. Wind speeds may approach marginal SCA-criteria, but confidence too low to issue a SCA at this time, especially with potentially stable conditions due to rain. Similar conditions expected Saturday night through Sunday night. Coastal low pressure will impact the waters Monday and Tuesday before moving out to sea during the middle portion of next week. A Small Craft Advisory may be needed for Monday through Tuesday night. High pressure over the central Atlantic will cause a south to southwest flow to return for the middle portion of next week.
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&& .TIDES/COASTAL FLOODING...
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A persistent onshore flow will develop behind a cold front Saturday through early next week. Elevated water levels are expected during this time...and minor flooding cannot be ruled out near times of high tide...especially Sunday into Tuesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...BJL

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