Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201848 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 248 PM EDT Thu Apr 20 2017 .SYNOPSIS...
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High pressure off the coast will move further east today. A cold front will drop into the Mid Atlantic Friday. Weak high pressure builds over the area Friday night before low pressure impacts the area Saturday night and Sunday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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A few showers have been developing over the past hour or two across the higher terrain...though, they are currently struggling to sustain themselves long enough to become thunderstorms as parcel accelerations remain rather weak. That should change over the next hour or two...as instability continues to increase over that area. Once scattered thunderstorms develop, they will gradually push east through the afternoon into the evening. CAPE/shear parameter space supports general thunderstorms (e.g., MLCAPE ~600-1000 J/kg...0-6 km EBS ~20-30 kts)...with perhaps a very brief and localized pulse-type strong thunderstorm. The best instability will be across the higher terrain...with much less expected east of the Blue Ridge. The higher shear totals (~30 kts by late afternoon) are mainly focused to our north across PA...but could slip into the northern tier of counties near the Mason-Dixon line by early evening. While still low probability, this area is the most likely to experience a stronger thunderstorm...if the activity in PA/WV can move into this area. Any thunderstorms should weaken shortly after sunset as instability wanes with the loss of daytime heating. While low confidence, there could also be some patchy fog tonight...particularly where it rains this afternoon/evening. Overnight lows near 60F.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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Cold front will push through the area Friday. Still some uncertainty with regards to timing...which will have a large impact on sensible weather across the area. Thunderstorms are expected along/ahead of the front. A few of these storms could become marginally severe...as instability increases compared to today (e.g., MLCAPE ~1000-1400 J/kg). Latest SPC Day 2 Marginal Severe risk was shifted further north into our area...which makes sense considering the timing uncertainty of the frontal passage and thus how much destabilization can occur. While slightly better than today...strong wind field remains to the north of the area...which results in only marginally favorable shear profiles for storm organization. Front will stall to our south late Friday into Saturday, leaving us with extensive cloud cover as we remain in the cool sector. Low pressure will approach the area from the SW during the day Saturday...allowing for overrunning precipitation to develop over the area during the afternoon and through the overnight. Uncertainty remains in regards to how far north the greatest QPF sets up...latest trends in the guidance continue to push the highest totals south...keeping them over central VA. 1-2 inches of rain are possible across central VA...with generally less than an inch expected north of I-66.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/...
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Low pressure at the surface will likely sink south of the area Sunday as the upper level low forcing it becomes cutoff from the westerlies. The low may meander off the coast for a couple days with high pressure building to the north. This would result in a cool and dreary pattern into the first half of next week. By the middle of next week, the low should move away and high pressure rebuilds to the southeast of the area, resulting in southwesterly flow and a marked increase in temperatures.
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&& .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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VFR expected at all sites through much of the today. Scattered thunderstorms are possible this afternoon into the early evening...with brief flight restrictions and gusty winds possible. There could be some patchy fog around tonight...though confidence is rather low at this time. Thunderstorms with gusty winds are possible Friday...with flight restrictions possible in any stronger thunderstorms...VFR expected outside of thunderstorm activity. Low clouds and periods of rain could impact the terminals Saturday afternoon through Sunday morning...with sub-VFR possible. Sub-VFR likely in low clouds/onshore flow (generally northeasterly around 10 knots) Sunday into Monday.
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&& .MARINE...
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While gradient winds are expected to remain below SCA values today/tonight, scattered thunderstorms are possible this afternoon into early evening (especially over the Upper/Middle Tidal Potomac). Brief/localized gusty winds are possible in the strongest thunderstorms...with a chance for a few Special Marine Warnings. Rain is expected for the waters Saturday. Generally sub-SCA conditions expected Saturday night and Sunday. SCA conditions possible Monday with increasing northeasterly flow. Northeasterly flow between high pressure to the north and low pressure to the south may push gusts into Small Craft Advisory territory early next week.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...DFH AVIATION...MSE/DFH MARINE...MSE/DFH

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