Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 210126 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 926 PM EDT Thu Apr 20 2017 .SYNOPSIS...
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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 /UNTIL 6 AM FRIDAY MORNING/...
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Thunderstorms were able to feed off of steep low-level lapse rates, decent CAPE (around 1000 j/kg), and 30-40 kt bulk shear to organize into a line that crossed the forecast area late this afternoon and evening. These storms entering a slightly more hostile airmass (less CAPE and a bit of CIN), likely aided by nightfall and the loss of daytime heating. Will be clearing the forecast area by 10-11 pm. Database will follow suit. The next area of thunderstorms are back in northwestern Pennsylvania. This area pre-frontal as well. There should be a weakening trend overnight, but the HRRR suggests echoes will approach the northwestern corner of the forecast area after midnight. Will have chance PoPs returning at that time. Most likely just showers, but with a little lingering MUCAPE, adequate shear and lapse rates...wouldn`t rule out an elevated rumble.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/... 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. && .LONG TERM /SUNDAY THROUGH THURSDAY/... 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. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/...
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Thunderstorms over BWI/MTN at this time. Otherwise, all activity has pushed east of the area. In metro Baltimore, activity will push east by 10pm. No additional storms anticipated tonight. Fog is another story. There is now sufficient low level moisture. Will be adding MVFR to TAFs shortly, and will reassess to see if IFR warranted. There is certainly a chance at that. 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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Weakening showers and thunderstorms reaching the Bay at this hour. We believe that gusts will be under 30 kt, but are monitoring in case additional Marine Warning will be warranted. A cold front will cross the waters Friday. Additional showers/storms...some with gusty winds...will again be a consideration. All should monitor in case another round of Marine Warnings are issued. 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...HTS SHORT TERM...MSE LONG TERM...DFH AVIATION...HTS/MSE/DFH MARINE...HTS/MSE/DFH

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