Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 171920 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 320 PM EDT Thu Aug 17 2017 .SYNOPSIS...
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A warm front will exit the area to the northeast tonight. A cold front will approach from the west Friday before crossing the region Friday night. Surface high pressure will build in Saturday, but an upper level disturbance will cross the region Saturday night. A warm front will advance back north across the region early next week, followed by a strong cold front in the middle of the week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Warm front is slowly advancing back north across the region and has currently cleared nearly all of the CWA and metro. Scattered storms have developed near and east of I-95, mainly in response to the passage of a weak vort max aloft. This convection is supported by decent CAPE (close to 1500 J/kg mixed layer) and modest shear (20-30 knots). However, a weak cap remains aloft. This cap should diminish as the day progresses, so still expect storm coverage to persist if not expand westward, towards the forcing provided by the terrain to the west, with individual storms moving northeastward over time. Given high PW`s now approaching 2 inches, the main risks today are isolated microbursts and localized flash flooding, with the outside risk of a tornado if a storm can get strong and organized enough and tap into the decent directional shear in the vicinity of the warm front. Overnight, we naturally lose the surface CAPE, but another weak shortwave looks to move across the region, which may keep generating isolated to scattered storms. Given diminishing surface CAPE, think coverage will diminish, but perhaps not end entirely overnight. Where skies briefly clear, we could get some patchy fog again. Lows will stay very warm with the high dew points, with 70s common.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/...
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Patchy fog will be a concern early Friday, but not expecting widespread dense fog, and it should break fairly quickly once the sun rises. Probably the biggest forecast concern is the threat of severe weather on Friday afternoon. While it is certainly not the best set up, very high PW`s in excess of 2 inches (potentially above 2.25 inches) and surface temps perhaps into the low 90s with surface dew points into the mid 70s and shear of 25-35 knots, and the surface cold front moving into it, will likely get some sort of line of storms going by early afternoon, which will then slowly push east across the region. Best time in the cities looks like mid-late afternoon, with storms lingering well into the evening east of I-95 into southern MD. Training storms could easily produce flash flooding as well. SPC has outlooked the region for marginal severe threat, while WPC has outlooked the region for slight risk of excessive rainfall. Front slides in later Friday night, with some drying and cooling. Lows will be a bit cooler than tonight, with some areas returning to the 60s. Surface high presses into the region Saturday and Saturday night, but a trough aloft may result in isolated storms late in the day into the evening. For now have kept POPS mainly NW of the cities, but these may need to be expanded south and east if confidence in storms grows. Highs Saturday will be in the 80s. Temps will cool a bit more at night, with 60s more common.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/...
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High pressure will keep dry conditions on Sunday as it slowly moves offshore Sunday night into Monday. Southerly flow settles in advecting moisture into our region Monday and Tuesday. A pressure trough at the surface could trigger showers and thunderstorms Monday and Tuesday afternoon and evening. A cold front will approach from the northwest on Wednesday and move across on Wednesday night. High pressure returns on Thursday. High temperatures during this period will be in the mid to upper 80s in Northern Maryland... to low 90s in Central Virginia at times... and 70s at higher elevations.
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&& .AVIATION /19Z THURSDAY THROUGH TUESDAY/...
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VFR overall through this evening, but scattered thunderstorms are a concern at all terminals until at least late this evening, given multiple shortwaves. Late tonight concern shifts to low clouds and fog potential, which while not as severe as Wednesday morning, is likely to reduce cigs and vis to MVFR if not high end IFR. We should see improvement Friday just in time for the next round of thunderstorms on Friday afternoon. This activity will be more organized with the cold front, with a better severe threat, along with a better chance of low cig and vis with storms. Generally should improve to VFR by Saturday morning, with a much lower (though not zero) risk of storms late Saturday as an upper level trough crosses the region behind the surface front. VFR conditions expected Sunday into Tuesday, with short periods of sub-VFR conditions mainly Monday and Tuesday afternoon due to possible showers and thunderstorms.
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&& .MARINE...
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Southerly channelling ongoing, with SCA in effect over middle Bay and lower Potomac. This will continue through the night and Friday, ending Friday night as the cold front slides southward into the area. Otherwise, main concern will be for thunderstorms which may produce gusty winds this afternoon and evening, and again Friday afternoon/evening. Strongest storms likely to be tomorrow, but even today any storm may produce gusts to 34 knots. Winds diminish and storms end Saturday morning, but a few isolated storms can`t be ruled out late Saturday as an upper level disturbance moves acoss the region. Mainly dry conditions expected Sunday into Tuesday, with periods of showers and thunderstorms possibly developing Monday and Tuesday afternoon. Wind gusts are expected to stay below the small craft advisory threshold.
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&& .TIDES/COASTAL FLOODING...
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Southerly winds are increasing anomalies and we currently expect Annapolis to hit minor tidal flooding thresholds overnight, thus we`ve issued the advisory for it. Think SW waterfront may also hit early Friday, but will let next shift re-evaluate before adding advisory there. Straits Point will also be close, but think the anomalies will not quite be there for the overnight cycle. With continued south winds, additional advisories may be needed Friday before the front bring a NW flow and allows anomalies to drop Saturday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Coastal Flood Advisory from 11 PM this evening to 8 AM EDT Friday for MDZ014. VA...None. WV...None. MARINE...Small Craft Advisory until 1 AM EDT Saturday for ANZ533-534- 537-541-543.
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&& $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...IMR AVIATION...IMR/RCM MARINE...IMR/RCM TIDES/COASTAL FLOODING...RCM

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