Area Forecast Discussion
Issued by NWS Sterling, VA

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470 FXUS61 KLWX 180128 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 928 PM EDT Thu Aug 17 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Scattered storms exist from Cumberland to Charlottesville, advancing northeast. These are likely tied to a theta-e ridge and developing modest low level jet. Even though hi-res guidance generally shows a weakening trend over the next several hours, would not be surprised if showers and storms persist/continue to develop due to the forcing and around 2000 J/kg of MUCAPE per mesoanalysis and 00Z IAD sounding. Will carry chance POPs through most of the night, as elevated instability will remain in place. The isolated severe threat should be waning though due to increasing surface inhibition. Precipitable water values near 2 inches and deep warm cloud layers suggest heavy rain will be the biggest threat, though so far storms have been exhibiting reasonable motion. Where skies clear, we could get some patchy fog again, especially in places which saw rain today. 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 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. The hot and humid airmass will result in high heat indices. Using the best performing model blends results in a heat index of 100 to locally 105 east of the Blue Ridge and upper 90s west. Best chance of reaching criteria will likely be along I-95 corridor which will have the longest heating before storms arrive. Will allow midnight shift to make the call on an advisory based on the latest cloud/convective trends. 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/... 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. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/...
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Scattered thunderstorms are a concern at all terminals until at least late this evening. Some weakening is seen in radar trends, but will not rule out storms reaching the metros due to modest low level jet forcing on elevated instability. Late tonight concern shifts to low clouds and fog potential. Given variable clouds and uncertainty in storm coverage, it is still difficult to place exact timing and severity of any restrictions. 18Z guidance suggests the best chance for low clouds will be along the I-95 corridor. Have generally held TAFs at MVFR for now, but IFR not out of the question. 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 channeling ongoing with occasional gusts to 20 kt. Expanded the SCA to Pooles Island on the Bay zones due to better mixing potential of a low level jet on the open waters. Have carried the northern two zones until 6 AM, when the jet should weaken. However, SCA conditions will still be possible farther south into Friday, ending Friday night as the cold front slides southward into the area. Further adjustments to the SCA are possible. Otherwise, main concern will be for thunderstorms. Can`t rule out a storm tonight, although the threat for strong storms is lowering. The strongest storms will likely be Friday afternoon into evening. 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 across 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 will be increasing anomalies up to one foot above normal. Have gone ahead and issued advisories for Straits Point and DC SW Waterfront since observations seem to be exceeding forecasts (advisory already in place for Annapolis). Alexandria will be close, but some time to make that decision since high tide is around 5:30 AM. The following high tide is lower astronomically, so am not seeing a threat for that one. With continued south winds, additional advisories may be needed Friday before the front brings a NW flow and allows anomalies to drop Saturday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory from 3 AM to 7 AM EDT Friday for DCZ001. MD...Coastal Flood Advisory until 8 AM EDT Friday for MDZ014. Coastal Flood Advisory until 1 AM EDT Friday for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory until 1 AM EDT Saturday for ANZ533-534- 537-541-543. Small Craft Advisory until 6 AM EDT Friday for ANZ531-532.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...ADS/RCM LONG TERM...IMR AVIATION...ADS/IMR/RCM MARINE...ADS/IMR/RCM TIDES/COASTAL FLOODING...ADS/RCM

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