Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 240139 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 939 PM EDT Tue May 23 2017 .SYNOPSIS...
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Low pressure will move northeast off the DelMarVa overnight. Upper-level low pressure will move toward the area Wednesday through Thursday, and then depart on Friday. High pressure will move across the area Friday night into Saturday, with a warm front approaching on Sunday.
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&& .NEAR TERM /THROUGH WEDNESDAY/...
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Latest surface analysis indicates that high pressure remains over the northern Mid-Atlantic this evening. A stationary front is draped southeast of the area. Dry air associated with the high pressure is continues slowing northward progress and intensity of rain, but not as much as many models expected it to. With the second trough moving northeast into the area and inducing cyclogenesis along the coast, overall chance of rain appears higher than guidance has suggested. While heavy rain resulting in any flooding concerns is certainly not expected, widespread rain looks more and more likely overnight, so will likely up POPS through at least 9Z to likely with the next scheduled grid update in about an hour. With saturated low levels, fog development is still possible. Lows will be in the 50s.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
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Most of the daytime hours of Wednesday will be dry as the coastal low moves away and the wedged high pressure continues to exert influence. A few showers or drizzle may linger through the day in the Allegheny Highlands. It will be cloudy across the entire area with temperatures in the upper 60s to lower 70s. Meanwhile, dynamic low pressure system will be winding up over the Ohio Valley. Strong low level jet/moisture transport will overspread the area Wednesday night, resulting in a widespread moderate to perhaps locally heavy rainfall. With strong theta-e advection aloft, there may be a few embedded rumbles of thunder as well. If any areas in the mountains overperform on rainfall tonight, it could set the stage for minor flooding concerns, but will have to let the preceding events play out first. Otherwise the system is progressive as the dry slot works northward Thursday morning, with amounts from this wave ranging from 0.5 to 1.5 inches. Jet energy will be moving off to the east on Thursday as upper level low moves overhead and takes on a negative tilt. Surface low looks to develop over northern parts of the area in response to the forcing. The amount of clearing and destabilization is in question, and if so, how much would overlap with the departing shear. So while showers will be probable by afternoon, the amount and intensity of thunderstorms is uncertain. Rain chances will diminish Thursday night as low/occluded front move to the east.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/...
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Closed upper level low will continue to move NE away from New England on Friday. Energy behind the upper trough may enhance some showers over our area. Conditions dry out into Friday night and Saturday as high pressure briefly builds over. Shortwave trough and an approaching warm front will bring rounds of showers Saturday evening and into Monday, as the front stalls near or over our CWA. Cold front approaches from the west Monday into Tuesday keeping the unsettled weather conditions over our area. High temperatures will be near normal mainly in the 70s and low 80s... some 60s at higher elevations.
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&& .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
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MVFR cigs have returned as wave of low pressure moves along the coast. Rainfall tonight will be generally light in nature. Considerable uncertainty still exists about tonight regarding flight cats, thought it is diminishing. While MOS guidance suggests IFR cigs develop, actual model RH fields show dry air advecting in from the northeast, causing cigs to lift. However, overall guidance appears to be overdoing the dry air somewhat. Have trended TAFs more pessimistic. Break in low clouds and precip during the day Wednesday, although remaining OVC. Next wave of rain will arrive Wednesday night and be moderate to possibly heavy at times. IFR or lower conditions seem likely. LLWS may be an issue with southeasterly winds at surface and southerly aloft. A break in rain is possible Thursday morning in addition to rising cigs, but additional showers and perhaps thunderstorms will occur Thursday afternoon. Precipitation exits Thursday evening but cigs may be slow to lift. Periods of sub-VFR conditions possible on Friday, becoming VFR Friday night into Saturday. Then sub-VFR conditions return Saturday evening and into Sunday with rounds of showers moving through our region. Gusts as high as 22 kt possible on Friday.
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&& .MARINE...
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Low clouds will result in poor momentum transfer though the midweek. Coastal low pressure will move past the Delmarva tonight. Extended Small Craft Advisories for the mid Bay and lower Potomac through 9 AM with expected slower progression of low pressure wave. Southeast winds increase Wednesday night in response to low pressure in the Ohio Valley. Mainted a Small Craft Advisory for larger waters of the Bay and lower Potomac where mixing should be better, although moderate rain may eventually limit this potential. There may be a lull on Thursday morning, but winds may increase again later in the day as low pressure passes by to the north. Scattered showers expected on Friday with breezy conditions, gusting as high as 22 kt, which would require a small craft advisory. Conditions dry out Friday night into Saturday before other rounds of rain move through into Sunday. Winds will be below the SCA threshold Saturday and Sunday.
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&& .TIDES/COASTAL FLOODING...
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Water levels are less than a half foot above astronomical normals this afternoon. However...coastal low pressure will strengthen the onshore flow a little this evening...so anomalies will increase a bit during this time. The flow should be light enough for water levels to remain below minor flooding thresholds for most areas...but it will be close for sensitive areas such as Straits Point in St Marys County and Annapolis in Anne Arundel County. Anomalies will have to be monitored through this evening. The low will move away from the area late tonight into Wednesday...but deep low pressure will cross the area Thursday, with southeast winds picking up Wednesday night. The early Thursday morning tide cycle will need to be monitored, as the preponderance of evidence suggests that minor flooding is likely. Minor flooding is possible during the high tide cycles Thursday afternoon through Thursday night...but confidence is lower since the wind is expected to turn west to southwest.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM Wednesday to 6 AM EDT Thursday for ANZ530>534-537>543. Small Craft Advisory until 9 AM EDT Wednesday for ANZ533-534- 537-541-543.
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&& $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...IMR AVIATION...ADS/IMR MARINE...ADS/IMR TIDES/COASTAL FLOODING...BJL

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