Area Forecast Discussion
Issued by NWS Sterling, VA

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025 FXUS61 KLWX 231906 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 306 PM EDT Tue May 23 2017 .SYNOPSIS...
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Low pressure will move northeast off the DelMarVa tonight. 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 TONIGHT/...
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Latest surface analysis indicates that high pressure remains over the northern Mid-Atlantic this afternoon. A stationary front is draped south of the area. Dry air associated with the high pressure is limiting northward progress and intensity of rain with the first wave along the front. Expect areas of light rain for the remainder of the afternoon. A second trough is located near the Smokey Mountains, which will help induce cyclogenesis along the coast this evening. Some uncertainty remains with the evolution of rainfall tonight, as well as a couple changes from the previous forecast. The main difference amongst models is how much the dry air wins out...with a wide range of QPF in the DC/Baltimore corridor. Even if rain does eventually overspread the entire area as suggested by several models, this corridor will also be farthest removed from forcing from both the coastal low and a shortwave trough over the Appalachians. This trough has resulted in a substantial bump up in POPs over the mountains tonight...and the overall exit of the two systems is a little slower than previously suggested. While some areas may see a period of moderate rainfall, guidance does not suggest there will be enough to have flooding concerns. With saturated low levels, fog development is still possible. Lows will be in the 50s.
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&& .SHORT TERM /WEDNESDAY 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 develop 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 /19Z TUESDAY THROUGH SUNDAY/...
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MVFR cigs have been spreading back to the northwest as wave of low pressure moves along the coast. Rainfall through this afternoon will be light in nature. Considerable uncertainty still exists about this evening into tonight. While MOS guidance still suggest IFR cigs develop, actual model RH fields show dry air advecting in from the northeast, causing cigs to lift. Intensity of rainfall also remains in question, especially across Baltimore/Washington. Have trended TAFs more optimistic, but did not go VFR until after dawn Wednesday. 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. Am maintaining Small Craft Advisories for the mid Bay and lower Potomac through 6 AM. Southeast winds increase Wednesday night in response to low pressure in the Ohio Valley. Have hoisted 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 ANZ531>534-537-539>541-543. Small Craft Advisory until 6 AM EDT Wednesday for ANZ533-534- 537-543.
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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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