Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 120125 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 925 PM EDT Tue Aug 11 2020 .SYNOPSIS... High pressure will settle to the south of our area through tonight. A cold front will approach late Wednesday into Wednesday night and remain stalled near our area through the end of the week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... T-storms are still ongoing this evening over portions of the Shenandoah Nat`l Park. Have received reports of trees down in southeast Rockingham County earlier around 815 PM. Storms should dissipate by midnight with another warm and muggy night ahead with lows in the 70s. Areas of dense fog are possible. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/... An active stretch of weather will be in store across the area for Wednesday and Thursday. During the day tomorrow, a cold front will drop southward from Pennsylvania into our area, before eventually stalling out overhead. The day will start out dry, with ample sunshine expected during the morning hours. This sunshine coupled with moisture transport into the area will allow instability to build, with 2000-3000 J/kg of MLCAPE in place to the east of the Blue Ridge by peak heating. Flow will be weak throughout the column, and deep moisture will be in place, so the threat for damaging winds is expected to remain low, but still non-zero, given the highly unstable atmosphere in place. The much greater threat tomorrow will be flash flooding. The combination of a stalled frontal boundary, precipitable water values in excess of two inches, deep warm cloud layers, slow storm motions and antecedent wet conditions suggests that instances of flash flooding are likely across the area tomorrow. As a result, we have issued a Flash Flood Watch for all areas to the east of the Blue Ridge, as well as Rockingham, Augusta and Page counties to the west from noon until 11 PM. Storms may initiate as early as noon, and could linger through the overnight. Thursday will be another very active day, with a similar overall setup to Wednesday. The main difference will be the presence of a shortwave tracking to our northwest through the Ohio Valley. The surface front will remain stalled over our area, but the shortwave may induce a bit more lift/overrunning over the boundary, as flow at 850 hPa turns southerly in response to the shortwave. Deep moisture will remain in place with precipitable water values holding over two inches, but instability values may be lesser compared to Wednesday. Nonetheless, the threat for flash flooding looks to remain elevated on Thursday, especially given that the ground will likely be saturated from rains on Wednesday. && .LONG TERM /FRIDAY THROUGH TUESDAY/... A very active period is expected during the long term period from Friday through Monday. A near stationary boundary will remain stalled near to over top our region through Saturday. Multiple lows will form and move along this boundary bringing continued showers and thunderstorms capable of producing heavy rainfall. A flow out of the south to southwest will lead to warm and humid air being advected into our region. The presences of a stalled boundary coupled with high PW`s will lead to a continued risk for flash flooding Friday through early Sunday. During this same period, warm and humid conditions will lead to modest CAPE with decent shear which could lead to a strong to severe thunderstorm threat, but that all depends on the positioning of the frontal boundary. There remains some uncertainty on whether the extended rain threat will continue through Sunday. The rain and flood threat for Sunday will depend on the upper level trough that approaches from the west and kicks the stalled boundary out of our area. If the trough shifts further south, the trough would kick off a new low on Sunday that brings further rain to our potentially water logged region, but if it stays further north rain threat shifts further south with the boundary. Another front will drop down into our area on Monday bringing another threat for rain and thunderstorms. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... The main aviation concern over the next couple days will be thunderstorm coverage. Overall coverage of storms today should remain isolated to widely scattered in nature. Coverage of storms is expected to increase moving into Wednesday and Thursday. For tomorrow, have introduced VCTS at all terminals due to high expected coverage of storms. At the three major hubs, tried to target a four hour window with the highest coverage, but storms could be possible starting as early as noon and could potentially linger through the night and right into Thursday. An extended period of rain and a south to southeasterly flow will likely lead to subVFR conditions Friday through Sunday with low ceilings being the main threat with periods of reduced visibility. Strong gusts will be possible with thunderstorms that move over terminals. && .MARINE... No significant synoptic wind issues expected through the next few days, but southerly channeling could cause winds to approach SCA levels this afternoon into tonight. Current thinking is that gusts will remain below SCA levels, but will continue to monitor winds through the remainder of the day. Otherwise, the main concern through Thursday will be thunderstorms. Coverage should remain isolated to widely scattered today, but will increase significantly Wednesday and Thursday, with gusty winds a threat from any storm. Best timing is afternoon and evening, though the risk may linger through the night Wednesday and Thursday nights. Special Marine Warnings may be needed for any stronger storms. The overall flow will lead to SubSCA conditions Friday through this weekend but thunderstorms could bring periods of gusty winds over our marine areas. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flash Flood Watch from Wednesday afternoon through Wednesday evening for DCZ001. MD...Flash Flood Watch from Wednesday afternoon through Wednesday evening for MDZ004>006-011-013-014-016>018-503>508. VA...Flash Flood Watch from Wednesday afternoon through Wednesday evening for VAZ025-026-029-036>040-050>057-501-502- 505>508. WV...None. MARINE...None. && $$ SYNOPSIS...LFR NEAR TERM...LFR SHORT TERM...KJP LONG TERM...JMG AVIATION...KJP/JMG MARINE...KJP/JMG

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