Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 270143 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 943 PM EDT Sat May 26 2018 .SYNOPSIS... A front will stall over the area early in the week then dissipate. Another front will cross the area during the middle of the week. The remnants of Alberto may pass to the west of the region late in the next week. && .NEAR TERM /THROUGH SUNDAY/... Most of the shower activity has dissipated this evening except for a small cluster over wrn MD and ern WV. These will probably persist for another hour or two. Main issue for tonight is development for low clouds as strongly suggested by most of the guidance. Tomorrow appears will be very active day with significant flash flooding potential. PWs will be at 2.0 inches which is at the 99.5 percentile and there will be training as mid level winds become parallel to sfc front. Large CAPE values will also help fuel storms. Based on collaboration with PHI, will hold off on a flash flood watch until the next shift. && .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... Another round of showers and heavy thunderstorms is likely on Sunday as a cold front drops southward out of Pennsylvania, running into the humid airmass over the Mid-Atlantic. The presence of the front and some increase in shear may lend to a little more storm organization, but the details will rely heavily on prior day`s convection. Training storms seem more likely on Sunday on a more widespread scale, though, given storm motions largely parallel to the low-level boundary. Convection may linger well into Sunday evening as the front will likely be slow to clear the area. The high pressure to the north forcing it through is forecast to only be around 1020 mb, which usually means it gets hung up over southern parts of the area, which could lead to another round of showers and thunderstorms Monday afternoon and evening across central VA to southern MD. && .LONG TERM /TUESDAY THROUGH SATURDAY/... To start out the long term period on Tuesday, a front will be located off to our south. A stray shower or two can`t be ruled out (especially over southern portions of the forecast area), but it appears as though most locations should stay dry. The main feature of interest throughout the long term period will be the remnants of Tropical Storm Alberto. The official NHC forecast has Alberto making landfall late Monday Night along the Gulf Coast. After landfall, Alberto will weaken, but it`s remnants will still have a large influence on our weather through the week. Also of interest are a trough currently located over the western CONUS and a stronger trough currently located in the Gulf of Alaska. Over the next couple days, the western CONUS trough will become largely cutoff from the polar jet and progress slowly eastward. Meanwhile, the stronger trough over the Gulf of Alaska will dive into the Pacific Northwest by Tuesday. Alberto is expected to track north on Tuesday into the lower Mississippi/Tennessee Valleys beneath an anomalously strong upper ridge. As Alberto progresses further north, the aforementioned western CONUS trough will be kicked out onto the Upper Plains by the stronger trough moving into the Pacific Northwest. That will place Alberto within the steering influence of the ejecting trough, causing it to initially continue north, before turning more to the northeast as the weakening trough tracks to it`s north. These complex interactions between the two troughs and remnants of Alberto suggest that there is a fair amount of uncertainty as to where the remnants will track. However, most model guidance (deterministic and ensemble) moves the system from the Tennessee Valley into the Ohio Valley by Thursday. Regardless of the exact details of the track, it appears as though we will remain to the east of the track. Southerly flow to the east of the system will transport a very tropical airmass into the region. The deterministic GFS and Euro, as well as their respective ensembles suggest that precipitable water values will approach or exceed two inches by late Wednesday into Thursday. Recent model runs have the best forcing for ascent associated with Alberto`s remnant circulation located off to the north of our area. However, this far out there is a fair amount of track uncertainty. At the very least, showers and thunderstorms will be possible on both Wednesday and Thursday. Given the near record precipitable water values, any storms that do form could produce heavy rainfall. In the wake of Alberto`s remnants, model guidance pushes a cold front through the region. There are considerable differences between the models with regard to the timing and sensible impacts of this frontal passage. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/... MVFR/IFR cigs likely to develop overnight as suggested by all guidance. Fog appears unlikely due to plenty of clouds. Similar conditions expected Sunday as a cold front drops south out of PA. Front likely stalls near CHO to southern MD Monday with more convection possible in this corridor. North of the boundary, winds will become northerly generally AOB 10 kts. The weak front will slip south through the area on Tuesday and remain nearby on Wednesday. While shower and thunderstorm coverage may be limited, they will be possible each day. && .MARINE... Winds 10-15 kt over the next several days. Winds and waves higher near thunderstorms. Light winds are expected Tuesday as the weak front sags southward through the area, although there could be a few showers and thunderstorms. Southeast winds may near SCA thresholds on Wednesday as the front lifts back to the north and high pressure moves off the New England coast. Additional thunderstorms will be possible. && .HYDROLOGY... More widespread training thunderstorms with heavy rain are possible Sunday as a cold front drops south out of Pennsylvania. A flash flood watch will likely be required Sunday afternoon into into Sunday evening. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...LFR NEAR TERM...LFR SHORT TERM...KLW LONG TERM...KJP AVIATION...ADS/LFR MARINE...ADS/LFR HYDROLOGY...LFR is the U.S. government's official web portal to all federal, state and local government web resources and services.