Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 281426 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1026 AM EDT Thu Jul 28 2016 .SYNOPSIS... A front will remain stationary along the Mason-Dixon line through Friday morning. An area of low pressure will ride along this front across the area tonight into Friday. The front will move offshore during the weekend, then high pressure will build in by early next week. && .NEAR TERM /THROUGH TONIGHT/... Surface features remain subtle this morning. Based on dewpoints, it appears as though weak stationary boundary stalled out somewhere near the Mason-Dixon line. Initial area of showers, coincident with nose of theta-e ridge, entering mountains west of the Shenandoah valley this morning have been dissipating. This moisture likely going toward moistening a dry layer around 700 mb. However, there has been ample cloud cover associated with this feature. These clouds will be incurring a delay in destabilization. That said, based on 12z LWX RAOB, we aren`t exactly stable right now. Anticipate insolation will further destablize atmosphere during the midday hours. Based on forecast surface conditions (going forecast and LAMP in good agreement, so used this in sounding analysis), there should be almost 2000 j/kg MLCAPE by late day, along with 30-35 kt effective shear. This should be more than enough to support convection, which is already noted in upper Ohio Valley. That area also has height falls and shortwave support to provide more than adequate forcing. In short, storms should become numerous by late day. At first, there may be a few discrete cells. Eventually, the form will evolve into an more consolidate area of showers and thunderstorms. Given foreast parameters, localized wind damage will be a concern. Reference MGRL/SLGT risk areas per SPC. After airmass saturates, the threat will evolve as well. Since moisture rich air will be converging upon a boundary, there is a threat of heavy rain across northern half of forecast area...specifically, eastern West Virginia, much of Maryland, and perhaps the northern Shenandoah Valley. This area best characterized as north of I-66 (VA)/ US-50 (MD). However, antecedent conditions actually quite dry. Going forecast, and current Flash Flood Watch, captures intersection of ingredients and flood guidance well. There are no plans to adjust Watch area at this time. Timing is uncertain still, due to a disparity amongst guidance. Certainly risk will be heightened for the second half of the afternoon through the evening. It is less clear how far into the overnight this will last. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... Trough axis and low pressure will be shifting east of the area by 18Z Friday. Have focused additional rainfall in the morning hours, although a few showers could linger into the afternoon. Airmass should be a little more stable, but ultimately that will depend upon how quickly the front can cross the area, how deep the attendent low will be, and where it will be positioned. If things progress slowly, there could be a few more decent thunderstorms in the middle of the day. Another shortwave trough is forecast to cross the area Saturday and aid in additional shower and t-storm development with pockets of heavy rainfall Sat afternoon and night. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Broad upper level troughing will gradually push across the area during the first part of next week. A building ridge over the Plains will begin to spread eastward on Wednesday. At the surface, the quasi-stationary front which has been a persistent feature of the local weather may get a shove south from low pressure moving across the area on Sunday. Showers and storms will be likely during this time. There is some question how far the south the front gets, but high pressure over New England will be in a favorable position to push drier air down the coast. There`s a better chance of dry weather for Tuesday- Wednesday. Temperatures will remain closer to normal through the first half of next week. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Numerous to widespread showers and thunderstorms expected mid-late afternoon, continuing through at least the evening. These storms may contain strong gusty winds, with the threat slowly evolving into a heavy rain threat. How long these storms last less certain. Best case scenario has an ending time by/near midnight. Worse case supports a continuation into the morning push Friday. There should be MVFR flight restrictions throughout at a minimum due to cigs. IFR likley near any storms. A lull in activity expected Fri afternoon and night before more rain moves into the area Sat-Sat night. Showers and thunderstorms will likely be present across the area on Sunday. A smaller risk of thunderstorms will linger into Monday. && .MARINE... Winds generally 10 kt or less today except higher in thunderstorms, which will be developing late today into tonight. Marine Warnings likely near thunderstorms, especially this evening. Winds strengthen late tonight and especially Friday as deepening low pressure departs the area. A Small Craft Advisory remains in effect for the Chesapeake Bay and lower Potomac for Friday. Low pressure will cross the waters Sunday, helping to push a front south of the area. At this time, gradient winds are forecast to stay less than 15 kt. However, thunderstorms will be likely Sunday, with the chance lingering into Monday, posing a threat of locally higher winds. && .CLIMATE... Record high minimums were set at DCA and IAD yesterday. DCA tied the record of 81F set in 1930. IAD set a new record with 75F breaking the previous record of 73F in 2006. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Flash Flood Watch from 5 PM EDT this afternoon through Friday morning for MDZ006-011-014-506>508. VA...None. WV...None. MARINE...Small Craft Advisory from 7 AM to 6 PM EDT Friday for ANZ530>534-537>543. && $$ SYNOPSIS...LFR NEAR TERM...HTS SHORT TERM...HTS/LFR LONG TERM...ADS AVIATION...HTS/LFR/ADS MARINE...HTS/LFR/ADS CLIMATE...ADS is the U.S. government's official web portal to all federal, state and local government web resources and services.