Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 280919 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 519 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 Thursday night into Friday. The front will move offshore during the weekend, then high pressure will build in by early next week. && .NEAR TERM /THROUGH TONIGHT/...
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A mid-level trough over the mid-south and TN valley will move ewd next 36 hrs crossing the area around midday Fri. This will induce an area of low pressure to form over srn VA that tracks northeast toward Long Island by Fri afternoon. There will be ample moisture over the local area with PWATs in excess of 2.0 inches, forcing and instability to fuel widespread showers and thunderstorms later today into Fri morning. There will be a severe wx threat with t-storms this afternoon into early evening and a heavy rainfall threat with isolated flash flooding tonight into Fri morning. 0-6km bulk shear of 35-40 kt and decent helicities will support a risk of isolated discrete supercells with perhaps a tornado before activity consolidates more during the evening. In terms of rainfall potential, the 00Z GFS shifted the axis of heavy rainfall further north into PA and NJ. The 00Z ECMWF shows two areas of significant rainfall one over PA near stalled out front and one over the CHO area. Lack of rainfall during the past 7 days especially in ern WV, nrn VA and MD and northward shift of QPF axis from models make me lean toward not issuing a flood watch at this time. Will wait a few more hours for additional ensemble guidance and WPC excessive rainfall outlook before issuing any flood watches. Based on RFC flash flood guidance, the area of greatest concern is from Winchester south to Augusta and Nelson counties and, of course the urban areas of DC and Baltimore.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... Trough axis and low pressure will be shifting east of the area by 18Z Fri with rain largely ending, although a few showers could linger into the afternoon. Another s/w trough is fcst to cross the area Sat and aid in additional shower and t-storm development with pockets of heavy rainfall Sat afternoon and night. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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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.
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&& .AVIATION /09Z THURSDAY THROUGH MONDAY/...Numerous to widespread showers and thunderstorms expected this afternoon into Fri morning with risk of strong gusty winds and heavy rainfall. MVFR/IFR conditions possible in heavy rain. 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...
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Winds generally 10 kt or less except higher in t-storms. Winds strengthen late tonight and especially Fri as low pressure departs the area. Small craft advisory has been issued for the Ches Bay and lower Potomac for Fri. 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.
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&& .HYDROLOGY...
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Based on additional ensemble guidance and new WPC excessive rainfall outlook decided to go with a flash flood watch for the urban corridor of Baltimore. Stationary front remains draped along the Mason-Dixon line and will likely to enhance rainfall across that area. While places to the south could see some brief heavy rainfall with any t-storms, the area around Baltimore is expected to see more persistent and organized convection. Given the flashy nature of a large urban environment a flash flood watch has been issued. Widespread 1+ inches of rain are expected in that area with isolated amounts of 3+ inches which could fall in a short period of time.
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&& .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...
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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.
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&& $$ SYNOPSIS...LFR NEAR TERM...LFR SHORT TERM...LFR LONG TERM...ADS AVIATION...ADS/LFR MARINE...ADS/LFR HYDROLOGY...LFR CLIMATE...LFR

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