Area Forecast Discussion
Issued by NWS Sterling, VA

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474 FXUS61 KLWX 281918 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 318 PM 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/...
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Given instability and shear in airmass (2000-3000 j/kg MUCAPE and 40 kt effective bulk shear), thunderstorms were able to develop rapidly this afternoon. These conditions remain favorable through the evening; thus Severe Thunderstorm Watch in effect until midnight. HRRR has been verifying fairly well thus far today. It has been consistent in depicting two waves...the first crossing area now, and the second evident in West Virginia. The latter should be crossing area late evening. Ingredients also in place for heavy rainfall. Hourly rain rates for the stronger cells have been 1-2 inches. Overall threat should morph from severe to flood. Expanded Flash Flood Watch. See Hydrology section below for more info. It is unclear how long the more intense rain will last. HRRR/RAP/WRF-ARW4 all suggesting a diminishing trend around midnight. GFS/ECMWF on the other hand keeping rainfall going longer due to height falls, shortwave passage, and development of a surface low. Will refocus on the evening, with reduced confidence in the overnight hours.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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Once low emerges from coast, which should be sometime Friday morning, there won`t be a lot of forcing for thunderstorms on Friday. However the front will still be nearby with embedded vort energy. Therefore its entirely possible that thunderstorms may return Friday afternoon. Will maintain chance PoPs for the afternoon and evening. Another shortwave will track across the area Saturday afternoon and evening. Anticipate that this afternoon`s scenario will repeat on Saturday, although the airmass likely won`t be quite as unstable. Current SPC outlook is for MRGL severe risk. Can forsee more heavy rain issues.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/...
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Quasi-stationary front remains near the area Sunday and will act as a foci for mainly afternoon/evening showers and thunderstorms in the warm (U80s) and humid atmosphere (dewpoints in L70s). Frontal boundary begins to sink southward as a "cold" front late Sunday into Monday with weak high pressure beginning to build into the area. This will act to keep temperatures near seasonable averages through much of the week. Proximity of frontal boundary just to our south keeps shower/storm chances in the forecast Monday (especially in the southern part of the forecast area). By Tuesday, front is forecast to move far enough south that shower/thunderstorm chances will be limited through midweek.
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&& .AVIATION /19Z THURSDAY THROUGH TUESDAY/...
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No flight restrictions at this time outside of thunderstorms. Storms will be converging upon the hubs late in the afternoon continuing into early evening. There may be a second round late evening. These are the periods that have the best chance at local IFR restrictions. It is unclear how the overnight will pan out. Some guidance souces suggest worst of the storms will be over. Am siding that way. However, may still have low ceilings (MVFR) if it does. Worse case supports a continuation into the morning push Friday. A lull in activity expected Fri afternoon and night before more rain moves into the area Sat-Sat night. Stalled frontal boundary across the area Sunday will lead to showers and thunderstorms. Front sinks just south of the area Monday, though showers/thunderstorms are still possible (mainly for southern terminals). Front far enough south by Tuesday that shower/storm chances should be limited.
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&& .MARINE...
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Winds generally 10 kt or less today except higher in thunderstorms. Some storms developed early in the afternoon. Threat will become more enhanced late this afternoon and evening. 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. Frontal boundary remains across the area Sunday, with thunderstorms possible into Monday. Any storm could produce gusty winds over the waters.
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&& .HYDROLOGY...
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Moisture rich air converging upon a boundary across the northern half of the forecast area this afternoon. Precipitable water levels up to around 2 inches. Storms that have developed have been producing rainfall in that range. Although antecedent conditions have been quite dry, the rainfall rates in warm rain processes appear to be sufficient to overcome this obstacle. This rainfall will be enhanced by a shortwave crossing the area this evening. Therefore, expect periods of moderate to heavy rain through the period. Have expanded Flash Flood Watch for northern 2/3rds of the area, consistent with where best ingredients reside. It is uncertain how long this threat lasts. HRRR has been consistent in depicting the final wave crossing area through midnight. On the other hand, GFS/ECMWF keeping threat through Friday morning. Am erring on the longer side for the watch. Will monitor situation, and cancel early if warranted.
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&& .TIDES/COASTAL FLOODING...
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Tides running about a half foot above astronomical normals. This places Annapolis right at minor threshold. Will be holding under for now. Would not rule out a short-fused Coastal Flood Advisory.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch through Friday morning for DCZ001. MD...Flash Flood Watch through Friday morning for MDZ003>006-011- 013-014-501>508. VA...Flash Flood Watch through Friday morning for VAZ026>031-040- 052>054-501-503>507. WV...Flash Flood Watch through Friday morning for WVZ050>053-055- 501>506. 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...HTS SHORT TERM...HTS LONG TERM...MSE AVIATION...HTS/MSE MARINE...HTS/MSE HYDROLOGY...HTS TIDES/COASTAL FLOODING...HTS/JE

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