Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 270917 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 517 AM EDT Thu Apr 27 2017 .SYNOPSIS...
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High pressure will build across the southeastern United States resulting in a considerable boost to the temperature with above seasonal warmth expected by the weekend. An increase in southwesterly wind flow will bring enough moisture back into the area beginning today to promote widely scattered afternoon and evening showers and thundershowers over the mountains.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 500 AM EDT Thursday... High pressure is beginning to build over the southeast CONUS. This will stave off most of the energy from a vigorous short wave trough that is lifting northeast through Illinois and into the western Great Lakes. A surface front trails south from the upper trough and will be moving east today, the southern portion of the front weakening with time in response to the building ridge of high pressure over the Southeast. In spite of its weakening trend, it will create enough lift for showers and thunderstorms this afternoon and evening, particularly across our western CWA where SBCAPE is forecast between 1000-1500 j/kg. Some of this activity may make it east of the mountains and into the Piedmont late in the day. The Storm Prediction Center has the entire CWA outlooked for a marginal risk of severe storms today. The main threat would be from isolated damaging wind gusts. Model CAPE is forecast into the 1000-1500 j/kg range this afternoon, but diurnal gains in buoyancy may be muted by in increasing cloud coverage ahead of the front. If sufficient solar insulation does occur, then low/midlevel flow will encourage momentum transport capable of supporting locally damaging wind gusts. Temperatures today will be very similar to yesterday with highs in the 70s to lower 80s. Loss of daytime heating will result in a demise of the shower/storm activity before midnight. The actual surface front is then forecast to dissipated over our area as the building upper ridge of high pressure acts as a road block keeping the front from introducing any sort of change in airmass to the forecast area.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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As of 400 AM EDT Thursday... A broad and strong h5 ridge will be centered over the southeastern U.S. in this period with 588 dm heights over Florida building to 591+ dm just off the Carolina coast by Saturday. This will provide a very warm and fairly humid air mass across the southern Appalachian region, especially for late April. High temperatures will run 10 to 15F above normal both Friday and Saturday with highs in the upper 70s to low/mid 80s Friday to mid/high 80s Saturday. Not really threatening records but still unseasonably warm. Precipitation-wise do not expect to see much, if any. A strong upper trough will be digging southward into the southern high plains Friday/Saturday setting the stage for a heavy rainfall event well to our west, across parts of the southern and central Mississippi Valley. Forcing for precipitation in our area will be weak with only a chance for diurnally driven convection which will be widely scattered and not hydrologically significant. This will allow the last of our flooded rivers to continue receding. Models indicate decent ML CAPE of 1-2K Saturday afternoon but only modest shear but could allow a few strong to near-severe pulse thunderstorms in the afternoon.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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As of 400 AM EDT Thursday... Very warm and humid conditions continue Sunday with the chance for showers and thunderstorms, especially along the Blue Ridge. By Monday, the vertically stacked upper closed low over the Midwest will move into the Great Lakes region. This will push a vigorous cold front into the area from the west with a good chance of showers and thunderstorms through Monday afternoon into Monday night. High pressure will then build on for Tuesday with cooler weather and windy conditions. Moisture increases on Wednesday, with better chances of rain Wednesday night into Thursday.
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&& .AVIATION /09Z THURSDAY THROUGH MONDAY/... As of 210 AM EDT Thursday... Clear skies will promote good radiational cooling early this morning with ambient ground moisture from recent rainfall leading to some patchy valley (MVFR) fog, otherwise expecting widespread VFR conditions all sites through 18z/2PM Thursday before frontal showers and a few storms arrive along the western slopes of the Appalachians. Increasing clouds/showers are the result of an approaching cold front. Ongoing line of deep convection over the TN valley is forecast to weaken this morning before redeveloping along the outflow boundary as it moves into the western Appalachians this afternoon. Storm coverage the afternoon is expected to be scattered in nature...thunderstorm clusters as opposed to a solid line. Southerly winds will become a bit gusty from mid morning on per 35 to 40 knot LLJ ahead of the approaching front. Showers/Storms are forecast to linger until loss of daytime heating. The actual surface front is expected to wash out and dissipate as it crosses the mountains, so in spite of a wind shift as it crosses the mountains this evening, very little if any wind shift is expected over the piedmont east of the mountains where the winds will remain predominantly out of the south or southwest. Extended Aviation Discussion... Mainly VFR flying weather is expected for Friday and into the weekend as high pressure builds over the southeast states promoting temperatures which will be much above normal. Warming surface temperatures also means increasing atmospheric instability. As such, potential will exist for cloud build-ups and the risk for isolated to scattered thunderstorms along the spine of the Appalachians during the peak heating hours each afternoon. && .HYDROLOGY...
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As of 430 AM EDT Thursday... River Flood Warnings remain in effect for the Dan River and lower portions of the Roanoke River. The Dan continues to fall rapidly at Danville, thus the warning has been cancelled for Danville. Downstream at Paces, the river crested yesterday afternoon and is now falling slowly. At South Boston the river is just past the crest, which was at a stage of 30.25 feet at 12:30 AM this morning. Preliminary records indicate that this is the 9th highest flood on record at South Boston and the highest since March 23, 2003 when the river hit 30.34 feet. The Roanoke River at Randolph has crested and will begin falling this evening.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PM SHORT TERM...PC LONG TERM...KK AVIATION...PM HYDROLOGY...PC

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