Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 120605 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 205 AM EDT Sat Aug 12 2017 .SYNOPSIS... A cold front will make slow eastward progress from the Ohio Valley tonight. Full clearing of the cold front is not expected until Saturday. Periods of showers and thunderstorms are expected tonight through Saturday, a few of which may be heavy at times. The weather pattern remains unsettled on Sunday, before the next more widespread shower threat for Monday and Tuesday. High temperatures will be near to below normal with near to above normal low temperatures through early next week. && .NEAR TERM /THROUGH TODAY/... As of 321 PM EDT Friday... Regional composite radar mosaic and METARs depict scattered to numerous showers with embedded thunderstorms from eastern KY south- southeastward across the Tri-Cities TN area and into the Asheville/Charlotte NC vicinity. These showers and storms were moving northeastward in association with southerly convergence axis along the Blue Ridge. Another cluster of stronger thunderstorms was becoming apparent in association with a pre-frontal/lee trough in the eastern WV Panhandle. Primary surface cold front still is slow to progress eastward and is located from just east of Chicago southward into the Memphis TN/eastern AR area. Aloft, seasonably strong belt of broadly cyclonic mid-level flow in place across the Appalachians with a 500 mb jetstreak inferred in WV imagery in northern IL. For the rest of the aftn/tonight: Only a limited opportunity for further destabilization exists with sunset only a few hours away. Air mass presently is not overly unstable due to extensive cloudiness that has only recently begun to thin, with MUCAPE values between 500- 1000 J/kg and LI`s around -2 to -3. However, effective bulk shear values of 35 to 40 kts are in place due to increased mid-/upper- level winds. Expect ongoing cluster of showers and embedded thunder along the southern Blue Ridge to shift eastward into the VA/NC Piedmont through tonight. Another potential focus for thunderstorms may be on the trailing edge of the lee trough in the eastern WV Panhandle, into Greenbrier County and the southern Shenandoah Valley in VA. In either area, there may be a couple of stronger thunderstorms capable of wet downburst winds as these move eastward, given high PWATs and precipitation loading effects, along with heavy downpours. However, strong/localized severe storm coverage would be isolated. Feel this threat should exist until about 10 pm with a decrease in convective coverage thereafter. Recent high- resolution guidance after midnight shows something of a break in our northern and western counties, with scattered to numerous leftover showers mainly in the eastern/central VA Piedmont and into NC as pre-frontal trough shifts eastward. Still, there should be a generous degree of cloudiness for the overnight hours. This may limit any overnight fog as compared to prior nights, and the expected cloudiness keeps low temperatures tonight in the 60s to low 70s. For Saturday: The significant source of uncertainty for Saturday is the coverage and extent of early-day clouds/showers and afternoon clearing. Guidance today has slowed the cold front down toward a frontal passage during the late afternoon hours. While the coarser- resolution GFS shows a broad-brushed QPF field for the afternoon, a number of the finer-resolution solutions (ref: the 12z 3-km NAM and WRF NMM core simulated reflectivity) in the Saturday afternoon period support cells to broken lines of convection along the 850 mb frontal axis/northwesterly wind shift after 18z/2pm. Effective-layer shear magnitudes of 35-40 kts support potential organized cells, and both the GFS and NAM-based BUFKIT profiles in the lee of the Blue Ridge into the VA/NC Piedmont support a potential moderately unstable environment (MUCAPEs as much as 2000 J/kg). I say "potential" because the degree of cloud clearing will play a heavy role in affecting how much instability, if at all, can be realized. Feel above described parameter space would be a worst-case convective scenario, and given the extent of forecast cloudiness expected a more modest level of instability is current forecast thinking. That said, SPC did opt to expand the 5% severe/Marginal Risk for Saturday southward across much of the forecast area to the VA/NC border. Gusty to damaging winds could be possible, along with heavy downpours, should stronger thunderstorms develop tomorrow afternoon. Those with plans outdoors tomorrow should pay close watch to forecasts Saturday. Highs range from the mid/upper 70s to mid 80s. && .SHORT TERM /TONIGHT THROUGH MONDAY/... As of 320 PM EDT Friday... Models have slowed timing of deep moisture with frontal passage moving out Saturday night into Sunday, with a couple waves of low pressure expected to move from upstate SC Sunday morning to the VA tidewater. This should keep main threat of showers/few thunderstorms across our southern CWA. and eastern CWA by Monday morning. Front starts to shift back north as another low moves across the TN Valley Monday. Monday should be the wettest day of this period. Temperatures remain at or just below normal thanks to high pressure to the north and clouds/moisture hanging around most of the time. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 320 PM EDT Friday... Pattern remains somewhat active this period with similar setup as current pattern. A front slides southeast of the by midweek, before stalling over the Gulf Coast States. Upper flow amplifies somewhat with another surface low pushing into the central Plains by Friday. This will lead to the front to lift northward again as a warm front. For us there will be at least 2 periods where showers will be around the most, while temperatures stay near or below normal. Based on the models and pattern should be wetter Monday into Tuesday, drier Tuesday night-Thursday with high pressure in place, then return flow and isentropic lift over the warm front to bring next threat Friday. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
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As of 200 AM EDT Saturday... Still expect widespread MVFR ceilings, locally IFR mainly in the west, to become widespread across southwest Virginia, southeast West Virginia, and northwest North Carolina through the morning hours. Showers from Friday evening have drifted to the south in assocation with a weak frontal boundary. However, a wave of low pressure will ride along the front Saturday and as it approaches late tonight/early morning, expect light showers to redevelop across western areas. Thus, have included -RA and VCSH in western areas during the late night/early morning hours. This activity will shift east of the Blue Ridge in the 15Z-18Z time frame. All activity should be east of the TAF sites after 18Z. Cloud ceilings should improve to high end MVFR west of the Blue Ridge and low end VFR east of the Blue Ridge, but will likely remain BKN-OVC until early evening. Given the extensive cloud shield tonight, fog development is limited. However, most locations saw rainfall Friday leaving wet ground in place and indeed this is causing some fog to develop in locations such as BCB. Will need to watch, but possibility for localized IFR-LIFR cigs/vsbys in BR/FG exists through about 13Z. Will address clouds and fog Sunday morning better in the 12Z TAF set. Winds will be mostly calm through mid-morning, then gradually becoming W-WNW 4-6kts into the afternoon. Low to medium confidence in ceilings through 16Z, the medium confidence in ceilings through the remainder of the TAF valid period. Medium confidence in visibilities through the TAF valid period. Medium to high confidence in wind speed and direction through the TAF valid period. Low confidence in any thunderstorm potential today. Extended Aviation Discussion... Cold front will linger south of the area through Sunday, then return north on Monday. Mostly VFR conditions expected Sunday outside of late night/early morning fog/low clouds in the valleys and rural areas. Unsettled conditions Monday-Tuesday as the front returns north and a new front approaches from the west will keep periods of sub-VFR conditions in the forecast.
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&& .EQUIPMENT...
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As of 200 AM EDT Saturday Aug 12th... KFCX doppler radar will continue to be down for the rest of the month due to a failing bull gear. It will only be operational during this time frame for brief periods if an imminent significant widespread severe weather and/or hydrologic event occurs. Operating the system for any amount of time until the bull gear is replaced risks a catastrophic failure that would further extend system downtime. From the 15th through the 25th the radar will most likely be down completely as the repairs are expected to be made during this period.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...WP LONG TERM...WP AVIATION...AMS/RAB EQUIPMENT...RAB/WERT

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