Area Forecast Discussion
Issued by NWS Blacksburg, VA

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316 FXUS61 KRNK 171759 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 159 PM EDT Thu Aug 17 2017 .SYNOPSIS... Highly moist airmass in place and residual surface trough will be the trigger for showers and storms today into tonight. A cold front moving into the region on Friday sparks another round of showers and storms followed by clearing along with drier conditions for the weekend. && .NEAR TERM /THROUGH TONIGHT/...
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As of 158 PM EDT Thursday...Increasing shower and embedded thunderstorm coverage noted from the NC High Country north and west into southeastern West Virginia, ahead of an approaching pre-frontal trough and humid, moderately unstable air. Expect continued scattered to numerous convective coverage from the Alleghany Highlands southward to the NC foothills, a few of which could produce localized heavy downpours and localized gusty winds in stronger cells. Further south and east into the Piedmont/Southside/NC Piedmont, with relative lack of trigger/lift, it is less certain that storms may develop here. If so - either through local outflows, air mass there is even more unstable there per SPC analysis (~3000 J/kg with pocket of -5 LI), but may be isolated coverage. It`s become quite humid, as expected. Combined with temps running in the 80s to near 90 in spots, heat indices in the mid 90s have been common, with the highest in Danville into the low 100`s. Appears that heat index values should remain below advisory criteria, but taking frequent breaks from heat/humidity and keeping adequately hydrated is certainly encouraged. No sig changes otherwise. Previous near-term discussion issued at 301 AM follows... Showers continue to linger and drift slowly east from the Mountain Empire into the NC piedmont, but activity is isolated, and radar trends showing weakening. This is associated with higher theta-e values and moisture convergence. High-res models showing showers arriving/developing through mid morning along a LYH/DAN corridor, while should stay dry elsewhere, though a few shower could start popping over the mountains by noon. Fog will be patchier than yesterday as more clouds around, some dense fog possible though given higher moisture content in the air. Today, 5h ridge axis stays overhead into early afternoon, then shifts east toward evening. Underneath, the flow will stay out of the southwest, with PWATS around 1.5 to 2.4 inches, from northwest to southeast. An unstable airmass and some low level theta-e ridging and convergence should act to fire up showers and storms this afternoon, though upper level support is limited but some difluence by late afternoon may increase lift and coverage. Severe threat is limited but cannot rule out an isolated strong storm, as SBCAPES approach 2000 J/KG, though GFS showing computed CAPES of 3-5k, seems overdone. Models are not agreeing on which area has the best shot of storms, varying from GFS in the mountains, NAM piedmont, and the ECMWF area wide. High-res models this afternoon favor development in the west, then more coverage along/east of the Blue Ridge by 21z. At the moment based on upper support and front to the north, a higher chance exists from the NC and far SW Va mountains, northeast toward Lynchburg/Buckingham, with smaller more widely scattered threat over the NC piedmont. Should be less sunshine than Wednesday but still sunshine will be out at times to heat the airmass into the 80s. A few locations east and south of Danville and Appomattox could reach around 90F. With the dewpoints in the lower 70s, anticipate heat indices upper 90s. Tonight, upper trough axis remains to our west, and expect lower threat of showers/storms later in the evening but upstream vorts in southwest flow will bring another shot of showers/storms toward WV/far SW VA by dawn Friday. Will not stay completely dry overnight as weak sfc trough/moisture convergence also lingers in the piedmont, but coverage should be widely scattered. Temperatures staying muggy with lows in the mid to upper 60s mountains, to lower 70s east.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 300 AM EDT Thursday... Upper low over Great Lakes Friday morning will rotate northeast Friday afternoon through Saturday night. Weak shortwave trough travel to our north Friday allowing a surface cold front to slowly travel east toward the region. Residual showers preceding the boundary across the west early Friday should jump into the eastern lee trough during the afternoon where deeper convection appears possible. Progged instability are still quite strong east of the mountains where models suggest a narrow band of shortwave energy crossing into the piedmont late Friday. Late day timing could allow for better convergence, heating and instability from the Blue Ridge east especially if the low level flow backs southwest, and progged very high 850 mb theta ridging materializes. SPC in the day two convective outlook has placed our area in general thunderstorms, while a marginal risk is to our northeast where surface-3km lapse rates could exceed 7 C/km. We can not completely rule out a strong to severe thunderstorm with healthy cape values. High temperatures Friday will range from the upper 70s in the mountains to the lower 90s in the Piedmont. Temperatures will be highly dependent of downslope flow, clouds cover and convection. Surface cold front will push east into the Atlantic ocean Friday night as the upper trough digs into the Ohio Valley. Low temperatures Friday night will vary from around 60 degrees in the mountains to the lower 70s in the piedmont. Shortwave trough in the Ohio Valley will move east Saturday and will pass just north Saturday night. How far east the push of drier air goes Saturday remains in question. There may be enough low level moisture with heating to trigger an isolated shower or thunderstorm in southeast portion of the forecast area. High temperatures Saturday will generally range from the mid 70s in the mountains to around 90 degrees in the piedmont. High pressure will build south across our area Saturday night. Low temperatures Saturday night will generally be in the 60s. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 130 PM EDT Wednesday... Exiting upper trough by late in the weekend will evolve into strong ridging across the region for early next week as Bermuda high pressure links with building heights over the southeast states. Appears this in combination with weak surface high pressure ridging in from the north and lack of upper support, will act to keep a lid on most organized convection for Sunday into Monday. However given close proximity of at least low level moisture near the residual front over far southern sections, will need to leave in an isolated diurnal convective mention espcly southern Blue Ridge for now. Otherwise will be leaving things mostly dry with more clouds far south and less northern half into Monday. Ridging will begin to flatten Tuesday in advance of the next digging 500 mb trough that looks to push another cold front toward the region by midweek. Return moisture ahead of the boundary and less cap should support a scattering of showers/storms Tuesday afternoon especially mountains. Expect a bit more organized convection ahead of the front and under cooling aloft Wednesday when will have medium to high chance pops. Better ridging and warming 850 mb temps to around +22C suggests a rather warm/hot period with highs mostly 80s, except around 90 or warmer piedmont, before cooling slightly on Day7 per more showers around. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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As of 158 PM EDT Thursday... Generally VFR conditions, though noting increase in shower and thunderstorm coverage along the southern Blue Ridge northward to just east of Bluefield. This is in association with a pre- frontal through. Given humid airmass, brief and localized IFR to LIFR visibility in heavy downpours can be expected, along with localized gusty winds and lightning in thunderstorms. Indicated VCTS for now and will be amending TAFs with TEMPOs as no threat presently imminent. Best chance from Blacksburg, Lewisburg, Roanoke through late aftn and perhaps Lynchburg into early evening. Outside of storms, winds should be southwest to west- southwest 3-7 kts. For tonight, lingering showers or storms in the central Piedmont should dissipate. May be a brief period of clearing between, and with weakening wind field, could see a large coverage of patchy fog. Indicated IFR vis patchy fog at Blacksburg and Roanoke where cloud clearing could be greatest, with MVFR vis patchy fog at Danville and Lynchburg where there could be a greater coverage of clouds. It`s less certain on fog coverage in the WV TAFs, where southwesterly winds may remain up enough ahead of cold front to mitigate development. Into Friday, should have mix of flight categories from VFR to potentially leftover IFR cigs/vis. Best chance MVFR/IFR cigs in the eastern Piedmont/Danville. These should gradually erode ahead of a cold front. Renewed thunderstorm activity ahead of the beginning as early as late morning/early afternoon, though better shot exists after 18z Friday. Indicated VCTS to indicate potential storms around, though VCSH indicated at Lewisburg and Bluefield given earlier frontal timing and less opportunity for destabilization. Extended Aviation Discussion... Another round of scattered afternoon showers and thunderstorms Friday associated with the primary cold front, though VFR prevails outside of showers or storms. Dry advection and light to moderate west to northwest winds behind the front Friday night could keep overnight fog coverage limited, but a potential sub-VFR ceiling is possible at Lewisburg and Bluefield. Mainly VFR conditions for the weekend under weak high pressure.
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&& .EQUIPMENT... As of 755 PM 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 17th through the 27th the radar will most likely be down completely as the repairs are expected to be made during this period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL/WP NEAR TERM...AL/WP SHORT TERM...JH/KK LONG TERM...JH AVIATION...AL/WP EQUIPMENT...RAB/WERT is the U.S. government's official web portal to all federal, state and local government web resources and services.