Area Forecast Discussion
Issued by NWS Blacksburg, VA

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027 FXUS61 KRNK 171357 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 957 AM 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 955 AM EDT Thursday...Mostly sunny skies presently exist across the forecast area in what is a rather soupy, humid air mass (dewpoints in the upper 60s to middle 70s). Made some downward reductions to Sky cover forecast to reflect these conditions, which should last through much of the rest of the morning. Looking at area RAOBs reveals precipitable water values in the 1.5 to 1.8 inch range, a little bit lower than models were progging, but it`s expected that these should rise through the afternoon by almost another half-inch. Air mass is weakly capped (surface-based CIN is < 100 J/kg), and continued insolation should foster deepening cumulus. High-resolution guidance consistency has been rather poor, particularly on timing, though where they do agree area-wise is on a band of showers and thunderstorms in association with a subtle convergence band over eastern KY/eastern TN moving northeastward toward the western Appalachians/western side of the Blue Ridge. This also coincides with leading edge of better mid-level height falls from distant upper-level disturbance. Timing on this - as mentioned - is rather broad, between 16-22z/noon to 6pm. Couldn`t rule out an isolated thunderstorm further east into the Piedmont. Though a few stronger/SPSable type thunderstorms are envisioned with better prospects for stronger convective instability, localized heavy rainers and smaller footprints of heavy rain should be the primary hazard with thunderstorms. Tried to better shape PoPs this way for the afternoon but many locations stand to be dry. 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 /14Z THURSDAY THROUGH MONDAY/... As of 715 AM EDT Thursday... Any fog will be gone by 13z with VFR conditions through most of the day. May see some MVFR at BLF this morning though. High-res models continue to show showers/storms starting in WV late this morning then shifting east to the piedmont in the afternoon. Have VCTS at all sites for now. Best coverage should be in the LWB-LYH corridor but models still disagreeing on this so kept any predominant wx out of the tafs. Showers/storms may linger into late evening in the piedmont, but will take VCTS after 00z. Added IFR fog at LWB/BCB late tonight. Winds will generally be light through the period mainly out of the south to southwest. 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. && .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...MBS/WP EQUIPMENT...RAB/WERT

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