Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 190558 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 158 AM EDT Wed Jul 19 2017 .SYNOPSIS... An upper low will continue to slide east across the region overnight into Wednesday before exiting Wednesday night. As the system drifts south of the region on Thursday, it will herald a stretch of even warmer and more humid weather for the end of the work week into the upcoming weekend, along with a slowly increasing threat for additional showers or thunderstorms. && .NEAR TERM /THROUGH TODAY/... As of 900 PM EDT Tuesday... Clusters of deep convection continue to linger this evening within the upper vortex that is all but stuck across the region per latest water vapor loop. This in conjunction with a lingering surface trough along the mountains and residual instability likely to keep at least isolated convection going a while longer despite loss of heating. Short term guidance also shows some redevelopment from north to south with outflow so leaving in some chance to slight pops until just before midnight at this point. Otherwise trend of becoming partly cloudy under debris clouds and then some fog/stratus on track with lows mainly in the 60s. Previous discussion as of 315 PM EDT Tuesday... An upper level low is currently positioned over the Blue Ridge and Shenandoah Valley region. In conjunction with an airmass that is moderately unstable, this has promoted scattered showers and thunderstorms within about 40 miles either side of the Blue Ridge. More isolated storm coverage exists in the Alleghany Highlands. Storm motions have been highly erratic with weak winds aloft, heavily influenced by terrain and/or convective outflow with DCAPEs of 800-1100 J/kg. However they will move into a more unstable environment present off the Blue Ridge foothills. Strongest reflectivity cores will be capable of gusty to at times damaging winds in this environment, along with periodic instances of hail up to 1" in diameter. With no other significant sustaining force, storms should begin to diminish in number and strength after sunset with many areas trending dry by mid-evening. In its place will be clearing skies and light to calm winds, which should help generate areas of patchy fog overnight tonight. The Greenbrier and New River Valleys, along with any location that saw rain this afternoon. Lows tonight mainly in the 60s. For Wednesday, aforementioned upper low will slowly pivot into the eastern VA through the day. Daytime heating should afford some isolated to scattered showers and thunderstorms again, with the best chance east of the Blue Ridge, and particularly in the Piedmont. Synoptic-scale subsidence aloft may keep areas west of the Blue Ridge on the dry side despite warm and somewhat humid conditions. Even in the Piedmont, coverage may only be isolated to scattered. Highs on Wednesday should be in the mid 80s to mid 90s. Apparent temperatures in the central VA Piedmont may near 100 degrees when the muggy dewpoints are factored in. && .SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/... As of 315 PM EDT Tuesday... Weak upper closed low pressure forecast to drift slowly south into southern GA during the short term period as an upper ridge of high pressure builds over the system and east into the mid- Atlantic region from the central plains. Increasing temperatures in the lower and mid troposphere will help to boost daytime high temperatures well into the 80s to near 90 in the mountains, with mid-upper 90s common in the Piedmont. Despite the increasing heat and humidity, dew point values are not expected to be high enough to push heat indexes to the 105F advisory level. Gradual warming of temperatures aloft and increasing weak subsidence on the backside of the upper low should limit extent of diurnally-driven thunderstorm development. However, despite the development of a weak capping inversion, an isolated brief shower/storm still can`t entirely be ruled out in the mountains and foothills, where differential heating and localized topographical forced ascent will be maximized, but most areas will remain dry - as has been the case for many weeks in some locations. Overall, expect a relatively dry, uneventful, and warm to hot short- term period, but not hot enough to threaten any day or nighttime records. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 315 PM EDT Tuesday... Very warm/hot weather expected to persist through much of the long-term forecast period. Weak upper closed low expected to drift west into AL over the weekend, then shear slowly northeast back toward the southern and central Appalachians by early next week. Meanwhile, a series of short wave trofs in the northern stream are forecast to scoot east through the Great Lakes, with the strongest on Sunday into Monday likely to push cool front/baroclinic zone south toward or through the Blacksburg forecast area later Monday or Monday night. Whether the surface front can progress this far south while pushing against a strong Bermuda high pressure system still remains somewhat in question, but the frontal proximity, combined with at least some moisture streaming back northeast in advance of southern closed/shearing low should support a slowly increasing threat for mainly diurnally-driven showers/thunderstorms into portions of the Blacksburg forecast area, especially in the mountains (Saturday) and into most areas Sunday into early next week as the front nears or moves southeast through the region. Little overall change in airmass expected until the possible frontal passage - with a continuation of relatively hot days with warm nighttime lows, but with only a very limited threat for any record breaking readings. Cooler air may arrive by Tuesday, driving temperatures back down 5-7F and closer to seasonal levels. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
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As of 145 AM EDT Wednesday... Most sites received at least some rainfall late in the day Tuesday leaving the ground wet/moist at sunset. With calm winds and mostly clear skies now, conditions are ideal for radiational cooling and fog. Indeed this is the case with LWB, BCB, and DAN already seeing 1/4SM FG or less at times and it is only 06Z. See no reason why these stations will not stay 1SM or less most of the remainder of the night and until after sunrise. LYH is also showing some fog development and would expect them to go down as well with time. Thus, LIFR- VLIFR visibilities and attendant ceilings can be expected throughout the night. The exceptions will be BLF and ROA where little to no fog is expected as not a favorable synoptic situation for such and radiational fog is rare at these locations, even after rainfall. After sunrise, conditions will improve quickly to VFR and should remain that way through the remainder of the TAF valid period. Not expecting a repeat of the widespread showers and thunderstorms that were observed Tuesday under the weak upper low as the upper low drifts further east-southeast and upper ridging moves eastward from the Plains. 500mb temps will warm from -8C to -10C to around -5C during the next 24 hours, which will leave a much more stable atmosphere in place. Showers and thunderstorms will be isolated today and confined mainly to areas east of the Blue Ridge closer to the upper low. Not enough confidence to include in the TAFs at this time. Winds will be light and variable through the TAF valid period and mostly calm at night. Some tendency for winds to be southeast across the Piedmont, but even there speeds will be 5kts or less. Medium confidence in visibilities and ceilings through 13Z, then high confidence in VFR conditions. Medium to high confidence in wind speed and direction through the TAF valid period. Aviation Extended Discussion... The pesky weak upper low will drift into the southeast states toward the weekend and then eventually retrograde westward along the Gulf Coast. Eventually it could figure into our weather once again early next week. However during the time frame of Thursday through Sunday, generally VFR conditions are expected as ridging aloft spreads eastward from the Midwest toward the Southeast U.S. A frontal system could approach the area late in the weekend with an increase in convection at that time. Late night and early morning fog can be expected at the usual sites (e.g., LWB, BCB, LYH) most likely each day through the period with IFR-LIFR conditions possible in the 08Z-13Z time frame each day.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL/JH NEAR TERM...AL/JH SHORT TERM...WERT LONG TERM...WERT AVIATION...AL/JH/RAB

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