Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 270229 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1029 PM EDT Wed Jul 26 2017 .SYNOPSIS... A front will move in from the northwest late this week, increasing chances of showers and thunderstorms across the area. Cooler and drier conditions will arrive behind it for the weekend. && .NEAR TERM /THROUGH THURSDAY/... As of 1010 PM: Cloud cover along an outflow boundary stretches from the Georgia mountains to near Lake Wylie attm. A couple of showers have formed near this boundary since sunset, but have been brief. Low-level flow has already veered to SE following high pressure moving off the Northeast Coast and is expected to continue veering overnight. Short-term guidance generally depicts occasional/isolated development as this flow and lingering mesoscale boundaries allow elevated instability to be realized. Consensus is greatest on activity firing near the southern-facing Blue Ridge Escarpment west of Hendersonville, and to a lesser extent along the northern Blue Ridge. I have revised PoPs to initially focus along the aforementioned outflow boundary but still transition to a Blue Ridge focus by early morning. Dewpoints remain elevated and did not mix out much today. Patchy fog is likely, but with the southerly flow being associated with some upglide, expanding cloudiness is expected to limit radiation and keep vsbys from being problematic. Min temps have been raised a bit in light of the juicy conditions. Moderate to strong instability is forecast to develop again Thu afternoon, especially across the western half of the area. Morning clouds and perhaps showers may again delay destabilization, but probably not to the extent that they did today, owing to the return of SW low level flow. Meanwhile, higher resolution guidance depicts an area of height falls approaching the southern Appalachians by early evening. The result should be an enhancement of the typical high terrain convective coverage, with 50-60 pops warranted across the Blue Ridge zones by late afternoon. Steering currents and outflow boundaries should take this convection at least as far east as the NC foothills during the late afternoon/early evening, with pops tapering down to a slight chance across the southeast quarter or so of the CWA. A few pulse severe storms appear much more likely on Thursday. In fact, increasing shear profiles could give storms a little more longevity by the end of the day. Otherwise, max Temps are expected to rebound to normal levels across the entire area. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 215 PM EDT Wednesday: Forecast confidence remains relatively low with regard to the severe weather potential on Friday. As for the details, a broad, cyclonically-curved upper jetlet will stretch out from the mid-Mississippi River Valley through the central and southern Appalachians Thursday night through Friday as heights fall upstream. Associated upper level divergence will rapidly improve the upper-level forcing during this period, while a cold front approaches from the northwest. A mid-level shortwave will likely cross the region from the west-northwest Thursday evening into early Friday morning, with the best evening PoPs stretched out along the I-40 corridor into the piedmont. Overnight convective chances will then ramp up in the western mountains given the potential for any middle Tennessee convection to make a run into the southern Appalachians before daybreak Friday. Deep layer forcing will then likely be the strongest just ahead of the cold front Friday afternoon and evening, but instability remains the primary question mark as cloud cover ahead of the upstream convection should thicken up in many locations. The best chance of moderate or better CAPE with decent insolation should be in the lower piedmont - generally southeast of the I-85 corridor and east of I-77 through Friday afternoon/evening. Westerly shear will also improve through the day Friday, but with surface to 6 km bulk shear values generally remaining less than 30 kt throughout. Precipitable water values will surge above 2 inches in many locales across the region on Friday, but improving westerly steering flow should help minimize hydro issues - except where any training occurs. Whether we end up with a Marginal or a Slight risk of severe thunderstorms going forward for Friday, the HWO mention still seems warranted given the potential. The surface cold front will cross the region from the northwest Friday night and settle along the southeast piedmont Saturday morning. Northwest flow moisture will likely linger in the mountain upslope areas through Saturday morning, where a few showers could continue, but deep layer drying will steadily work into the region from the northwest through the day. Isolated to scattered showers, with a few thunderstorms, could also linger in the lower piedmont Saturday until the drier air builds in in earnest there late day. Below climo temperatures are expected each afternoon, with pre-frontal clouds expected Friday and then cold advection on Saturday as a deepening east coast trough sets up over the region. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 200 PM EDT Wednesday: The medium range is still marked by a relatively high-amplitude pattern, with persistent troughing over the eastern CONUS to start the period. The trough will continue to amplify through the weekend and into early next week, resulting in temperatures five or more degrees below average. At the surface, a surface cold front will have cleared the area by the time the medium range picks up Saturday night, and confidence is increasing that a drier airmass will work its way into the southeast over the weekend and through early next week. With a surface ridge extending its influence over much of our area as well, convection should remain almost entirely suppressed Sunday, Monday, and possibly Tuesday as well. Temperatures will begin to moderate back to normal levels as the longwave upper trough flattens a bit, with pops making a return to near seasonal normals by the end of next week. && .AVIATION /03Z THURSDAY THROUGH MONDAY/... At KCLT and elsewhere: Guidance generally has trended toward fewer restrictions overnight, perhaps reflecting the lack of convection today and drier soils as a consequence. However it is still expected that low level winds will veer and pick up a bit overnight, resulting in isentropic upglide and some VFR level cloudiness. This should keep radiation fog at bay outside the deeper mountain valleys. Near the Blue Ridge, several sources continue to depict MVFR stratus forming with upslope enhancement toward dawn. Thus a mention is still warranted at KAVL/KHKY. Isolated showers could also regenerate overnight due to elevated buoyancy along and south of the Blue Ridge; this is chiefly handled with VCSH though it was included in the TEMPO at KAVL. Diurnal convection will develop tomorrow afternoon, with chances close to climatology. Improved shear could allow some clustering along outflow boundaries, which could threaten all sites with wind shifts if not squalls. Outlook: Diurnal convection and a threat of strong to severe wind gusts will return Friday. Then drier air will filter in from the north behind a passing cold front this weekend. Chances for morning fog and stratus will also be possible each day, mainly in the mountain valleys. Confidence Table... 02-08Z 08-14Z 14-20Z 20-00Z KCLT High 100% High 100% High 100% High 100% KGSP High 100% High 90% High 100% High 100% KAVL High 95% Low 45% Med 62% Med 60% KHKY High 100% Low 58% Med 65% High 100% KGMU High 100% High 94% High 100% High 100% KAND High 100% High 91% High 100% High 100% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...Carroll NEAR TERM...JDL/Wimberley SHORT TERM...HG LONG TERM...Carroll AVIATION...Wimberley

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