778 FXUS62 KGSP 210553 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 153 AM EDT Sat Aug 21 2021 .SYNOPSIS... An upper level low or trough will persist over the eastern seaboard, as Tropical System Henri moves over the offshore waters. Weak surface high pressure will set up over the southern Appalachians, with a lee trough of pressure persisting in the Piedmont. The upper-level pattern will remain relatively unchanged through the period. Isolated showers and thunderstorms will remain possible this weekend into early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 140 am EDT: Nocturnal convective activity along the I-77 corridor will gradually dissipate through the early morning hours as surface-based instability continues to wane. Fog development will need to be monitored closely through daybreak with less mid-level debris around, providing for better radiating conditions atop the fairly moist ground. Otherwise, an H5 closed low center over the central Appalachians will continue to spin north of the region, with various spokes of vorticity moving through the southern Appalachians and crossing the forecast area today. Forcing from these vorticity lobes may be better in the northern tier today, but moisture and instability will be better in the southern half, and that is where the better chance PoPs will be featured during peak heating as sbCAPE values climb to 2500+ J/kg. Initiation should get started near the mountains again, especially the southern escarpment area, but with a secondary focus along a weak lee surface trough. Maximum temperatures should be comparable to yesterday. Tropical System Henri will remain well east of the Carolinas today through tonight, with no impact to the region expected. Any diurnal convection will die out quickly this evening, with another night of patchy fog and some dense fog in the mountain valleys. Mins remain above climo. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 100 AM EDT Saturday: Dry air advection from NW-ly flow aloft will set the stage for very limited convective coverage on Sunday. A closed upper low over the mid-Atlantic will continue to lift northeast, while pulling TS Henri further west and potentially stalling over New England and portions of the Northeast. In our neck of the woods, subsidence aloft and rising heights will filter into the area from a deepening upper ridge over the Southern Plains and Lower Mississippi Valley, which allows for mostly dry conditions across the CFWA outside of isolated ridgetop convection and a pop up storm or two east of the mountains for Sunday. With ~590 dm thicknesses continuing to set up shop over the region, expect for the very warm and humid airmass to persist through the short term period. With little change in the overall synoptic pattern and mesoscale processes during the early part of the upcoming week, expect a very similar forecast for both Sunday and Monday. The main difference between both days will be temperatures. With the upper ridge extending further east into the southeastern CONUS on Monday, temperatures should uptick a few degrees from Sunday to Monday. Either way, temperature values will run above normal for both days. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 230 PM EDT Friday: The eastern periphery of the ridge remains over the Carolinas and Georgia at the beginning of the extended forecast period, but retreats back west middle of next week. This will allow the Bermuda high in the Atlantic to extend over the southeast CONUS. So the overall synoptic pattern middle of next week will have the Carolinas and northeast Georgia sandwiched between two high pressure systems. This will lead to increased moisture and the continuation of above climo high temps. PWAT values will increase Tuesday ranging from 1.70 to 2.0 inches. These values are expected to linger over the CWA through the end of the forecast period. Tuesday into Wednesday shower and thunderstorm chances increase due to a surface trough draped over the central Carolinas. This sfc trough should remain over the Carolinas throughout the extended. Both the GFS and ECMWF show the two ridges of high pressure working together to bring a tropical wave into the Carolinas Thursday into Friday. This would further increase shower and thunderstorm chances middle to end of the workweek. Confidence on the track of this tropical wave remains fairly low as this occurs towards the end of the extended forecast. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Decaying, weak convection across the NW NC Piedmont early this morning is unlikely to reach the KCLT airport. Anticipate mainly VFR debris type clouds in the area overnight. Low stratus and fog potential through daybreak will be best in the mountain valleys, with LIFR conditions likely at KAVL after 08Z. Expect FEW to SCT VFR cumulus to develop quickly today, with PROB30 TSRA for the mid to late afternoon hours at all but KHKY and KCLT where profiles will be drier. Light winds will favor northerly early in the period and then southerly the second half of the TAF period. Any convection will diminish quickly tonight. Other than valley fog, any restrictions through the period will be confined to thunderstorms. Outlook: A typical, diurnal summertime convective pattern is expected to prevail the rest of the weekend through the middle of next week, with scattered coverage at best. Mountain valley fog will be possible each morning, as well as at sites that receive heavy rainfall the previous afternoon. Confidence Table... 06-12Z 12-18Z 18-24Z 00-06Z KCLT High 97% High 99% High 100% High 100% KGSP High 100% High 96% High 100% High 100% KAVL Low 46% Med 73% High 100% High 100% KHKY High 100% High 100% High 100% High 100% KGMU High 100% High 96% High 100% High 100% KAND High 100% High 96% High 100% High 98% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...AP/HG NEAR TERM...HG SHORT TERM...CAC LONG TERM...AP AVIATION...HG