363 FXUS62 KGSP 261735 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 135 PM EDT Sat Aug 26 2023 .SYNOPSIS... Hot conditions persist today with increasing thunderstorm coverage ahead of a cold front. The front will bring cooler temperatures below normal through the period. Heavy rainfall likely Sunday through Tuesday. Mid to late week weather remains uncertain due to the potential for a tropical system to develop Gulf of Mexico. && .NEAR TERM /THROUGH TONIGHT/... 1130 AM Update: For this update, satellite shows a band of mid- level clouds streaming across the NC mountains and heading toward parts of the NC and SC Piedmonts. This shield of clouds could help keep temps from rapidly increasing over the next hour or two, depending on how long it remains. No major adjustments were made for temps outside of accounting for a degree or two reduction in the areas mentioned. Radar is clear and no convection anticipated from now through the next update. Another round of convec activity is in store this afternoon as waves of energy traverse the upper flow around a large-scale anti-cyclone situated over the srn plains. The airmass remains rather stagnant and sfc td/s will hover/mix into the uppers 60s while temps climb a few cats abv normal. Soundings across the Upstate show the LFC lowering to the LCL by 21z or so and this is when the CAMs begin firing off convection in a moderate sbCAPE environ. Convection will likely initiate a couple hrs earlier across the NC mtns. Deep layered shear remains quite low and these storms will be outflow driven as they near the I-77 corridor and likely become isol by the later afternoon. The biggest threat will be damaging outflow winds with some of these cells as a semi-organized line may form over the fthills arnd 23z due to the remnants of an upstream MCS pushing through. Isolated large hail is also possible due to deep instability noted on the profiles. The HRRR is showing another robust MCS reaching the NC mtns arnd 02z, but this is looking a little suspect with no support from the other hires models and will wait to make sigfnt adj/s to PoPs, if needed. Heat will once again be an issue across the wrn Upstate and NE GA where relatively high sfc td/s combine with u90 max temps to produce high HI values. A Heat Advisory is now in effect until 8PM EDT. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 245 am EDT Saturday: The robust 500 mb ridge/high center that has brought sweltering heat to much of the central and eastern CONUS over the past week will continue to retrograde westward toward the Four Corners region through Sunday. This will permit a broad trough to set up over the eastern U.S., with a weak surface cold front stalling across the Piedmont of the Carolinas. Shortwaves will move through this eastern trough on Sunday just as upper jetlet divergence improves atop the area. This forcing will combine with precipitable water values surging past two inches across much of the forecast area to produce likely PoPs for showers and embedded thunderstorms, with triggering most widespread along and near the Blue Ridge mountains. The best cooling (below climo) should occur along and north of I-40. Warm and moist profiles may curtail instability a bit, but a marginal risk of severe storms will continue as afternoon SBCAPE values reach 2000 J/kg and deep layer shear of 25+ kt persists. Hydro concerns should be limited to training activity, but this may set the stage for larger hydro issues with continued rainfall on Monday. The upper divergence divergence will persist, if not strengthen, Sunday night through Monday night as heights continue to fall from the northwest. At lower-levels, developing easterly surface winds and southerly 850 mb winds should aid upslope forcing and combine with the very moist profiles to raise the flooding potential with showers and embedded storms on Monday. A slight risk of excessive rainfall exists for Monday, with the magnitude of the threat dependent on how much wetting rainfall the area receives Saturday and Sunday. Meanwhile, surface high pressure centered to the northeast should help channel cooler air down through the western Carolinas, as T.S. Franklin moves northward well offshore from the coastal Carolinas. Maxes should be capped at 3 to 6 degrees below normal given the clouds, the expected rain coverage, and the northeasterly surface winds. An HWO mention of the heavy rainfall and hydrology threat will continue for the early week period. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 300 am EDT Saturday: Upper-level divergence and deep-layer DPVA forcing will continue through Tuesday as a 500 mb trough axis sharpens up west of the Appalachians. Southerly 950 to 850 mb flow will keep good moisture flux in place along with a continued heavy rainfall threat. The wet conditions should trim another degree or two from temperatures for Tuesday afternoon, with maxes running some 4 to 8 degrees below climatology in most areas. Attention increasingly turns to the eastern Gulf of Mexico for possible impacts on the mid to late week weather as a tropical cyclone is likely to form there and move northward. Timing of the mid and upper-level trough passage over the southern Appalachians will be critical for assessing the track of the possible tropical system. Recent model trends have suggested a slightly quicker eastward progression of the H5 trough, and perhaps a slower arrival of the system from the eastern Gulf, leaving more of the South and Southeast open to possible impacts from the tropical system. The 00Z GFS notably adjusted the track westward, but the Canadian and ECMWF camps continue to cluster more strongly along a coastal GA/SC track. These trends will need to be carefully monitored for potential impacts in the Wednesday to Thursday timeframe. Another concern to monitor is that the lingering surface boundary in the vicinity of the Carolina Piedmont could provide a focus for a predecessor rainfall event if tropical moisture from the Gulf system reaches the area. For now, will follow a blended model approach to the forecast for Wednesday through Friday, with very limited confidence on the sensible weather, wind timing, and other impacts. This will mean fairly solid chance PoPs in the Wednesday to Thursday period, higher across the southeast part of the area, with drying on Friday. Max temps remain below climo through the period. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... At KCLT and elsewhere: Mostly VFR conditions at all terminals through the forecast period. The exception is KAVL with MVFR/IFR possible with low VSBY due to fog from 7z-12z. Thunderstorm activity is expected today at a few of the NC terminals with a TEMPO at KHKY (21z-00z) and KCLT (23z-02z). Winds should remain light and N/NE through the end of the period. Gusts to 15kt expected at KAVL this afternoon. Sunday brings another round of TS activity across most of the area. Expect PROB30 to be added in future issuances. Confidence is moderate to TS affecting KCLT, therefore a PROB30 was added for the 19z-23z timeframe on Sunday. Outlook: Shower and thunderstorm chances continue Sunday into early next week as a cold front remains stalled south and east of the forecast area. Mountain valley fog and low stratus development will remain possible each morning around daybreak. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory until 8 PM EDT this evening for GAZ018-026-028- 029. NC...None. SC...Heat Advisory until 8 PM EDT this evening for SCZ010-011-019- 104-105. && $$ SYNOPSIS...CP/HG NEAR TERM...CP/SBK SHORT TERM...HG LONG TERM...HG AVIATION...CP