947 FXUS62 KGSP 260544 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 144 AM EDT Wed Jun 26 2024 .SYNOPSIS... High temperatures will peak today ahead of an approaching cold front which will bring shower and thunderstorm chances to the area this afternoon into Thursday. Hot and increasingly humid and unsettled conditions are expected Friday through Sunday as another cold front impacts the area this weekend. Typical summer weather is expected behind the front to start the next workweek. && .NEAR TERM /THROUGH TODAY/... As of 100 AM EDT Wednesday...No major changes were needed this update as the forecast remains on track. Did populate the latest OBS into the current forecast to get temps and dewpoints back on track. Also tweaked PoPs and thunder chances through the period based on the latest high-res guidance. Starting to see some upper cirrus track into the southern NC mountains but clear skies prevail elsewhere, along with dry conditions. Otherwise...dry high pressure will remain dominant across the area, before upper heights begin to fall ahead of an incoming weak cold front this afternoon. With upper dynamics increasing and a sw/ly sfc flow developing, pre-frontal convective activity will have an increasing probability of occurrence by this afternoon and early evening. Not expecting much coverage, mainly isolated activity and confined to the mountain ridges due to limited moisture transport until the very late period. The models have backed off on a precip response with the ECMWF now dry thru the period and the CAM models not enthused outside of discrete ridgetop cell generation. Thus, PoPs were adjusted down, especially across the non-mountain zones through the period. Lows will linger a few degrees abv normal, while highs bump a category or so warmer most areas with readings in the upper 90s across the non-mountain zones and lower 90s within the mountain valleys. Still not anticipating much of a heat index issue as sfc dewpoints vertically mix out a little better (~800 feet higher than this afternoon) due to the warmer sfc temps. && .SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/... As of 230 PM Tuesday: The upper-level pattern will evolve Thursday into Friday as a shortwave trough axis slowly drops south and east of the area and heights build ahead of another shortwave trough entering the Upper Midwest. This will allow the heat ridge to expand east across the Southeast and into the Atlantic for Friday. Guidance has been a bit inconsistent from run to run regarding the progression of a cold front through the area for Thursday with continued disagreement on whether or not it will clear the CWA. Given that the causative upper-level energy is shearing out over Southeast on Thursday, blended guidance is a reasonable solution at this time, which stalls the front somewhere over or just to the east of our Piedmont zones. However, as mentioned in previous discussions, the front will become increasingly ill defined. It will therefore serve more as a weak focusing mechanism for scattered to numerous showers and general thunderstorms on Thursday, but we lack sufficient forcing and instability with poor mid-level lapse rates, minimal SBCAPE, and anemic bulk shear values. Therefore, QPF and any strong to severe storm threat is insignificant. Highs on Thursday are generally unchanged from earlier packages, but the moderation behind the front will be a bit more noticeable given the increasing forecast max temps on Wednesday. Highs will be 4-6 degrees "cooler" for Thursday, but still 2-4 degrees above normal. The influence of the front will be a bit more discernible in the mountain valleys, with highs 5-7 degrees cooler than Wednesday, though these highs will still be slightly above normal. Guidance continues to advertise the cutting off of a weak mid-level low as upper energy departs the area later Thursday into early Friday and heights build to the north and west. Therefore, weak surface low development along the remnant front over the central Carolinas is possible during this timeframe, which may serve to maintain chance PoPs near the proximity of the front overnight, primarily east of the mountains. On Friday, the upper-level ridge axis will settle over the Southeast as surface high pressure propagates across New England. Resultant northeasterly flow early on Friday should help to push whatever remains of the front south and east of our area and usher in a more seasonable airmass, though highs will still be about 2 degrees above normal. The transient nature of the high will allow near-surface winds to veer easterly then southeasterly during the day, which should promote modest subtropical moisture advection. PWs gradually begin to increase during this time as evidenced by increasing moisture content below 650mb in forecast profiles. Therefore, even with mixing, dewpoints are expected to remain elevated into the afternoon and moist upslope flow should promote scattered shower and thunderstorm development across the area with terrain enhancement over the mountains. Some convection should be able to produce much-needed rainfall in isolated locations. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 230 PM Tuesday: The extended period will feature the strengthening of the upper-level anticyclone, potentially reaching 598-599 dm over the Southern Plains in the Sunday-Monday timeframe. The ridge will stretch over our area for Saturday, but height falls in response to the next shortwave over the Upper Midwest will erode the ridge overhead later Saturday into Sunday. An attendant cold front will be dropping south and east across the Ohio Valley on Saturday, as we continue to advect moisture into the column. PWs continue to climb during the day on Saturday in response, with guidance suggesting values will approach 2 inches. As a result, SBCAPE values remain unimpressive for significant convection, but ample moisture and increasing mid-level and low-level forcing from the approaching shortwave and cold front suggests scattered to numerous showers and storms are possible Saturday afternoon. High temperatures should be similar to Friday though they may be influenced by higher convective coverage. Regardless, dewpoints are not expected to mix out much if at all during the afternoon hours given the moisture aloft and this would result in notably uncomfortable humidity values during peak heating, with heat index values reaching the low 100s across much of the Piedmont. Expect chance PoPs to persist overnight Saturday into Sunday given the approaching cold front and ample low-level moisture. The front is expected to directly impact the area on Sunday, but as with previous packages, there remains about a 12-18 hour timing discrepancy between deterministic guidance. The faster GFS passage would cause the best combination of heating and forcing to be south of our area, so this is something that needs to be watched. Nevertheless, the overall impact remains the same, with numerous to widespread showers and thunderstorms expected during the day. With several days of moisture advection in place, this frontal passage looks to be the best opportunity for appreciable rainfall we have had in quite some time. Given mean flow and anomalous PWs exceeding 2 inches across much of the Piedmont, isolated areas of heavy rain are certainly possible. Unfortunately, GFS and ECMWF ensemble probabilities of an inch or greater remains well below 20 percent for most of the forecast area. It looks like the worst combination of heat and humidity continues to be on Sunday. The latest National Blend guidance nudges afternoon dewpoints even higher, reaching the lower to even mid-70s in isolated locations. This continues to be on the upper end of the guidance, but if these values verified heat index values would approach Advisory criteria (>105) in the Charlotte metro and Upper Savannah River Valley. However, a mix out of even a couple degrees in afternoon dewpoints would notably impact the heat index values; therefore, confidence in Advisory-criteria indices six days from now remains too low to add to the HWO at this time. Following the front on Sunday, we expect modest improvements in humidity and highs closer to normal for Monday and Tuesday, but an active upper-level pattern supports continued above climo PoPs each afternoon. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Key Messages... 1) Mostly Dry and VFR through the 06Z TAF period 2) Isolated SHRA/TSRA Will Develop Ahead of a Cold Front this Afternoon and Early Evening 3) The Cold Front Tracks Over the Terminals Overnight into Daybreak Thursday Potentially Bringing -SHRA to the Terminals At KCLT and elsewhere: Mostly dry and VFR across the terminals through the 06Z TAF period. Sfc high pressure remains in place through this morning while a cold front approaches out of the northwest. Winds are mostly calm to light and VRB this morning, although KCLT continues to see a light S'ly wind. Winds should generally remain light and calm to VRB through mid-morning. Some cirrus is noted pushing into the southern NC mountains as of 06Z but this is not impacting KAVL at the moment. Cirrus should continue to gradually spread east through the 06Z TAF period. Isolated SHRA/TSRA looks to develop ahead of the cold front this afternoon and early evening, mainly across the NC mountains. Thus, maintained the mention for VCSH at KAVL, with dry conditions elsewhere through the TAF period. Winds will gradually pick up out of the S/SSW later this morning into early this afternoon. Speeds should remain light this morning, becoming 4-7 kts east of the mountains by this afternoon/early evening. A sparse VFR cumulus field should develop once again this afternoon before dissipating early this evening. Winds will go light again tonight, remaining S/SSW east of the mountains. KAVL will likely see winds go calm to light and VRB again this evening into tonight. The cold front will track over the terminals overnight into daybreak Thursday and may lead to a line of -SHRA. However, the bulk of the precip looks to hold off until the very end, or just after, the 06Z TAF period. Outlook: A cold front will continue tracking over the terminals early Thursday before stalling south of the area on Friday, keeping SHRA/TSRA chances around the region. Another cold front will impact the terminals this weekend keeping unsettled weather around. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...AR/JMP NEAR TERM...AR/MPR/SBK SHORT TERM...JMP LONG TERM...JMP AVIATION...AR