textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 18z TAFs.

KEY MESSAGES

1. Hot and humid conditions linger through Sunday with heat index values ranging from 100 to 105 degrees each afternoon. Slightly cooler temperatures return next week which should keep heat indices mostly below 105 degrees. 2. Coverage of mainly diurnal showers and thunderstorms increases through Sunday. A few storms could become severe both days with damaging winds the main hazard. Mainly diurnal shower and thunderstorm chances linger through Monday with a brief lull in activity on Tuesday before a cold front brings the potential for another round of severe weather on Wednesday.

DISCUSSION

Key message 1: Hot and humid conditions linger through Sunday with heat index values ranging from 100 to 105 degrees each afternoon. Slightly cooler temperatures return next week which should keep heat indices mostly below 105 degrees.

Better convective coverage and associated cloud debris, along with lower thickness values, will allow for slightly cooler temperatures this weekend ranging from the low to mid 90s. However, dewpoints will be more elevated, ranging from the low to mid 70s. This will allow hot and humid conditions to linger through the weekend with triple digit heat indices expected each afternoon east of the mountains. Union County NC looks to be the only location with significant coverage of 105 heat index values today. Isolated locations elsewhere could briefly see 105. Therefore, have issued a Heat Advisory only for Union NC for now. Sunday will be a few degrees cooler which should keep any 105 heat index values limited to isolated locations.

Individuals should prepare for elevated heat through the weekend. Stay hydrated, take frequent breaks in air-conditioned or shaded areas, and never leave children or pets unattended in vehicles.

Key message 2: Coverage of mainly diurnal showers and thunderstorms increases through Sunday. A few storms could become severe both days with damaging winds the main hazard. Mainly diurnal shower and thunderstorm chances linger through Monday with a brief lull in activity on Tuesday before a cold front brings the potential for another round of severe weather on Wednesday.

An upper trough will dig into the Great Lakes region and the northeastern CONUS through Sunday as a cold front approaches the area. Better moisture advection ahead of the front and increasing low-level convergence will support an uptick in coverage of convection both days. With sbCAPE reaching the 2,000-3,000 J/kg range during peak heating and deep layer shear increasing slightly to 15-25 kts each day, convection should become more organized. Steep low level lapse rates and high PW values will create a threat for damaging downbursts. Thus, the SPC Outlooks showing a Marginal and Slight risk across the GSP CWA for both days appear warranted.

Convective chances linger through Monday but confidence on the severe threat is low at this time. Should see a brief lull in coverage on Tuesday before a cold front brings better convective chances back Wednesday. Strong to severe storms are possible ahead of the front on Wednesday, but confidence on the severe threat this far out remains low.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Sct convection has developed over the NC mtns during the past few hrs, and is expected to gradually spread eastward thru the afternoon and into the evening. To account for this, I have VCSH with PROB30s for TSRA at KCLT and the Upstate terminals. I have TEMPOs for TSRA at KAVL and KHKY as they will likely have more coverage in their vicinity. Otherwise, activity should diminish and move east of our fcst area later this evening. Another round of mtn valley fog/stratus is likely overnight, but I'm not confident enough that it will reach KAVL to include any restrictions in the TAF. Tomorrow should be similar to today, with sct convection initiating over the higher terrain and spreading east/south as the day wears on. Outside of the mtns, winds will remain SWLY thru the evening with some low-end gusts at the Upstate terminals and KCLT. They will go light and VRB to calm later tonight and pick back up from the SW tomorrow afternoon.

Outlook: Good coverage of afternoon/evening SHRA/TSRA continues thru Monday with less coverage expected on Tuesday before increasing again on Wednesday. Low stratus and/or fog will be possible each morning, mainly in the mtn valleys.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Heat Advisory until 8 PM EDT this evening for NCZ082. SC...None.