textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00z TAF issuance.

KEY MESSAGES

1. Severe threat returns Tuesday afternoon and evening ahead of a cold front. The main hazards with any severe storms that develop are damaging winds and large hail. Make sure you have multiple ways to receive warnings. 2. Hot and humid conditions are expected Tuesday with heat indices climbing into the triple digits for most locations east of the mountains. A Heat Advisory was issued for portions of northeast Georgia for Tuesday from Noon to 8 PM. Stay hydrated, limit time outdoors and spend as much time in air conditioning as possible. Never leave people or pets alone in a closed car. Look before you lock! 3. Less humid, drier, and cooler behind the front Wednesday through Friday. Even cooler conditions expected over the weekend with shower and thunderstorm chances returning.

DISCUSSION

Key message 1: Severe threat returns Tuesday afternoon and evening ahead of a cold front. The main hazards with any severe storms that develop are damaging winds and large hail. Make sure you have multiple ways to receive warnings.

Convection will be mostly suppressed today but cannot rule out a few isolated showers or thunderstorms, mainly across northeast GA and the SC Upstate this afternoon and evening but confidence is low. With the "Ring of Fire" pattern lingering over the region, another storm complex looks to make a run at the GSP forecast area overnight into early Tuesday morning. The Day 1 Marginal risk was removed from the GSP CWA which makes sense as the severe threat looks minimal if any with the 12Z CAMs depicting the system weakening rapidly as it pushes towards/into western North Carolina.

There is a much higher concern for severe weather Tuesday afternoon and evening as a cold front tracks across the GSP forecast area. However, the severe threat will be highly dependent on how quickly leftover convective debris cloudiness clears out prior to peak heating. If cloud cover lingers longer than expected and cuts into peak heating then the severe threat could be limited somewhat. Regardless, we will have a very favorable environment for severe storm development for this time of year. Storms will be more organized than we typically see during the summer, with supercells possible thanks to an impressive amount of deep layer shear for summertime (~30-40 kts) and 2,500-3,500+ J/kg of surface instability developing during peak heating. Any storms that develop in this enviroment will have the potential to become severe and will be capable of producing damaging wind gusts and large hail up to the size of golf balls. Thus, the Day 2 SPC Severe Weather Outlook showing a Slight risk across nearly the entire GSP forecast area is warranted. The severe risk appears to be the highest across the central and northern NC mountains, the NC foothills/Piedmont, and the northern/eastern South Carolina Upstate. Confidence on the severe threat elsewhere is lower. Timing for severe storms looks to be 2 PM to 8 PM Tuesday.

Key message 2: Hot and humid conditions are expected Tuesday with heat indices climbing into the triple digits for most locations east of the mountains. A Heat Advisory was issued for portions of northeast Georgia for Tuesday from Noon to 8 PM. Stay hydrated, limit time outdoors and spend as much time in air conditioning as possible. Never leave people or pets alone in a closed car. Look before you lock!

Tuesday will be the hottest and most humid day of the week, especially east of the mountains and across the mountain valleys, thanks to increased SW flow ahead of the front. Highs east of the mountains will climb into the lower to mid 90s with heat indices ranging from the upper 90s to lower 100s. Hot and humid conditions are also expected across the typically warmer mountain valleys with highs ranging from the upper 80s to lower 90s and heat indices climbing into the lower to mid 90s. Breezy W/SW winds ahead of the front as well as afternoon/early evening storms should provide a bit of relief from the heat and humidity. The upper Savannah River Valley would have the best chance to see heat indices reach Heat Advisory criteria (which starts at 105 degrees) so a Heat Advisory was issued for Elbert, Hart, Franklin, and Stephens counties in northeast Georgia from noon to 8 PM Tuesday. Confidence on reaching advisory criteria is lower across the southwestern SC Upstate so will hold off on an advisory for these locations for now and will let the mid shift reevaluate if the current advisory needs to be expanded. Both heat indices and high temperatures Tuesday could be limited somewhat by convective debris cloud cover from the overnight/early morning MCS lingering longer than expected and/or by diurnal convection.

Key message 3: Less humid, drier, and cooler behind the front Wednesday through Friday. Even cooler conditions expected over the weekend with shower and thunderstorm chances returning.

The cold front should be south of the GSP forecast area by daybreak Wednesday, stalling over the Southeast through Friday. Dry high pressure will build in from the north behind the front allowing lower humidity, drier conditions and cooler temperatures to return for the rest of the workweek. Highs Wednesday through Friday will end up near normal to just above normal for most locations. The front will slowly creep back northward over the weekend bringing shower and thunderstorm chances back to the forecast area. Even cooler conditions can be expected over the weekend with highs dropping below normal to kick off August.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Mostly dry and VFR thru the bulk of the 00z TAF period. Convection associated with a cold front moving thru the region is expected to impact our fcst area tomorrow aftn/evening. Confidence wrt the timing of the main line of storms still remains fairly low at this time. As such, I have PROB30s for TSRA at all TAF sites from roughly 18 to 24z tomorrow. If/when confidence increases, we can transition to TEMPOs at most sites. Otherwise, we could see some patchy fog/low stratus in the mtn valleys overnight, but it's unlikely to develop at KAVL. Winds should go light and VRB to calm over the next few hrs and remain that way into the morning. Outside of the mtns, winds will pick up from the SW by late morning and become gusty by the early aftn. At KAVL, winds will pick up from the WNW by the early aftn tomorrow with low-end gusts expected for the rest of the day/evening.

Outlook: Dry and mostly VFR conditions return Wed thru Fri, with shower and thunderstorm chances and their associated restrictions returning over the weekend. Low stratus and/or fog will be possible each morning, mainly in the mtn valleys.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from noon to 8 PM EDT Tuesday for GAZ018-026-028- 029. NC...None. SC...None.