textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00Z TAFs.

KEY MESSAGES

1. Hot and muggy conditions as well as shower and thunderstorm chances linger through the weekend. A few severe storms capable of producing damaging wind gusts are possible Sunday afternoon. 2. Heat risk will steadily increase through next week, with heat indices potentially exceeding 100 degrees across much of the Piedmont and foothills...especially from mid-week on.

DISCUSSION

Key message 1: Hot and muggy conditions as well as shower and thunderstorm chances linger through the weekend. A few severe storms capable of producing damaging wind gusts are possible Sunday afternoon.

Diurnal convection has mostly diminished evening but there's a decent signal in convection-allowing models and associated ensemble guidance that activity will return in the southern Appalachians at some point during the 09-15Z Sunday time frame. Considering the time of day/limited instability and lack of stronger shear to maintain a high degree of organization, the potential for severe weather will be low in our area during this time, but can't rule out some isolated strong-to-damaging wind gusts as this activity makes its initial push into the area tomorrow morning. The possibility...if not likelihood of morning convection creates a considerable amount of uncertainty regarding diurnal convective trends Sunday afternoon. Remnant outflow could create renewed potential for stronger convective development across the Piedmont if timing of convection is late enough to interact with destabilization...or if convection is early enough in the day to allow for good recovery. Sunday PoPs peak at likely over the mountains Sunday morning, with generally token chance PoPs carried in all areas during the afternoon.

Temperatures will be a little above normal through Sunday night. Heat Index is forecast to at least approach the century mark across the southern tiers of forecast zones Sunday afternoon, but it is unlikely that the heat index will reach Heat Advisory criteria, which starts at 105, outside of a few localized areas.

Key message 2: Heat risk will steadily increase through next week, with heat indices potentially exceeding 100 degrees across much of the Piedmont and foothills...especially from mid-week on.

An anticyclone and associated ridge will build across the TN and OH Valleys early in the new work week, with the center of the anticyclone forecast to build into the southern/central Appalachians by the end of the week. This will allow the heat to steadily build across the Southeast, with temperatures of around 5 degrees above climo during the first half of the week forecast to increase to almost 10 degrees above normal by the start of the holiday weekend. Maximum daily Heat Index is expected to be around, or a little over the century mark, but still below Heat Advisory levels through Wednesday, with greater potential for criteria to be met Thursday through Saturday. This is all quite uncertain at this time...and serves as a reminder that H.I. is very sensitive to dewpoints...which sometimes mix out more than anticipated during subsident regimes.

Otherwise, the more suppressed synoptic pattern is expected to result in limited coverage of diurnal convection...especially from Tuesday onward, when isolated to perhaps widely scattered activity is forecast to be largely confined to the mountains.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Shower and thunderstorm activity has mostly waned as of 00Z. Most terminals east of the mountains should remain dry and VFR this evening into the overnight hours, with KHKY being the exception as MVFR fog may develop at the terminal overnight. KAVL will also see the potential for MVFR fog to develop overnight, with rain chances returning around daybreak and lingering through the early afternoon hours. Thus, went with a PROB30 for SHRA at KAVL from 10Z-15Z with prevailing SHRA from 15Z-18Z and then a PROB30 for TSRA from 15Z-18Z. Both KAVL and KHKY will have the best potential to see MVFR cigs develop Sunday morning. Terminals east of the mountains will not see TSRA and associated restrictions return until the afternoon hours, but KHKY could see some SHRA around the terminal during the morning hours so went with VCSH starting at 13Z. Have PROB30s for TSRA east of the mountains Sunday afternoon but confidence on timing is low as high-res guidance sources are not in agreement regarding exact timing. Low-end gradient wind gusts may return Sunday afternoon east of the mountains but confidence is low. Thus, only have gusts mentioned at KAND for now. Wind direction east of the mountains will remain SW/WSW through early Sunday afternoon before toggling more WNW/NW by mid to late Sunday afternoon.

Outlook: Diurnal showers and thunderstorms are expected mainly across the mountains next week. Mountain valley fog/low stratus will be possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.