textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 18Z TAFs. Otherwise, little change of significance has been made to the forecast.

KEY MESSAGES

1. Shower and thunderstorm chances linger through the weekend. Isolated severe storms capable of producing damaging wind gusts are possible, mainly during the afternoon/evening. 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: Shower and thunderstorm chances linger through the weekend. Isolated severe storms capable of producing damaging wind gusts are possible, mainly during the afternoon/evening.

Convection continues to steadily expand in coverage and intensity this afternoon within a moderately unstable environment characterized by sbCAPE of 2000-2500 J/kg. By late afternoon...scattered-to-numerous coverage is expected across much of western NC...where westerly flow will result in a combination of upstream convective clusters moving into the CWA and MCVs embedded in the mid-level flow initiating new convection. While deep layer shear is seasonably modest, 0-3 km shear of 15-25 kts should be sufficient to allow for small clusters of convection to organized along outflows...posing a threat of locally strong-to-briefly damaging wind gusts, with pulse updrafts and cell mergers posing an additional risk of brief microbursts.

Diurnal convection should generally wind down this evening. However, convection is forecast to blossom and become at least somewhat organized this afternoon/evening within a very unstable air mass across the mid-Miss Valley on the east side of building upper ridge. There's a decent signal in convection-allowing models and associated ensemble guidance that remnants of this activity will move into 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/evening. 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 ties of forecast zones Sunday afternoon, but it is unlikely that occurrence of H.I. criteria of 105 will be reached on anything more than a localized basis.

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 /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Convection will continue to gradually increase in coverage and intensity through late afternoon. Convection is most likely at the western NC terminals, and TEMPOs for TSRA are carried at those TAF sites at some point during the afternoon/evening. Probabilities are lower at the upstate SC terminals, and Prob30s are carried this afternoon/evening at KGMU/ KGSP. Gusty and erratic outflow winds and briefly reduced visby are expected with any TSRA. The convective potential is lower at KAND, but isolated SHRA were already developing in the general area as of early afternoon, so opted to start them off with a VCSH.

Otherwise, SW winds of 8-12 kts are expected this afternoon everywhere except KAVL (where lighter winds are expected). Convection should diminish across much of the area by mid/late evening. While VFR conditions are forecast for the overnight, can't rule out some areas of fog and/or low stratus developing, especially in locations that receive appreciable rainfall this afternoon/ evening. However, this is very uncertain at this point, so VFR is generally forecast with the exception of some MVFR fog at KAVL. High resolution guidance has consistently depicted a complex of decaying convection moving into the area from the NW at some point Sunday morning, so VCSH or Prob30s for -TSRA are carried at most sites at some point between 12-18Z Sunday.

Outlook: Isolated-to-widely scattered diurnal convection is expected Sunday afternoon and again Monday. After Monday, diurnal showers and storms are expected to become more confined to the mountains. Mountain valley fog/low stratus will possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

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