textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 18z TAFs.

KEY MESSAGES

1. Hot and humid conditions linger through the weekend with heat index values ranging from 100 to 105 degrees each afternoon. 2. Coverage of mainly diurnal showers and thunderstorms will increase through weekend as a weak cold front moves through our area.

DISCUSSION

Key message 1: Hot and humid conditions linger through the weekend with heat index values ranging from 100 to 105 degrees each afternoon.

Broad upper ridging over the eastern two-thirds of the CONUS has allowed for very warm thicknesses to settle across the area. In response, hot and humid conditions will prevail as heat indices reach into the triple digits today across the Piedmont and foothills. Deep boundary layer mixing will keep dewpoints at bay with upper 60s to lower 70s, while max temperatures reach into the mid and upper 90s. As a result, heat advisory criteria of 105 degrees will only impact a very small sample size as zonal coverage won't be enough to justify a Heat Advisory. Better convective coverage and associated cloud debris will help temperatures drop a few ticks lower Saturday and Sunday (low to mid 90s), while dewpoints remain more elevated (low to mid 70s) compared to today with better moisture advection and less mixing in the boundary layer. Hot and humid conditions will prevail with triple digit heat indices expected during peak heating across the Piedmont and foothills this weekend. Due to slightly lower temperatures, the current forecast keeps heat indices just below advisory level criteria over the weekend. A cold front will move into the region late Sunday into Monday, leading to more seasonable-like temperatures and heat indices through the first half of next week.

Heat index values will likely be 100 or higher outside of the mountains, so individuals should prepare for elevated heat risk over the next few days. 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 will increase through weekend as a weak cold front moves through our area.

Plume of 700-850mb moisture advection and subtle shortwave will help to kick off diurnal convection from the OH/TN Valley and the Deep South this afternoon/evening. Terrain enhancement and lower LFC's will support convective initiation across the mountains, northeast Georgia, and the western Upstate. The atmosphere will remain capped elsewhere. A few storms that develop may become strong to severe with locally damaging winds as the primary threat.

An upper trough will dig into the Great Lakes region and northeastern CONUS from Saturday through Sunday with an attendant cold front. Better moisture advection ahead of the front and increasing low-level convergence will support an uptick in convective coverage on Saturday and especially on Sunday with the frontal boundary draped across the OH Valley and Mid-Atlantic by then. Diurnal instability and slightly more deep layer shear (15-25 kts) should help to produce more loosely organized convection capable of producing damaging winds and small hail. The marginal risk for Saturday and marginal/slight risk on Sunday is justified and wouldn't be surprised to see the slight risk expanded further west from the Charlotte area on Sunday based on the latest guidance. With the front in place to the northwest, convective coverage will peak during the typical diurnal timeframe, but help to keep lingering nocturnal convection in the area Sunday night. Depending on the placement of the boundary and how much the atmosphere has been overturned from this weekends convection, Monday could see another round of showers and thunderstorms, with a few becoming strong to severe, but confidence is lower on this development.

Convective chances decrease on Tuesday to below normal climo PoPs, while convective chances increase yet again by next Wednesday as another front moves into the area, where SPC has already highlighted the eastern zones in a 15% slight risk outlook on D6.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Low VFR cu has developed across the area with showers and thunderstorms ongoing across the southwest mountains of North Carolina. Expect the coverage to increase between the 18Z and 00Z update. As a result, placed a PROB30 for TSRA and associated restrictions across KAVL and the Upstate sites as the activity moves downstream from the original initiation locations. Winds have toggled more east to southeasterly and should turn southerly later this afternoon for all terminals but KAVL where it remains northwesterly through much of the period. Winds go light and variable later this evening and overnight, but mountain valley fog/low stratus expected once again before daybreak Saturday. Hinted at this chance at KAVL, but will most likely depend if rainfall makes it to the terminal and the rest of the French Broad Valley this afternoon/evening. Expect low VFR cu to develop again tomorrow afternoon, with an uptick in shower and thunderstorm chances. Placed a PROB30 for TSRA and associated restrictions for KCLT and KAVL Saturday afternoon. The rest of the TAF sites will likely need at least a PROB30 for TSRA and associated restrictions for tomorrow afternoon/evening during the 00Z update.

Outlook: Even better coverage of afternoon/evening SHRA/TSRA is expected this weekend with a few strong to severe storms possible each day during peak heating hours. Coverage then diminishes early next week. Low stratus and/or fog will be possible each morning, mainly in the mountain valleys.

CLIMATE

RECORDS FOR 07-17

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 95 1980 72 1917 77 1887 54 1939 1891 KCLT 100 1986 74 1989 80 1881 62 2004 1887 1896 1903 1886 KGSP 103 1887 76 1930 76 1934 60 1886

GSP WATCHES/WARNINGS/ADVISORIES

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