textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Confidence continues to increase in dangerous heat Thursday through at least Saturday. A heat advisory has been issued for portions of the area for Thursday and additional advisories will likely be needed through the weekend.

Updated the aviation discussion for the 18z TAFs.

KEY MESSAGES

1. Dangerous heat continues to build through the week and into the holiday weekend, with heat indices likely exceeding 105 degrees across much of the Piedmont and foothills. Daily thunderstorms will also be possible across the mountains. 2. The heat threat begins to wane late weekend and into early next week with a return to a more traditional mid summer pattern featuring seasonable temperatures and daily storm chances.

DISCUSSION

Key message 1: Dangerous heat continues to build through the week and into the holiday weekend, with heat indices likely exceeding 105 degrees across much of the Piedmont and foothills. Daily thunderstorms will also be possible across the mountains.

A digging western trough extending from the Pacific Northwest through the Great Basin continues to further strengthen a highly anomalous upper ridge over the Tennessee Valley. The ridge will slowly migrate east across the Southern Appalachians and into the Coastal Plain tomorrow through at least Saturday. 850mb temperatures are forecast to rise into the 24-26 C range, which is above the 99th percentile for early July. This anomalous warmth will foster very hot high temperatures in the upper 90s to low 100s tomorrow through at least Saturday. Some locations across the Piedmont may see air temperatures peak as high as 102-104 degrees. Mountain valleys will be hot as well with temperatures rising into the low to mid 90s with the only relief found at the very highest mountain tops where highs will be in the low 80s.

The biggest question that remains is how much moisture mixes out and resulting implications on heat indices. Forecast soundings continue to depict a dry and deeply mixed boundary layer extending to at least 700mb, if not higher. Surface dewpoints mix out into the upper 50s to low 60s, which would help to keep heat index values in check. This is a more aggressive solution, however, and current observations today show notably more boundary layer moisture hanging on across the area. Forecast dewpoints tomorrow through the weekend were blended down with part raw model guidance, but even these lower values still support heat advisory criteria across the Charlotte metro area into the I-40 corridor and portions of the Savannah River Valley in northeast Georgia. As such, a heat advisory has been issued for these areas from noon to 8pm tomorrow. Additional heat advisories will likely be needed through the rest of the holiday weekend. Precautions should be taken if spending time outdoors as this heat will be dangerous with quick onset of heat illness.

Additionally, afternoon thunderstorms will be possible across the mountains each day. The environment will be favorable for any storms that develop to become severe. This afternoon's environment is rather high end with over 5000 J/kg of surface-based CAPE, steep low- level lapse rates and large DCAPE. Any deeper storms will be capable of damaging winds and large hail. Forecast confidence is lower, however, in regards to coverage with guidance having a hard time depicting forcing mechanisms and how much will get going. This pattern will repeat the next several days with activity mainly confined to the mountains. An isolated storm cannot be ruled out into the foothills, but is much less certain.

Key message 2: The heat threat begins to wane late weekend and into early next week with a return to a more traditional mid summer pattern featuring seasonable temperatures and daily storm chances.

By Sunday, the upper ridge weakens and shifts offshore with south/southeasterly return flow from the Gulf/Atlantic. 850mb temperatures will still be rather warm and could support another day of dangerous heat into Sunday, but it will be highly dependent on cloud cover and potential for greater coverage of afternoon thunderstorms. Perturbed northern stream flow is progged to sag into the Ohio Valley with embedded shortwave perturbations passing near the area, including a more well defined shortwave trough around Tuesday/Wednesday. This will support a return to a more typical summer pattern with seasonable temperatures in the low 90s and daily thunderstorms across the region. As with any summer storms, at least some of these may become severe with damaging winds and small hail.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR conditions are expected to prevail at most terminals through the TAF period. An afternoon cumulus field has developed across much of the area, but cloud bases are expected to remain high enough to not result in any restrictions. Isolated to widely scattered afternoon thunderstorms are possible mainly across the mountains with a PROB30 group maintained at KAVL. Heading into tonight, the main focus will once again shift to mountain valley fog and low stratus. The setup doesn't quite look as ideal as this morning, but at least some degree of visibility and ceiling restrictions can be expected. Any fog should quickly mix out after sunrise with another cumulus field developing through the late morning hours.

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

CLIMATE

RECORDS FOR 07-02

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 93 1970 65 1943 70 2018 44 1988 1954 1931 KCLT 101 1931 64 1943 76 1991 56 2008 1970 1931 KGSP 101 1954 66 1943 77 1931 53 1899

RECORDS FOR 07-03

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 93 1970 65 1988 70 2018 50 2008 1897 1931 1937 1932 KCLT 99 1931 70 1988 76 1925 56 2010 KGSP 99 2016 70 1988 77 1925 56 1984 1970 1953

RECORDS FOR 07-04

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 97 1948 68 1976 70 2016 51 1986 KCLT 99 1993 70 1968 76 1993 55 1933 1955 KGSP 100 1993 70 1988 75 2018 58 2021 1996 1933

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from noon to 8 PM EDT Thursday for GAZ026-028- 029. NC...Heat Advisory from noon to 8 PM EDT Thursday for NCZ036-037- 056-057-069>072-082. SC...None.