textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Confidence is increasing that a more typical summer pattern will return late weekend into early next week with seasonable temperatures and daily storm chances.

Updated the aviation discussion for the 18Z TAFs.

KEY MESSAGES

1. Heat risk will steadily increase through the week, with heat indices likely exceeding 100 degrees across much of the Piedmont and foothills, especially by mid to late week. 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: Heat risk will steadily increase through the week, with heat indices likely exceeding 100 degrees across much of the Piedmont and foothills, especially by mid to late week. Daily thunderstorms will also be possible across the mountains.

The synoptic pattern starts off rather amplified today with a deep trough digging through the Great Basin while an anomalous upper ridge continues to build over the Tennessee Valley. With time, the upper ridge will migrate into the Southern Appalachians by late week through the 4th of July holiday weekend bringing a round of hot temperatures to the area. 500mb heights and 850mb temperatures will both be in the 99th percentile for early July, which will support hot afternoon high temperatures in the upper 90s to low 100s. Even mountain valleys will be warm with highs in the low to mid 90s.

While temperatures will definitely be hot, the main uncertainty is whether we will reach heat advisory criteria with heat indices of 105 degrees or greater. Forecast soundings continue to depict a deeply mixed boundary layer extending to nearly 700mb. This deep mixing will support aggressive mixing out of dewpoints with surface dewpoints potentially falling as low as the upper 50s to low 60s. The lack of a more humid airmass will generally preclude heat index values reaching advisory criteria. That being said, precautions will still need to be taken as air temperatures in the low 100s will still pose a risk of heat related illness.

Additionally, diurnally driven thunderstorms will be possible across the mountains each afternoon where terrain effects will be sufficient to overcome an otherwise unfavorable environment for convective initiation. What storms do develop will have the potential to become strong to severe owing to moderate to high instability, steep lapse rates, and modest DCAPE. The main threat would be for locally damaging winds and small hail.

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 associated with the holiday heat wave will have weakened and will be shifting off the Atlantic coast as weak northern stream flow dips farther south into the Ohio Valley. As such, afternoon high temperatures will return to the low to mid 90s and more in line with seasonal averages for early July. This should also keep heat indices in check. In addition, the return of active northwest flow and moisture return from the Gulf/Atlantic will support at least scattered to numerous afternoon showers and storms each day. This will likely be further amplified by several passing weak shortwave troughs. As with any summer storms, at least several may become strong to severe with locally damaging winds and small hail.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Mainly VFR conditions will prevail at all terminals through the TAF period. A few isolated to widely scattered showers and thunderstorms will be possible across the mountains this afternoon. As such, will maintain a PROB30 group at KAVL for associated thunderstorm restrictions. Otherwise, conditions are expected to remain quiet outside of the mountains. Another round of mountain valley fog and low stratus is expected tonight. Restrictions will once again be possible at KAVL through 01/13z. Winds will generally be light and variable through the period, but mainly out of the northeast.

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