textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Forecast Heat Index has trended down slightly for Wed and Thu afternoons, but 100-105 H.I. is forecast across much of the Piedmont and foothills.

KEY MESSAGES

1. Unseasonably hot and muggy conditions continue through the week. Relief may arrive early next week. 2. Showers and thunderstorms are forecast over the mountains each afternoon this week, but coverage and intensity should remain limited. Chances for showers and storms begin to increase for all areas over the weekend.

DISCUSSION

Key message 1: Unseasonably hot and muggy conditions continue through the week. Relief may arrive early next week.

An upper anticyclone centered over the western Ohio Valley this afternoon will gradually build toward the southern Appalachians by Thursday. This will result in unseasonably hot conditions...with max temps forecast at 7-10 degrees above climo each day through Saturday. Forecast soundings depict rather deep afternoon mixing with somewhat dry profiles through at least mid-week, which will result in surface dewpoints decreasing at least slightly during the hottest part of the day. Thus, confidence is low that widespread Heat Advisory criteria will be met over the next couple of days...or at least low enough to preclude issuance of any headlines at this time. Nevertheless, plenty of H.I. of 100-105 is forecast across the Piedmont, portions of the foothills, and perhaps even the lowest mountain valleys. By the end of the week, even if Heat Advisory criteria aren't forecast to be met, thought may need to be given to impact-based heat products, owing to the attritional effects of multiple days of unseasonably hot weather...especially with the holiday weekend looming.

By the start of the weekend, the global models suggest the anticyclone/associated ridge will begin a quick retrograde toward the central Conus, as a long wave trough sets up just off the East Coast. Attendant frontal zone is forecast to enhance convective chances/cloud cover and lower temperatures early next week, with near-normal temps forecast by Monday.

Key message 2: Showers and thunderstorms are forecast over the mountains each afternoon this week, but coverage and intensity should remain limited. Chances for showers and storms begin to increase for all areas over the weekend.

The latest mesoanalysis depicts moderately unstable air across the forecast area this afternoon, with radar imagery revealing a few pin-prick showers developing over the high ridge tops of western NC. Anticyclonic flow aloft will limit the overall coverage of convection this afternoon, as well as on Wednesday afternoon, but 20-30 PoPs are warranted along and just downstream of the ridge tops both days.

Although the center of an upper level anticyclone will become increasingly centered over the southern Appalachians during the latter half of the work week, forecast soundings over the mountains are uncapped each afternoon, while conditions will be increasingly hot and unstable. A slight uptick in diurnal mountain convection is therefore forecast Thu/Fri. By contrast, forecast soundings outside the mountains are at least weakly capped and/or feature high LFCs, so PoPs southeast of the Blue Ridge remain < 20% each afternoon through Friday.

Upper heights fall over the weekend, with the anticyclone/ridge forecast to quickly retrograde to the central Conus, and a trough forecast to deepen off the East Coast. This may introduce a weak frontal boundary to the region, with an overall increase in convective potential forecast for the entire CWA Saturday, with solid chances for diurnal convection continuing through at least Monday.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: FEW high-based cumulus will linger for a time tonight though skies eventually should be SKC, aside from mountain valleys where fog/stratus is expected to play out similarly to how it did Tuesday morning. Hence, at KAVL retained TEMPO LIFR mention at daybreak. Aside from KAVL, expect VFR thru the period. Winds should be light overall with anticyclone centered over the region through Wed; used VRB liberally on that account. There however should be a slight tendency toward northerly winds at KAVL/KCLT, and E to SE winds at the SC sites, by peak mixing/heating Wed afternoon.

Outlook: Isolated to scattered mountain convection expected through Friday, then expanding to all areas for the weekend. Fog and low stratus will be possible each morning, primarily in the mountain valleys.

CLIMATE

RECORDS FOR 09-02

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 91 2020 65 1928 70 2020 45 1967 2011 KCLT 99 1932 67 1928 74 1993 53 1967 1947 1941 KGSP 100 1912 65 1928 73 2020 50 1887 1947 1932

RECORDS FOR 09-03

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 92 2020 66 1994 70 1993 44 1967 1920 KCLT 98 1925 69 1994 75 1993 52 1967 1990 1941 KGSP 97 1962 65 1994 74 2015 52 1967 1944 1925

RECORDS FOR 09-04

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 95 1954 67 1913 70 1925 45 1952 KCLT 100 1925 70 1974 75 1925 54 1952 1886 KGSP 100 1954 67 1920 76 1925 52 1892 1925 1905

GSP WATCHES/WARNINGS/ADVISORIES

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