textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Backed off on dewpoint and resulting heat index forecast for Wednesday and Thursday based on verification from Tuesday. Still expect low-100s heat index each afternoon across the low terrain, but with less coverage of 105+ and thus lowered expectations for heat advisories.

Updated the aviation forecast for the impending 12Z TAF issuance.

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.

595dm upper anticyclone now positioned over the western Tennessee Valley. Wednesday's forecast looks like more or less a repeat of yesterday's, with hot and muggy conditions expected as the upper anticyclone remains largely unperturbed. Today's heat index values didn't quite reach forecasted values, despite accounting for some amount of afternoon mixing, so adjusted dewpoints down more aggressively for Wednesday...which precludes the need for a Heat Advisory today, and calls Thursday and Friday's heat index forecasts into question. Nonetheless still expecting widespread triple digit heat indices across the low terrain through at least Friday.

Long-range ensembles still depict a series of weak shortwaves beginning to erode the subtropical ridge from Saturday through Monday, culminating in a weak surface front crossing the area Saturday into Sunday, ushering in a cooler and drier air mass. Thus, still have lower temperatures forecast for Sunday and into the start of next week.

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.

Not much change to the convective forecast over the next several days, with PoP confined to the mountains where the ridgetops will act as a trigger. Lapse rates expected to remain fairly poor through Friday, limiting intensity of showers and storms, and weak capping east of the mountains should mean little if any coverage east of the mountains.

Still think that this weekend, the combined effects of deteriorating upper ridging and a weak cold front crossing the region will result in an uptick in coverage and possibly intensity of convection by Saturday and through the remainder of the weekend. Still too soon to speculate on any severe potential. A return to seasonal diurnal convection is possible by Monday if a weak surface high sets up over the western Atlantic, as depicted in the bulk of ensembles at this point.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: VFR on tap for the 12Z TAF period. Valley fog/stratus has developed as expected, but has thus far failed to make it to KAVL...and the window is fast closing for it to happen. Light and variable winds and SKC elsewhere through morning. FEW/SCT high-based cu expected again tomorrow, with variable 2-4kt winds expected through the period, perhaps favoring the S/SE during the afternoon hours. Isolated ridgetop showers and perhaps a thunderstorm or two will be possible across the mountains, but coverage is still expected to be too low to justify a mention at KAVL. SKC continues into tonight, with another round of valley fog and stratus possible.

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.

GSP WATCHES/WARNINGS/ADVISORIES

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