textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated to reflect the recent 18Z TAF issuance.

KEY MESSAGES

1. Diurnal showers returns on Friday and Saturday, before afternoon convection becomes more suppressed Sunday and into next week. A few storms may become strong to severe on Friday afternoon, producing isolated damaging wind gusts. 2. Hot and humid conditions return for most of next week. Dangerous heat index values will return to many areas by Wednesday or Thursday.

DISCUSSION

Key message 1: Diurnal showers returns on Friday and Saturday, before afternoon convection becomes more suppressed Sunday and into next week. A few storms may become strong to severe on Friday afternoon, producing isolated damaging wind gusts.

As of a little after midnight, convection has waned across the entire region, with a couple of lingering light showers around the NC Blue Ridge Escarpment but dry conditions elsewhere. Already seeing some patchy fog developing across the NC Piedmont, mainly in locations hit hard by rainfall Thursday evening. An ill-defined frontal boundary appears to be draped across eastern Tennessee, beneath cyclonic upper flow aloft as depicted in recent water vapor imagery and RAP analyses.

The next couple of days' weather will be driven by the evolution of these features. At the surface, we can expect to remain within a warm sector-like environment through at least Saturday afternoon, while aloft, a series of weak shortwaves will cross the western Carolinas, embedded within a deeper trough axis extending from the Tennessee Valley north into Ontario. Strong upper flow (for this time of year) will be present on at least Friday afternoon, if perhaps less so on Saturday afternoon.

Friday afternoon should feature some 2000+ J/kg sbCAPE within a weakly-sheared environment of 20-25kts of deep layer shear...particularly farther east across portions of the Upstate and the I-77 corridor. There'll be enough shear, at any rate, to support a few semi-organized segments and an isolated damaging wind threat...as well as sufficient DCAPE for downbursts even from non- organized cells. The CAMs generally suggest that convection will fire over the ridgetops during the early afternoon, then progress eastward through late evening. Saturday's environment looks less- sheared, but just as unstable...supportive of a couple strong storms, perhaps even a severe storm or two, but overall less impressive. Coverage looks like it'll become increasingly sparse on Saturday and Sunday as heights across the area begin to rise, retreating to the mountains entirely by Monday afternoon as the supporting trough lifts north.

Key message 2: Hot and humid conditions return for most of next week. Dangerous heat index values will return to many areas by Wednesday or Thursday.

By the first part of next week, long-range ensembles are in good agreement that any lingering weakness in the subtropical ridge will break down. In its place, broad anticyclonic upper flow will develop across the Southeastern United States, setting the stage for another hot, muggy week. Guidance features the development of a Bermuda High-like pattern across the western Atlantic as early as Tuesday, resulting in the steady advection of better low-level moisture into the area through the remainder of the week.

The result still looks like a return to above-normal temperatures and triple digit heat index by Wednesday...with the second half of the week likely featuring 100-105 degree heat index across much of the low terrain. Some uncertainty remains on the details, but confidence on a hot week is increasing.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Afternoon convection has already developed across the mountains early this afternoon and is expected to expand in coverage through the evening as storms move east. This will bring the potential for TSRA and associated visibility and ceilings restrictions at all terminals. Thereafter, quiet conditions are expected to return for much of the overnight. Attention then shifts to the potential for another round of patchy fog and low stratus, especially in favored mountain valleys. IFR to LIFR restrictions will be possible. Any fog should mix out quickly after sunrise with low stratus lifting and scattering as well bringing a return to VFR by mid morning. Winds will be light out of the southwest today and then northeast tomorrow.

Outlook: Widely scattered showers and storms are expected over through weekend, mostly over the mountains, then entirely over the mountains by Monday. Mountain valley fog and low stratus will be possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

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