textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

PoPs have trended down 10-20% across most of the Foothills and Piedmont Tuesday through Thursday. No significant changes to the temperatures or heat index.

Updated the aviation discussion for the 18Z TAFs.

KEY MESSAGES

1. Scattered showers and thunderstorms are expected across the area each day through Monday, with highest coverage in the mountains. A few storms could pose a threat of isolated flash flooding and perhaps one or two severe storms producing damaging wind gusts. 2. Temperatures will climb back above normal on Monday, after which it will be hot and humid each day through the end of the week. Some Piedmont/Upstate locations could have afternoon heat index between 98-103 beginning Monday.

DISCUSSION

Key message 1: Scattered showers and thunderstorms are expected across the area each day through Monday, with highest coverage in the mountains. A few storms could pose a threat of isolated flash flooding and perhaps one or two severe storms producing damaging wind gusts.

Not much change to the pattern over the next couple of days, with upper ridging reasserting itself from the east, while broad troughing sets up across the Great Lakes to the Northeast US. Temps will warm to a category or two above normal, with continued muggy dewpts in the upper 60s to lower 70s east of the mountains within a Bermuda High setup. This should result in moderately high instability each aftn thru Monday. Shear will remain in the 15-20 kt range, which will be marginal for updrafts to organize around some outflows. Steering flow has picked up a tad, and should veer more out of the west/northwest starting Sunday, allowing for more mountain-initiated convection to track out into the Piedmont. Still expect the bulk of coverage to remain in the mountains. A few pulse severe storms producing damaging wind gusts. As for heavy rain threat, PWATs remain high, and storms could merge or train over some areas. So, isolated excessive rainfall will remain on the table in this environment.

Key message 2: Temperatures will climb back above normal on Monday, after which it will be hot and humid each day through the end of the week. Some Piedmont/Upstate locations could have afternoon heat index between 98-103 beginning Monday.

Models are in good agreement on the upper pattern shifting to a broad ridge over the Southern Plains and persistent broad trough over the eastern CONUS for most of next week. This will set up a slightly drier west-northwesterly flow regime across the CWFA from Tuesday thru at least Thursday. The latest NBM has come in noticeably drier for these periods, especially east of the mountains, thanks in part to downslope flow. This also results in slightly warmer max temps. I think the NBM dewpts might be too high during the aftn hours each day, as dewpts should mix out at least to the upper 60s to lower 70s, rather than mid 70s. So while heat index values will likely push into the 98 to 103 range across much of the Piedmont, confidence on needing a Heat Advisory remains low. PoPs are mainly confined to the mountains near the TN border, where convection upstream may try to organize into an MCS and track toward the Southern Appalachians. Confidence remains low on that, but forecast trends will be monitored closely. The 12z GFS and Canadian deterministic models continue to show a front sagging south into the area by Friday, as the eastern upper trough deepens. However, the 00z ECMWF and the LREF suggest upper ridging will actually build back in from the west and keep the front to our north. The latest NBM has PoPs increasing into the 30-50% chance range east of the mountains. We'll which trend holds.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Scattered showers and thunderstorms are already developing across western NC, warranting a TEMPO for TS at KAVL over the next 3 hours or so. The latest guidance has trended toward more convection surviving east toward KCLT and KGSP/KGMU. Thus, have added PROB30 for TS at these sites, mainly for the 23z-02z time frame. Before then, VCSH is expected mainly north and west of KCLT. Convection should steadily wane this evening. Expect mtn valley fog again tonight, with a TEMPO for LIFR conditions at KAVL in a similar time frame to last night. Some guidance is hinting at IFR stratus beginning to form across the Upstate around or just before sunrise, then could expand northeast toward KCLT and KHKY. Confidence remains low, but given some MVFR stratus forming the last few mornings, will add mention of some few to sct low clouds in the Upstate for tomorrow morning. Whatever low clouds that develop should lift to VFR around 15-16z.

Outlook: Scattered diurnal showers and thunderstorms, with associated flight restrictions are expected each day through early next week. Patchy fog and low stratus will be possible each morning, especially in the mtn valleys and anywhere that received significant rainfall the previous day/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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