textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated to reflect the 00Z TAF issuance.

KEY MESSAGES

1. Evening showers and storms continue, with more expected Saturday afternoon and evening. A few storms may become strong to severe again Saturday afternoon, producing isolated damaging wind gusts. 2. Hot and humid conditions return for most of next week. Dangerous heat index values may return to some areas by Wednesday or Thursday.

DISCUSSION

Key message 1: Evening showers and storms continue, with more expected Saturday afternoon and evening. A few storms may become strong to severe again Saturday afternoon, producing isolated damaging wind gusts.

A large upper trough will continue to slide across the Mid-Atlantic into New England with the associated trough axis extending down the Appalachians. At the surface, a frontal boundary was analyzed on the east side of the Appalachians per low-level ThetaE fields. Most of the storms have pushed off to the south and east, and a cooling inversion has developed that should preclude any additional strong storms while the lingering stratiform rain and scattered showers/storms gradually dissipate. Storms should remain progressive enough to preclude a threat for flash flooding, especially with lower PWATs around 1-1.5", but isolated heavy rainfall rates could lead to nuisance ponding of water/low-end flooding.

By tomorrow, the trough lifts through New England into the Canadian Maritimes with the trough axis also shifting offshore. In its wake, a flat upper ridge extending from the Desert Southwest into the southern Plains will begin to build into the area with rising heights. At least isolated to widely scattered afternoon showers and storms can be expected, but coverage will be notably lower compared to today. However, as with any summer storms, an isolated severe storm cannot be completely ruled out.

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

Heading into next week, a stout upper ridge centered over the Southern Great Plains will shift eastward with rising heights across the Southern Appalachians. Guidance continues to peg a ~597dam H5 ridge parked over the region by mid to late week. The result will be a late summer heat wave with afternoon high temperatures climbing into the low to mid 90s. Heat indices will creep upwards as well with upper 90s to low 100s becoming common. Heat advisories could be warranted by Wednesday into Thursday, but confidence in such is low this far out. Additionally, increasing subsidence with rising heights will gradually shutoff coverage of diurnal convection. Any afternoon storms will become confined to the mountains with isolated to widely scattered coverage. It's worth noting, however, that convective signals are unreliable at this time range in the summer and the presence of any nearby boundaries or backdoor fronts could quickly bump up rain chances, even outside of the mountains.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Still dealing with some lingering deep convection mainly south of I-85, but more impactful is the trailing stratiform rain region over KCLT. Flight category should remain VFR with mainly light precip falling out of a mid-level ceiling, but a rogue lightning strike cannot be ruled out, thus the one hour TEMPO for TSRA. The deeper convection to the south continues to move away and weaken, so the lightning threat should be over by 01Z at the latest. Lots of convective debris will continue with VFR conditions and wind directions variable for the rest of the evening. By late evening, expect to see some fog and low stratus developing where the precip fell. Expect this to be variable in ceiling and vis with lots of amendments, but for now we will go with some prevailing MVFR and will amend off that. Low cloud ceilings could easily persist thru the late morning if they develop. Another round of showers and storms is expected Saturday afternoon, so PROB30s were carried at all sites. Wind should be NE to SE.

Outlook: Scattered showers and storms are expected 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.