textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 06Z TAFs.

KEY MESSAGES

1. A slow moving front to our west will produce multiple rounds of showers and thunderstorms capable of producing heavy downpours and isolated flash flooding through Sunday evening. A low- end severe weather risk is also possible Sunday afternoon. 2. A more typical late summer pattern returns next week, but daily thunderstorm chances may remain elevated.

DISCUSSION

Key message 1: A slow moving front to our west will produce multiple rounds of showers and thunderstorms capable of producing heavy downpours and isolated flash flooding through Sunday evening. A low- end severe weather risk is also possible Sunday afternoon.

An active stretch of weather is underway as a closed upper low and associated shortwave trough slowly slides across the Mississippi Valley and into the Ohio Valley. Progression of this feature will be rather muted as it remains sandwiched between a sprawling upper ridge over the Desert Southwest and another building ridge just off the east coast over the Atlantic. Deep-layer south/southwest flow is entrenched across the Southern Appalachians with a plume of anomalously deep moisture advecting over the region. PWATs on the order of 2.2-2.4", which is near daily records, are noted east of the mountains. Gradual height falls in conjunction with an approaching frontal boundary have instigated numerous bands of convection this evening with one currently situated along the southern and eastern escarpments with additional convection across north Georgia and east Tennessee. This activity will fester through the overnight, well past the traditional diurnal peak, within a modestly forced and very saturated environment. The main concern will be for locally heavy rainfall from training storms and very efficient rainfall rates. Locally heavy amounts of 3-4" have been observed across northern portions of the Upstate and into the NC foothills. Additional rainfall across these areas, along with training over the southwest NC mountains, will be the main focus for any isolated flash flooding. Of note is an area of very impressive rainfall that fell near the NC/VA border north of the Triad. While not a concern for us, it indicates the high ceiling for locally heavy rainfall where upwards of 6-7" fell in less than 3 hours. Any totals of this magnitude will be highly localized, but very impactful.

Heading into the afternoon hours, additional rounds of showers and storms will likely develop despite copious amounts of cloud cover. It won't take much to initiate numerous to widespread coverage, especially across the eastern two thirds of the area, within the exceptionally moist airmass. Subsequent rounds of heavy rainfall will once again pose a threat for isolated flash flooding. An isolated strong to marginally severe storm or two will also be possible as deep-layer vertical wind shear increases to 20-30kts. Thermodynamic parameters are rather lackluster, however, with a dearth of DCAPE and near moist adiabatic lapse rates and tall/skinny CAPE profiles. Nonetheless, a couple water loaded downbursts aren't out of the question.

Key message 2: A more typical late summer pattern returns next week, but daily thunderstorm chances may remain elevated.

As we head into next week, guidance is in generally good agreement in the evolution of the synoptic pattern. With time, the main trough axis lifts northeast as a Bermuda ridge builds offshore. There are indications, with the greatest support from the GFS, that a weak upper low will close off over the Southern Appalachians within the base of the trough. Even if this doesn't occur, the region will remain in a broad area of weakness between two prominent ridges. While temperatures return to near normal, the pattern will likely support a continuation of above climo PoPs each day, especially should the closed upper low solution verify. The plume of exceptionally deep moisture will have shifted east of us by then, but sufficient PWATs will remain that an isolated hydro concern or two cannot be ruled out any given day.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: A myriad of restrictions will plague the forecast at all terminals through the TAF period. Ongoing SHRA/TSRA over far western NC and northeast GA will continue to make inroads into the area through the overnight and could bring associated restrictions at most terminals, especially along and west of I-26. Regardless, at least isolated to widely scattered SHRA will be possible at any point through the overnight. In addition, low stratus will continue to overspread the area with IFR becoming common with pockets of LIFR not out of the question. Fog is lower confidence with sufficient winds to keep the boundary layer mixed. Ceilings will slowly lift through the morning with an eventual return to MVFR by the afternoon hours with some terminals briefly returning to VFR. Additional rounds of showers and storms will develop again by early afternoon with more restrictions across the board. Expect another bout of low stratus again Sunday night into Monday morning.

Outlook: Shower and thunderstorm chances, along with associated flight restrictions, are expected to linger through at least the middle of next week. Patchy fog and low stratus will be possible in the mountain valleys each morning, and wherever heavier rainfall fell the previous evening.

GSP WATCHES/WARNINGS/ADVISORIES

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