textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 12Z 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 /12Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Another busy TAF forecast is expected to pan out again today with a mixed bag of restrictions throughout the period. Ongoing SHRA will continue through the morning with a brief lull before at least one more round of afternoon/evening storms, if not two. Associated visibility and ceiling restrictions can be expected with any of this activity, especially lower vis from very heavy rainfall rates. A widespread deck of low stratus is also entrenched across the area with MVFR to IFR ceilings common. With time, ceilings will slowly lift to MVFR by this afternoon with perhaps several hours of VFR through the mid to late afternoon hours into the evening. This will be short lived, however, as another bout of low stratus with IFR ceilings is expected tonight into early Monday morning. Fog may also be more of an issue tonight, but confidence is low right now.

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.