textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00Z TAFs.

KEY MESSAGES

1. A stalled frontal boundary to our west produces multiple rounds of showers and thunderstorms capable of producing heavy downpours and isolated flash flooding. A low-end severe weather risk is also in store for Sunday. 2. Typical summertime weather returns next week, with hot and muggy conditions and afternoon showers and thunderstorms.

DISCUSSION

Key message 1: A stalled frontal boundary to our west produces multiple rounds of showers and thunderstorms capable of producing heavy downpours and isolated flash flooding. A low-end severe weather risk is also in store for Sunday.

Scattered showers and thunderstorms have developed across the eastern half of the forecast area this evening. Activity should continue to blossom as an outflow boundary pushes westward ahead of this activity. The severe threat will remain very low but additional strong, sub-severe storms are possible. The bigger threat still appears to be heavy rainfall, with PWATs as high as 1.9-2.1" through tonight as a 20-30kt LLJ advects more and more moisture into the region. This is near the high end of climatology, and more than enough for efficient rainfall production by any better storm developing in this environment.

Some of the hi-res guidance still hints at a round of showers redeveloping in the predawn hours Sunday. By far the bigger story, however, will be the afternoon Sunday. By this point in time, the upper trough (or low, depending on your choice of model) should have drawn close enough that more appreciable synoptic forcing as well as strong upper flow in general will cover the area. As a result, much better deep-layer shear, on the order of 20-30kts, should be in place over much of the area. As much as 2500 J/kg sbCAPE can be expected across much of the area. The hi-res guidance doesn't quite have a handle on the evolution of Sunday's convection...but pattern recognition alone says we can expect more widespread activity and some degree of organization lending itself to a low-end damaging wind threat. Still, as moist as profiles are, think any actual severe will be isolated, and the greater potential will once again be for training cells across the area, and resultant isolated flash flooding.

Key message 2: Typical summertime weather returns next week, with hot and muggy conditions and afternoon showers and thunderstorms.

Long-range ensembles continue to feature either a closed upper low or a weakening upper trough sandwiched between ridges over the western Atlantic and central CONUS, resulting in some level of moisture return persisting through the week. Without any clarity on the synoptic pattern, it's tough to say what this will mean for the convective forecast; right now it would look like a return to diurnal showers and storms is likely, but if a true closed low persists for days, we could be under the gun for more widespread coverage than normal by day, and some persistence of PoPs into the overnight.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Messy 00Z TAF period as scattered SHRA/TSRA chances should continue into late tonight. Thus, went with TEMPOs for TSRA and associated restrictions this evening into late tonight across the terminals. The SC Upstate terminals and KAVL will have the best potential to see SHRA, possibly even TSRA, develop overnight into daybreak Sunday but confidence is low so went with PROB30s. TSRA will return again Sunday afternoon/evening so have PROB30s across all terminals towards the end of the TAF period. Overnight cigs should drop to IFR/LIFR while vsbys drop to MVFR/IFR. Brief improvement is expected with both cigs and vsbys where any SHRA/TSRA develop overnight. Restrictions will be slow to lift Sunday morning. Vsbys should return to VFR by late Sunday morning or early Sunday afternoon but cigs likely will not lift to VFR until late afternoon Sunday. Winds will be SE/S through the period with low-end gusts possible starting Sunday afternoon.

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.