textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes to the public forecast this afternoon.

The aviation forecast was updated to reflect the recent 18Z TAF issuance.

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.

Water vapor imagery as well as this morning's 12Z RAOB analysis depict a 574dm upper low located over central Illinois, atop a more or less vertically-stacked low pressure system evident on surface maps. A broad blocking ridge is located over the western Atlantic. And though subtle, there would appear to be a weak boundary located along I-85, at the northern terminus of a deep moisture plume advected into the area overnight. Widespread cloud cover has only very recently begun to clear out effectively, and recent runs of the HRRR have responded by backing off on coverage of showers and thunderstorms for this afternoon. Still...already seeing some showers break out across the extreme southern and eastern portions of the Upstate...and wouldn't be surprised if some of this activity continues to strengthen over the coming hours.

The upper low will only slowly make tracks eastward over the next 48 hours, deterred by the blocking ridge over the Atlantic. As a result, there'll be limited forcing for ascent through this evening, and it'll be difficult to effectively trigger much convection. Whatever cells successfully develop will encounter an environment largely inhospitable for severe weather: only some 1000-1500 J/kg sbCAPE is forecast across the northern 2/3s of the CWA (perhaps less, should lingering cloud cover hinder daytime heating much longer), shear will be quite limited, and DCAPE virtually nonexistent. Mid-level lapse rates will be practically moist adiabatic, resulting in mostly weak updrafts with only limited buoyancy. Rather, the bigger threat would appear to be heavy rainfall, with PWs 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 /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: As a plume of moisture continues to advect into the western Carolinas, lowered ceilings have been very slow to improve, with IFR still reported at a number of terminals up until an hour ago, and still widespread MVFR ceilings at 1500-2500ft. Still think there'll be improvement to VFR in the next 1 to 2 hours. Despite the lingering cloud cover, some convection has already begun to initiate south of I-85, and this activity should begin to expand as daytime heating improves and instability increases. Hi-res guidance continues to struggle with the evolution of today's activity...but generally still think that a round in the afternoon will be followed by a possible second round of rainfall around and after sunset. Thereafter, even as coverage of SHRA/TS decreases overnight, another murky night should set in, with IFR to LIFR ceilings and patchy fog redeveloping across the terminal forecast area. It's unclear how convection will develop on Sunday, but think that at least scattered coverage is likely based on current guidance.

Outlook: Shower and thunderstorm chances, along with associated flight restrictions, are expected to linger through at least the middle of next week. In addition to the usual daytime diurnal convection, some nights may feature lingering showers and thunderstorms into the first part of the overnight. Patchy fog and low stratus will be possible in the mountain valleys each morning, and wherever heavier rainfall developed the previous evening.

GSP WATCHES/WARNINGS/ADVISORIES

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