textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

1. Dry weather with comfortable humidity levels will continue through Tuesday. Then humidity increases along with daily shower and thunderstorm chances returning Wednesday and continuing through the rest of the week.

DISCUSSION

Key message 1: Dry weather with comfortable humidity levels will continue through Tuesday. Then humidity increases along with daily shower and thunderstorm chances returning Wednesday and continuing through the rest of the week.

The upper pattern still consists of a large anticyclone over New Mexico and an upper low over the eastern Great Lakes. This pattern effectively persists through Tuesday, even as the upper low migrates eastward into Maritime Canada. Northerly low-level winds will advect dry continental air into the region today, and even though winds may favor southwest again Tuesday, they will be light and incapable of significant moisture return. Vertical profiles remain dry/subsident and unfavorable for deep convection, so PoPs remain near zero through Tuesday night. Daytime temperatures should run near normal. Afternoon mixing will maintain low dewpoints in the lower 60s, with upper 50s across much of the mountains and in the better-mixed Piedmont locations. Low temps tonight or Tuesday morning will be a few degrees below normal again on account of the lower dewpoints and good radiational cooling conditions. Slight moisture return does begin Tuesday night, so Wednesday morning temps will be closer to normal.

On Wednesday, just as heights trend upward behind the Maritimes low, another well defined shortwave rotates over Lake Superior, in turn strengthening cyclonic flow over the eastern CONUS. Winds turn more decidedly S to SW over our region as the stalled front south of our area is reactivated. Temperatures stay similar but afternoon RH will be more typical for this time of year. The moistening low levels make profiles again favorable for deep convection, and scattered showers/storms return to the forecast Wed afternoon; PoPs have increased to likely (~60%) over the mountains on this cycle. The cyclonic flow overlying the sfc baroclinic zone easily can result in continuing nocturnal shower/storm activity, and warrants maintaining 30-50% PoPs for most of the area into Thu morning. Some of the higher res models, including the NAM, depict convection in the Ozarks or Mid-South on Wednesday forming an MCV which could further enhance development. Even if that doesn't occur, relatively slow storm motion, potential for clustering, and elevated PWATs suggest a localized flash flood threat. On Thursday the main shortwave axis swings across the Ohio Valley carrying a cold front into that area; broadly convergent flow remains over the CWA and PWATs continue to increase across the Piedmont, with lower 70s dewpoints again likely across the Piedmont. Though upper forcing and convective parameters don't clearly increase, model QPF response is greatest that day and we end up with 60-80% PoPs.

The Great Lakes shortwave slides by to our north Thursday night, possibly pulling the cold front across the Appalachians, though with only subtle height rises thereafter, and the upper pattern gradually dampening out, it's hard to believe that effective airmass change will occur. The more likely scenario would appear to be the front stalling across the region, with drying occurring only gradually Friday through the weekend as the upper flow becomes more zonal, with a broad 500mb ridge taking shape over the southern CONUS. Daily precip chances remain high chance to likely, above climatological chances, on Friday, but trend slowly lower Saturday and Sunday. Temperatures remain near normal Friday- Saturday but with elevated dewpoints making for muggy conditions, though heat index does not look likely to exceed 100.

AVIATION /18Z MONDAY THROUGH SATURDAY/

At KCLT and elsewhere: VFR under mostly SKC this afternoon, aside from FEW cumulus at 050-070. Winds are light, generally NW across NC but VRB or occasionally SW over Upstate SC, but all locations should trend N to NE when not calm tonight. Mountain valley fog is likely to develop late tonight though most likely won't reach KAVL. Flow above the PBL is expected to turn southerly Tuesday and KAVL should flip to the SE during the morning after onset of mixing. Elsewhere mixing is more likely to result in VRB if having an impact at all, so no additional line to flip winds group appears justifiable at this time.

Outlook: SHRA/TSRA could return to the terminals Wednesday afternoon. A better chance of SHRA/TSRA is forecast Wednesday night through Friday. Mountain valley fog and low stratus is possible each morning. A chance of afternoon and evening SHRA/TSRA may persist this weekend.

GSP WATCHES/WARNINGS/ADVISORIES

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