textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion, otherwise no significant changes.

KEY MESSAGES

1. A few strong to severe thunderstorms again today and Saturday with damaging wind gusts being the main threat, along with isolated flash flooding where storms persist for a few hours. 2. Temperatures will climb back above normal on Sunday, after which it will be hot and humid each day through mid-week. Some locations could have triple digit heat index in the afternoon starting Monday, mainly along and southeast of I-85.

DISCUSSION

Key message 1: A few strong to severe thunderstorms again today and Saturday with damaging wind gusts being the main threat, along with isolated flash flooding where storms persist for a few hours.

The western Carolinas will remain in a relative weakness in the overall upper pattern in between the large anticyclone centered over the Four Corners region and the Atlantic Subtropical Ridge. The mid-level trof axis should remain to the west through Saturday before either lifting out to the north or filling on Sunday. Typically, when we have this pattern in the summertime that means above normal chances of mainly diurnal showers and storms, so the forecast will reflect that over the mtns at least, with likely/categorical precip probs. The model blend might be a bit deficient in the probs to the south/east of I-85 today and Saturday, given the pattern, so a minor upward nudge might be in order. The main concern for both days will be a few strong to severe storms and isolated flash flooding if the past few days is any indication. The afternoons should have roughly 2500 J/kg of sfc-based CAPE and favorable sfc delta thetaE, though lapse rates will be modest at best, but this should be enough to allow for some strong outflows that could become severe. As for the excessive rain potential, we note that the forecast area remains in the Marginal Risk through 12Z Sunday, which is reasonable given the continued high precipitable water above 2 inches outside the mtns and fcst soundings showing deep warm cloud depths and low storm velocity. It is August after all, so this tracks. As for temperatures, they should stay right around normal for this time of year.

Key message 2: Temperatures will climb back above normal on Sunday, after which it will be hot and humid each day through mid-week. Some locations could have triple digit heat index in the afternoon starting Monday, mainly along and southeast of I-85.

From Sunday into Monday, the Atlantic Subtropical Ridge will build westward again, flattening the flow aloft and bringing our region more under the influence of the Bermuda High. This should do two things for us. One, precip chances will drop back to climo or less each day, favoring the mtns in the afternoon. Two, high temps should climb back above normal starting Sunday and then reaching the mid-90s over the Piedmont and Upstate zones by Tuesday. Combined with an increase in dewpoint, that should push the afternoon heat index above 100F in some spots, mainly around metro Charlotte, assuming a frontal boundary remains to our north through the period. Confidence is marginal this far out, but another round of heat advisories is possible. Eventually, by late in the week, an upper trof is expected to dig over the east, which could either bring us back into a storm track from the west or allow the boundary to drop in from the north.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Waking up to widely varying and scattered ceiling restrictions across the fcst area, such that any terminal could have a brief restriction anywhere from LIFR to MVFR. Most will probably stay VFR. The ceiling situation should sort itself out with some daytime heating. This could pop up a low cloud MVFR ceiling for a few hours until the cloud base lifts above 030. Otherwise, a continued light SSW flow again today,with more diurnal thunderstorm activity. A PROB30 will be employed this afternoon at all terminals, until a consensus is reached on the most likely time for storms. The activity should wind down again shortly after sunset, after which we will have convective cloud debris and eventually another chance of fog development where rain fell in the past 24 hours.

Outlook: Scattered showers and thunderstorms, with associated flight restrictions through early next week. Patchy fog and low stratus are possible each morning, especially in the mtn valleys and anywhere that received significant rainfall the previous day/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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