textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes for this forecast cycle.

KEY MESSAGES

1. Scattered diurnal convection will develop this afternoon through Sunday, posing a continued threat of isolated flash flooding and perhaps a couple of pulse severe storms producing microbursts. 2. Temperatures will climb back above normal on early next week, after which it will be hot and humid each day through mid-week. Some Piedmont locations could have afternoon heat index between 99-103 beginning Monday.

DISCUSSION

Key message 1: Scattered diurnal convection will develop this afternoon through Sunday, posing a continued threat of isolated flash flooding and perhaps a couple of pulse severe storms producing microbursts.

Convection is steadily increasing in coverage and intensity over the southern Appalachians this afternoon within a moderately unstable air mass, while activity outside the mountains has so far been very spotty. Convection-allowing guidance continues to indicate that this trend of convection being heavily mountain-centric will continue through the afternoon. This makes sense, as steering currents are largely parallel to the mountains, upper ridging associated with a western Atlantic anticyclone is steadily building westward, and the atmosphere over the Piedmont is a bit drier than on Thursday. Still, sbCAPE of 2000+ J/kg and low LFCs warrants a token 20-30% chance across much of the Piedmont. Buoyancy is adequate to support a few pulse storms producing strong-to-severe microbursts. Steering currents are bit more brisk (15-20 kts) compared with recent days, but small scale areas of cell training and/or anchoring of storms on terrain features remain possible, posing a continued threat of locally excessive rainfall/flash flooding.

There's very little to hang one's hat on in terms of notable changes to the overall pattern and atmospheric regime on Saturday/Sunday: moderate afternoon destabilization again expected, and SW steering currents keeping the bulk of diurnal convection initiating over the high terrain being contained to the mountains. Having said that, guidance features a little more in the way of activity across the Piedmont/foothills Saturday, so PoPs are slightly higher southeast of the Blue Ridge. The potential for a few strong-to-severe pulse storms and localized flash flooding will continue through the weekend. Temperatures are forecast to be within a degree or so either side of climo.

Key message 2: Temperatures will climb back above normal on early next week, after which it will be hot and humid each day through mid-week. Some Piedmont locations could have afternoon heat index between 99-103 beginning Monday.

The attempt of the subtropical ridge to reassert itself over the region next week will be somewhat thwarted by periodic height falls progressing across the northeast Conus. The result of this battle will be a generally flat upper flow pattern atop the Southeast and Mid-Atlantic for much of the week. Nevertheless, deep layer WSW flow will result in temps climbing 2-3 degrees above normal by Monday...more or less remaining there through the end of the week. Dewpoints will be near seasonal norms...perhaps slightly above...and this will likely result in afternoon Heat Index nearing...possibly exceeding 100 across portions of the Piedmont each afternoon. Having said that, the forecast pattern is not conducive for a protracted excessive heat event, and any occurrence of Heat Advisory criteria being met is likely to be localized and brief. Typical mid-summer diurnal convective coverage is expected each day...generally scattered across the mountains and isolated elsewhere.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Diurnal convection will continue to expand across the mountains through mid-afternoon, with tempo TSRA warranted at KAVL beginning at 18Z. Steering flow from the SW will bring some mountain convection into the NC foothills during late afternoon/early evening, justifying a Prob30 for TSRA at KHKY. Prob30s are also carried at KGMU/KGSP, but the latest high resolution model guidance continues to depict little-to-no convection in the Piedmont, so no mention of convection is carried at KAND/KCLT. Conditions late tonight/early Saturday will again be favorable for development of low stratus and/or fog, with the best chances being in the mountain valleys, and IFR/tempo LIFR conditions are forecast at KAVL by daybreak. Signals for restrictions are weaker at the other sites, and other than MVFR visby at KHKY, the potential for restrictions is merely hinted at with SCT005 at the other sites. SSW winds of 5-10 kts are expected this afternoon, becoming light/vrbl or calm after sunrise.

Outlook: Scattered diurnal showers and thunderstorms, with associated flight restrictions are expected each day 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.