textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for 12z aviation package.

Similar thinking for isolated excessive rainfall potential today, diminishing Saturday.

Revised details about possible convective complexes making a run at the area Monday or Tuesday nights.

KEY MESSAGES

1. Potential for at least localized excessive rainfall continues through Saturday. 2. A return to typical mid-summer weather begins Sunday and continues through Tuesday, with hot temperatures and at least isolated diurnal convection expected. Another cold front may reach the area in the middle of next week.

DISCUSSION

Key message 1: Potential for at least localized excessive rainfall continues through Saturday.

A stationary front will remain south of the forecast area through Saturday, with at least weak southerly upglide continuing atop it. This front is situated near the base of a broad trough extending across the northeastern quadrant of the CONUS. PWATs in the frontal zone will remain at or above 2 inches through today along with at least a few hundred joules of MUCAPE for parcels elevated above the frontal inversion. A few moderate-intensity showers are possible nearer the front this morning, mainly over the Savannah Valley and adjacent parts of the Upstate; otherwise precip this morning will mainly be just drizzle or sprinkles. Our area will not have the degree of instability today that was seen on Thursday owing to extensive stratus, and for at least part of the day we will be in a relative lull in upper forcing between embedded shortwaves. Hence diurnal convective coverage is expected to be a bit less than Thursday, with the heavy rain concern greatest in those areas nearest the front--in our southern CWA. CAM response is rather muted/noisy-looking during the day. However, steering flow aloft will remain westerly, quasi-parallel to the front, and this suggests any convection that does fire will be capable of exceptionally high rainfall rates. WPC maintains a Slight Risk over the southern half of the CWA. By late afternoon some more weak vort maxes will approach from the west and in turn some CAMs suggest convective development mainly during the evening. The excessive rain threat will be somewhat mitigated by the best coverage today appearing in our south which generally saw the least rain on Thursday. That is a reason why no Flood Watch is in effect for today.

The aforementioned weak vort lobes promote a deepening of the upper trough such that the 500mb pattern is more amplified for Saturday. Some drying does occur aloft which will shift the band of frontal moisture still further south. Weak downslope flow in low levels may help scour out clouds particularly in the NW CWA and in turn convection should reflect a somewhat more diurnal trend.

Key message 2: A return to typical mid-summer weather begins Sunday and continues through Tuesday, with hot temperatures and at least isolated diurnal convection expected. Another cold front may reach the area in the middle of next week.

With northwest flow continuing aloft Saturday night into Sunday, drying continues and the frontal zone should continue to progress southward. The upper pattern continues to amplify with a broad 596+ DM anticyclone remaining over the Rockies/Plains with heights actually falling over the northeast coast. Temperatures trend warmer Sunday-Tuesday, a little above normal, as the ridge builds to our west and as SW flow is re-established. Diurnal convection should be possible over the mountains each day which could propagate into the foothills/Piedmont. Especially hot and humid conditions will develop in the Midwest with a NW-SE oriented thermal gradient suggested by the upper pattern. Models differ in the timing of the shortwaves rounding the ridge and diving into the trough but it would appear we have at least a chance of an MCS reaching the NC mountains between Monday night through early Wednesday. Shear appears favorable but the degree of instability east of the mountains may be a limiting factor. An upper low could form within the trough by the middle of next week, which could bring a cold front to the area and/or clouds/precip; these possibilities seem to account for model blend temps trending cooler Wed-Thu.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Widespread IFR stratus, with a few holes of only MVFR, across the terminal area early this morning north of a stalled front. A few patches of -DZ or -SHRA will occur; heavier precip may lead to improvement though if/where -DZ forms cigs could fall to LIFR. Slight improvement in cigs/vsby is expected as winds weaken diurnally over the front, though predominantly IFR cigs should prevail through the day. SHRA are expected to redevelop during afternoon, perhaps with a few embedded TSRA near KAVL and KAND. PROB30s are retained at all sites, with IFR vsby likely in moderate to heavy rain. Mainly breezy NE winds today except SE at KAVL. Restrictions should worsen with IFR to LIFR setting back in Friday night after the aftn/evening activity diminishes, winds go nearly calm, and some -DZ may return. Development of drier NW winds aloft should lead to gradual improvement thru Saturday morning.

Outlook: Convection will redevelop diurnally Saturday afternoon. As the front finally settles further south, some drying is in order for Sunday and Monday, so convection and restrictions look less likely. Low stratus and/or fog will be possible each morning in mountain valleys and any locations which receive heavy rainfall by day.

GSP WATCHES/WARNINGS/ADVISORIES

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