textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Heat Advisory for today was expanded to Greenwood, Abbeville, Anderson, and Oconee (non-mountains) counties.

SPC Day 1 tornado threat was expanded and now includes all of western NC, the northern SC Upstate, and extreme northeast GA.

KEY MESSAGES

1. Severe weather is possible this afternoon and evening with the main hazards being damaging winds and large hail, but there will also be an isolated tornado threat. Make sure you have multiple ways to receive warnings. 2. Heat and humidity peak today. A Heat Advisory remains in effect for portions of northeast Georgia until 8 PM and was expanded to include the western South Carolina Upstate. Stay hydrated, limit time outdoors and spend as much time in air conditioning as possible. Never leave people or pets alone in a closed car. Look before you lock! 3. Less humid, drier, and cooler behind the front Wednesday through Friday. Even cooler conditions expected over the weekend with shower and thunderstorm chances returning.

DISCUSSION

Key message 1: Severe weather is possible this afternoon and evening with the main hazards being damaging winds and large hail, but there will also be an isolated tornado threat. Make sure you have multiple ways to receive warnings.

We'll have one more day in the active, northwest-flow "ring of fire" pattern we have been locked into since late last week. A loosely organized cluster of storms currently over West Virginia, Kentucky, and northeastern Tennessee will approach the area late this morning. Could not rule out a few strong, sub-severe storms but the severe threat should remain low through the morning hours.

Cloud cover associated with the prefrontal WAA, and/or cirrus debris from the morning storms, still could inhibit convective development later in the day, though without a strong cold pool from an MCS it's hard to see that completely capping off profiles and thus is unlikely to preclude a second round of activity nearer the actual cold front, this afternoon and evening. PoPs are lower in the afternoon for the southwest quadrant of the CWA, where the morning activity (and arguably strong downslope flow) has the most potential to keep things quiet. PoPs are highest in the afternoon along the TN border for upslope, but also in the eastern Piedmont south of I-85, where morning activity is likely to have the least effect. However we will have to monitor convective trends and surface obs throughout the morning to evaluate the likelihood of the second round taking shape. By late afternoon or early evening, assuming no inhibition, 2000-3000 J/kg of SBCAPE still may overlap with 40+ kt of northwesterly 0-6km bulk shear (and strongly curving hodographs). As the cold front looks to advance SE'wd through the NC Piedmont accompanied by the shortwave, discrete supercells remain the favored mode there. Damaging winds and large hail perhaps approaching 2" in diameter are certainly on the table this afternoon, but 0-1km helicity exceeds 100 m2/s2 in some areas so there will be an isolated tornado threat, especially across the Charlotte metro. Farther west, the frontal orientation is more E-W and a more linear mode may result, though still with wind/hail risk from embedded supercell structures. SPC Day 1 Outlook keeps the Slight Risk for virtually the whole CWA, but with the probabilistic contours more suggestive of the greatest threat being in our eastern zones.

Key message 2: Heat and humidity peak today. A Heat Advisory remains in effect for portions of northeast Georgia until 8 PM and was expanded to include the western South Carolina Upstate. Stay hydrated, limit time outdoors and spend as much time in air conditioning as possible. Never leave people or pets alone in a closed car. Look before you lock!

Increased SW'ly flow will allow highs to end up around 4-8 degrees above normal this afternoon. Afternoon temperatures will climb into the lower to mid 90s east of the mountains and the upper 80s to lower 90s across the typically warmer mountain valleys. Heat indices east of the mountains will climb into the upper 90s to lower 100s, with a Heat Advisory criteria in effect through 8 PM for Elbert, Hart, Franklin, and Stephens counties in NE GA as heat indices from 105 to 108 degrees can be expected. Went ahead and expanded the Heat Advisory to include Greenwood, Abbeville, Anderson, and Oconee (non- mountains) counties as these locations will see heat indices from 105-108 degrees as well.

Key message 3: Less humid, drier, and cooler behind the front Wednesday through Friday. Even cooler conditions expected over the weekend with shower and thunderstorm chances returning.

Behind the front, temperatures trend back to around climo on Wednesday, with lower humidity that should keep indices below 100, and much more stable conditions such that precip chances remain near zero, except for the TN/NC border area where upsloping could kick off a shower or two. Appreciably drier air filters in for Thursday that brings temps slightly lower but heat indices more noticeably lower. Similar conditions last through Friday. The Upstate and NE GA, being warmer, actually see RH fall to around 35% Thursday and Friday, which is exceptionally low for this time of year. A few spots with enhanced mixing could go even lower.

The Western ridge / Eastern trough pattern lingers into next weekend, though not as unstable upstream and MCS threats do not look to recur. Precip/storms may return Saturday, when a reinforcing shortwave or upper low rotates into the Eastern trough and reactivates a warm front over the Southeast; PoPs are now likely for Sunday. The 00z GFS and ECMWF actually seem to occlude the low to our west by Sunday, whereas the Canadian GDPS maintains an open wave, which is more similar to earlier solutions. Thus confidence and PoPs decrease Monday given the spread in the speed of the low pressure system. Temperatures do remain mild, likely at least a little below normal.

AVIATION /13Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Winds will be light/VRB at the beginning of the period but will pick up from the SW 13-16z with diurnal mixing (exc. NW at KAVL) and gusts of 20 kt are expected to be common around the area by midday. Conditions are favorable for storms to form to our north and west during the morning, ahead of an approaching cold front. A weakening band of SHRA perhaps with an embedded TS may reach the sites between 15-18z; included VCSH and an uptick in high altitude cloud cover around that time everywhere except KAND. With that activity looking unlikely to bring a cold pool to the area, confidence has increased for TSRA impacts later in the afternoon to early evening. Went with PROB30s at KAVL and the SC sites in part due to less confidence on timing, though confidence has increased on occurrence; if timing confidence increases these PROB30s could easily be replaced with TEMPOs. At KCLT and KHKY the timing confidence is better and promotion to TEMPO has already been done. Environmental factors suggest TSRA will easily produce strong outflow winds and torrential downpours, pose a threat of localized damaging winds, and with greater than normal hail potential than most storms in this area this time of year. VRB gusts to 30 kt and IFR vsby in the conditional groups. Activity should shift out of the region this evening as the front passes, and winds shift to NW at all sites behind the front. A few SHRA could develop near KAVL overnight due to upslope flow into the mtns. Post-frontal drying could inhibit fog formation tonight but can't rule out some shallow fog after 06z especially where heavy rain falls today.

Outlook: Dry and mostly VFR conditions return Wed thru Fri, with shower and thunderstorm chances and their associated restrictions returning over the weekend. Low stratus and/or fog will be possible each morning, mainly in the mtn valleys.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory until 8 PM EDT this evening for GAZ018-026-028- 029. NC...None. SC...Heat Advisory until 8 PM EDT this evening for SCZ010-011-019- 104.