textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Increased confidence for isolated severe weather on Sunday.

KEY MESSAGES

1. Highly isolated showers and thunderstorms this evening, then greater coverage of storms across the area Sunday. Confidence has increased that thunderstorms will pose a damaging wind threat Sunday, with some potential for swaths of tree and minor structural damage, mainly in Northeast Georgia and the western Upstate. Hot and muggy through Sunday, but cooler Monday. 2. Drier and slightly below-normal temperatures Monday and early next week, with moisture and rain chances increasing in the second half of the week.

DISCUSSION

Key message 1: Highly isolated showers and thunderstorms this evening, then greater coverage of storms across the area Sunday. Confidence has increased that thunderstorms will pose a damaging wind threat Sunday, with some potential for swaths of tree and minor structural damage, mainly in Northeast Georgia and the western Upstate. Hot and muggy through Sunday, but cooler Monday.

Subtle stalled frontal boundary across the Carolinas this afternoon mainly evident in a gradient of PWAT and dewpoint from the mountains to the Piedmont, but very subtle. We appear to lack any significant forcing mechanism this afternoon aside from whatever convergence is present along the front. Profiles are relatively dry aloft, similar to yesterday, when convection seemed only to fire with aid of a cold pool boundary and/or shortwave, which are not present this afternoon. Thinking is the dry profiles make deep convection generally unlikely, though SBCAPE of 1500-2000 J/kg is analyzed per SPC Mesonalysis, along with 1000+ DCAPE and less than 20 kt of 0-6km shear. Highly isolated thunderstorms remain possible this afternoon and evening. It would appear if any fire, they could pose a threat of strong to severe 40-60 mph winds. A few isolated showers/storms could develop this evening over the lower/eastern Piedmont (Union SC to Union NC) well after peak heating, if sea breeze moves as far inland as some models depict. Heat indices in the lower Piedmont are likely to top 100 this afternoon but any instances of 105 should be isolated.

Sunday, as upper low swings across Ontario within a broader trough, a cold front will advance across the Ohio Valley during the afternoon. The current stalled front looks to lift north and/or wash out by morning. Moisture advection ahead of the front should bring higher PWATs into our area. 0-6km shear will rise to 20-30 kt in the stouter upper flow around the trough, with SBCAPE similar if not higher owing to temps trending slightly warmer and more humid again. DCAPE and sfc-midlevel delta-theta-e values remain high in the morning, though the increasing lower level moisture diminishes that over the course of the day. CAMs have largely depicted a prefrontal trough promoting convective development to our west and some degree of upscale growth along that axis, and/or along cold pools. Concern for organized storms, posing primarily a wind threat, has increased. CAM consensus is best over north GA into the western Upstate. SPC upgraded most of our CWA to a Slight Risk on the midday Day 2 update today, with the initial GA/SC area of concern being combined with an expanded risk areas in the Mid-Atlantic. The CAM consensus still could shift and thus changes in placement remain possible when this period becomes Day 1 tonight. Looks like another day with heat indices in the 100-104 range along and southeast of I-85. Convection looks likely to end soon after sunset assuming the activity will mainly be cold-pool driven; a few showers could keep going later into the night along the TN/NC border with NW flow developing, in areas not worked over by convection earlier in the day.

Key message 2: Drier and slightly below-normal temperatures Monday and early next week, with moisture and rain chances increasing in the second half of the week.

The surface cold front should settle past the area Sunday night and confidence has increased slightly that we will be on the cool/dry side of the front Monday, such that PoPs now remain 15% or less through the afternoon. Temps return to near normal and heat index should remain well below 100. The front will remain stalled to our south for a day or two. Small PoPs return to the forecast in some of our SE zones at times Monday night through Wednesday, which are plausible in that as the base of the upper trough approaches from the west, it could initiate warm upglide over the front. These chances also simply could be explained by old model runs, which showed the front not clearing the area before stalling, being part of the blend. Most likely those periods will be dry, especially in the northern zones. Temperatures trend still cooler Tuesday, topping out a few below normal, returning to near normal Wednesday.

Confidence decreases for the second half of the week as the front may interact with a weak disturbance exiting the western Gulf, and/or as the next upper trough approaches from the NW along with a cold front. A return to more active weather, with periodic chances for showers/storms, appears to be a good bet. A majority of guidance depicts a tight gradient over the area Thursday ahead of the trough, which resulted in robust gusts falling out of the NBM, and could indicate some organized severe weather threat around that time. Confidence is low, as noted. Currently not expecting heat indices to be a concern.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Winds remain relatively light over the area this afternoon, with directions trending toward SW by late afternoon. An isolated SHRA/TSRA cannot be ruled out anywhere, but only KAVL and KHKY had chances warranting a mention. Light/VRB winds for the most part overnight. Radiation fog setup possible at KAVL especially if convection passes over the field this afternoon, but at least light fog possible even if not, so included TEMPO for MVFR vsby and SCT clouds at IFR level. Winds pick up from the SW late Sun morning. Chances for SHRA/TSRA minimal 12-18z Sun, but will have to monitor for potential strong TSRA and winds late Sun aftn. For now just VCSH at KCLT as a hint.

Outlook: Relatively quiet Monday thru midweek. Valley fog and low stratus is possible each morning, primarily in the mountain valleys.

GSP WATCHES/WARNINGS/ADVISORIES

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