textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

High temperatures trended slightly cooler for Monday and Tuesday.

Heat index / apparent temperature has trended 1 to 2 degrees cooler for Sunday, Monday, and Tuesday.

Updated the aviation discussion for the 18Z TAFs.

KEY MESSAGES

1. Hot, humid conditions continue Sunday and into the new workweek. Those with outdoor plans should take steps to remain hydrated and take breaks indoors when possible. Never leave children or pets unattended in hot vehicles. 2. Scattered showers and thunderstorms are expected each afternoon through Tuesday, with a low-end threat of severe weather, mainly due to damaging winds. 3. The pattern becomes increasingly unsettled the latter half of the week, resulting in more showers and thunderstorms, but not necessarily confined to the afternoons.

DISCUSSION

Key message 1: Hot, humid conditions continue Sunday and into the new workweek. Those with outdoor plans should take steps to remain hydrated and take breaks indoors when possible. Never leave children or pets unattended in hot vehicles.

Hot and muggy conditions are continue across the western Carolinas Sunday afternoon, but should be somewhat less pronounced today than the previous two days. Competing effects are expected: guidance broadly depicts overall lower low-level thickness values across most of the area compared to the last two days, with the core of synoptic-scale ridging now centered to our east; however, several CAMs feature a weak lee trough in place through the afternoon (there's already some evidence of a low-level convergence band setting up from perhaps the Shelby/Rutherfordton area, northeast up to I-40) which could result in higher downstream temperatures over localized portions of the NC Piedmont in particular.

There's also a big question mark in the form of afternoon convection (see key message #2) today, which would further disrupt afternoon high temperatures.

All things considered, the likelihood of hitting true Heat Advisory criteria across much of our area is pretty low for this afternoon. So, no plans to issue one with the afternoon forecast package for Sunday. Heat and humidity will continue each day this week, but generally looks like the highest-impact temperatures are over with. Forecast temperatures look to remain above normal by a category or two through the week (so, mid-90s outside the mountains) but with heat index values struggling to reach into the triple digits.

Key message 2: Scattered showers and thunderstorms are expected each afternoon through Tuesday, with a low-end threat of severe weather, mainly due to damaging winds.

Persistent subtropical ridging is expected to remain in place for much of this week. Guidance broadly agrees this will keep upper troughing and any associated synoptic-scale forcing shunted well to our northwest today through Tuesday, before the pattern gets a little more progressive the latter half of this week (see third key message). Through at least Tuesday, we'll remain within a moisture- rich air mass directly exposed to the Gulf, with a frontal zone stalled well to our north. Mainly diurnal convection can be expected each afternoon/evening. Each day, initiation can generally be expected over the NC mountains, but confidence on convective evolution remains limited as is typical on these pulse days.

For Sunday, there looks to again be a low-end severe weather risk. Veering of low- and mid-level flow has resulted in some improvement of 700-500mb RHs, and a resultant decrease in DCAPE compared to the last few days. Seeing some evidence of a developing lee trough along and just east of the NC Blue Ridge Escarpment...just as predicted by recent runs of the HRRR and RRFS...and in addition to the typical ridgetop initiation, could see the convergence zone ahead of the trough serving as a focus for further initiation. Indeed, most members of the 12Z HREF depict a loosely-organized band of showers and thunderstorms pushing out of the mountains by mid-afternoon and tracking eastward across the NC Piedmont and northern SC Upstate. This is clearly not shear-driven (most of the CAMs feature a paltry 5-15kts of deep layer shear), but rather may be focused by the lee trough initially, then outflow-driven thereafter. In a 1500-2500 J/kg sbCAPE environment, though, we can expect some hefty updrafts, and with even a marginal 600-800 J/kg sbCAPE, a low-end damaging wind threat is within reason. Agree with SPC's Marginal Risk of Severe Weather for today.

The convective setup looks a little more scrambled on Monday. We won't have the strong cross-boundary component to winds over the mountains, and without the presence of a lee trough / convergence zone, the CAMs depict more ridge-driven initiation and look less organized overall. Deep layer shear may become marginally higher by Tuesday afternoon, as a cold front settles across Tennessee and Virginia, and upper flow becomes elevated by an approaching shortwave. Values of 15-20kts as depicted in the GFS, GDPS, and their ensembles would support at least a modicum of organization, even though any frontal passage will likely be delayed until at least early Wednesday morning.

Key message 3: The pattern becomes increasingly unsettled the latter half of the week, resulting in more showers and thunderstorms, but not necessarily confined to the afternoons.

The pattern should become more active the second half of next week. Synoptic guidance agrees on the gradual retreat of subtropical ridging Wednesday and Thursday, which should allow us more opportunity for some shortwaves to make it into the area. Indeed, the operational models all feature a few low-amplitude trough axes crossing the area during the Thursday-Saturday timeframe. Confidence remains quite low on timing and details of any such features, but in general, would expect the potential for more synoptically well-forced convection late in the week, should upper features align well with the thermodynamics. The environment during this period looks like it'll feature better shear as well, so there's potential for more organized convection as opposed to the typical summertime pulse convection we've been dealing with recently.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: Mostly VFR conditions expected to prevail through the 18Z TAF period, except where thunderstorms develop this afternoon. Already seeing widespread initiation cross the mountains, and to a lesser extent, the Piedmont and Upstate. Expect scattered to widespread TSRA through much of the afternoon and early evening...handled by a mix of TEMPOs and PROB30s at all six TAF sites...before coverage wanes through the evening. Overnight, dry conditions are still expected...and some mountain valley fog could develop again, with some of the guidance hinting at MVFR to even brief IFR restrictions at KAVL. VFR should return after sunrise Monday. Another round of convection is expected Monday afternoon. Winds should remain southwest through late evening, becoming light and variable overnight, then picking up again out of the southwest tomorrow.

Outlook: Mainly afternoon/evening shower and thunderstorm chances will continue through at least Tuesday. The pattern remains unsettled Wednesday and beyond, but confidence is lower on how things will unfold, and what the convective potential will be. Low stratus and/or fog will be possible each morning in the mountain valleys and in locations that see appreciable rainfall the previous afternoon/evening.

CLIMATE

RECORDS FOR 07-05

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 96 1948 71 1976 70 2024 46 1967 2018 1933 KCLT 101 2024 66 1892 78 2024 57 1967 KGSP 101 2024 70 1976 78 2016 58 1967 1933 1892

GSP WATCHES/WARNINGS/ADVISORIES

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