textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 12Z TAFs. Updated PoPs to reflect rain chances for this morning, mainly south of I-85.

KEY MESSAGES

1. Afternoon/evening shower and thunderstorm chances return tomorrow and continue through the weekend, with isolated severe storms possible Saturday and Sunday. 2. Elevated heat risk is forecast by the first half of next week, with triple digit heat indices possible across most of our lower terrain by mid-week.

DISCUSSION

Key message 1: Afternoon/evening shower and thunderstorm chances return tomorrow and continue through the weekend, with isolated severe storms possible Saturday and Sunday.

Quasi-zonal flow aloft and continued low-level south to southwesterly flow will provide weak moisture advection as a subtle shortwave shifts across the Deep South and Southeast this morning and through the daytime period. This has helped develop a few showers south of I-85 and should continue to push northeast towards the Charlotte area over the next couple of hours. With better moisture advection, expect for the atmosphere to destabilize more compared to the past couple of days and lead to diurnal convection across the CWFA. Better PoPs are forecasted to be east of the CWFA, but isolated coverage will become realized across the mountains and portions of the Piedmont, with any lingering convection waning off shortly after sunset.

Weak shortwave trough to the north and the presence of a frontal boundary will set the stage for better diurnal convective coverage on Saturday. Strong destabilization with more established moisture and a slight increase in low-level shear (0-3km: ~20-25 kts) should help develop outflow driven cell clusters that could promote a few strong to severe storms, with the best chance being over western NC during the afternoon and evening hours.

The flow aloft becomes more anticyclonic later Saturday and Sunday as a deepening anticyclone develops over the Lower MS-Valley and turns the flow aloft over the CWFA northwesterly. This is a typical "Ring of Fire" set up where storms develop upstream and move downstream within the mean flow and send one or more rounds of potential MCSs. Confidence is rather low on timing and exact location of a potential MCS, but this can become interesting, especially if one can take advantage of max instability during daytime heating, which can't be ruled out on Sunday and will need to be monitored. However, temperatures will warm up to 5 degrees above normal by Sunday as higher heights filter in thanks to the deepening upper/mid anticyclone, with heat indices getting very close to triple digits, mainly in the southern tier of the CWFA. One caveat to this forecast is if a MCS moves into the CFWA earlier in the day on Sunday, lingering convective debris will disrupt max temperatures from reaching their full potential and will lower PoPs in regards to the diurnal convective coverage as well with less destabilization.

Key message 2: Elevated heat risk is forecast by the first half of next week, with triple digit heat indices possible across most of our lower terrain by mid-week.

A strong upper/mid-level anticyclone will set up shop over the MS Valley late Sunday and Monday, while gradually propagating eastward during the new workweek and will engulf most of the eastern CONUS, especially by Tuesday and beyond. Afternoon highs will reach 4-8 degrees above normal each day starting Monday as the heat really cranks up by Wednesday and Thursday. Drier air aloft will help to suppress the atmosphere, leading to less diurnal convection and more in the way of mixing surface dewpoints with better low-level subsidence. In this case, heat indices will approach 100 degrees during peak heating, but confidence for Advisory Criteria is relatively low at this time, but best chances would be in the Lakelands and Charlotte Metro. Can't rule out daily isolated afternoon showers and storms over the high terrain, but any mentionable PoP (15-30%) will remain confined to the mountains through the end of the forecast period.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR conditions will prevail through the forecast period. Can't rule out mountain valley fog, but not expected to impact KAVL through daybreak. Light and variable winds will pick up out of the south to southwest at 4-8 kts later this morning and into the afternoon. Expect an increase in VFR-level stratocu or mid clouds before daybreak, especially for the Upstate and CLT. A few showers continue to push south of I-85, but current trends has this area of showers moving toward CLT, so placed a VCSH in the prevailing line and keep it through 15Z. Diurnal convection is expected for this afternoon and early evening, with the best chances being in KAVL and KCLT, where a PROB30 is in place for TSRA and associated restrictions. Decided to go with VCTS during peak heating for the Upstate sites as confidence is too low in regards to thunderstorm coverage for anything more than that, but will need to be reevaluated through the morning. Any SHRA or TSRA that develops should dissipate or move away from all terminals by 00Z Saturday. Lingering convective debris and an increase in overall cloud cover should stick around into the overnight period.

Outlook: Afternoon/evening shower and thunderstorm chances and associated restrictions continue through the weekend. Diurnal convective chances diminish a bit early in the new work week, as upper level high pressure strengthens over the region. Early morning fog/low stratus is possible each day, especially in the mountain valleys.

GSP WATCHES/WARNINGS/ADVISORIES

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