textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to forecast thinking.

KEY MESSAGES

1. Scattered diurnal thunderstorms each afternoon through Friday, primarily across western North Carolina. Shower and thunderstorm coverage increases Saturday across the whole forecast area and remains elevated into early next week. Isolated severe storms may produce damaging winds this afternoon and again Thursday. 2. Generally hot and humid conditions through Saturday before cooler and less humid conditions return Sunday into early next week. Those with outdoor plans should stay hydrated and take frequent breaks in air- conditioned or shaded areas. Never leave children or pets in unattended vehicles.

DISCUSSION

Key message 1: Scattered diurnal thunderstorms each afternoon through Friday, primarily across western North Carolina. Shower and thunderstorm coverage increases Saturday across the whole forecast area and remains elevated into early next week. Isolated severe storms may produce damaging winds this afternoon and again Thursday.

While a typical summertime Bermuda High pattern continues over the Southeast at large, a weak, almost stationary frontal zone is seen north of the upper ridge near the NC/VA border. This front extends between an upper low in the mid-Mississippi Valley and low pressure off the Jersey Shore. Moisture pooling in proximity to the frontal boundary should maintain elevated dewpoints this afternoon and marginally higher convective potential particularly in our NC zones. The MS Valley low will drift slowly northeastward tonight into Wednesday, beginning to open into a more progressive shortwave, while the East Coast low moves farther out into the Atlantic. The frontal zone shifts slightly farther north in response. Dewpoints look to mix out slightly lower Wed afternoon, and coverage is forecast to be slightly lesser as well. Coverage may however tick back upward over the mountains Thursday afternoon as the weakening shortwave drifts overhead. Steering flow through Thursday is west-southwesterly; ridgetop convergence as usual will be a driver for convective initiation with propagation mainly toward the northeast as a result. Hence, through Thursday, convective coverage looks to remain very sparse over the lower Piedmont--with PoPs generally below 15 percent--especially over the more southern GA/SC zones. Weak shear suggests mainly pulse-storm threats today and Wednesday, and thus the possibility of isolated damaging microbursts. Deep-layer shear increases with the arrival of the shortwave Thursday, so storms that day may have slightly more potential to organize along cold pools though not necessarily resulting in higher severe risk.

A shortwave is shown to pass north of the CWA Friday, possibly with a weak trailing convergence zone which could maintain some nondiurnal convective activity near the TN border Friday night. Heights concurrently rise over the central CONUS, with a subsequent shortwave moving past the developing ridge and bringing a better defined cold front south through the CWA, such that a more active northwest-flow regime sets up Saturday through Monday. The ingredients don't quite come together to necessarily put us in an MCS track, but we may at least have the exposure to disturbances originating upstream which could promote convective development locally. As of now the most active day appears to be Sunday when most locations in the CWA receive a 60-70% PoP.

Key message 2: Generally hot and humid conditions through Saturday before cooler and less humid conditions return Sunday into early next week. Those with outdoor plans should stay hydrated and take frequent breaks in air- conditioned or shaded areas. Never leave children or pets in unattended vehicles.

Seasonably hot and humid conditions will continue through Saturday. Temperatures are hovering a little above normal today, with dewpoints remaining elevated in the vicinity of a stalled frontal boundary just north of the area. As that boundary shifts northward for Thursday, dewpoints potentially will trend a little lower, though temperatures trend a degree or two higher; these offset such that heat index tops out at similar levels, generally in the upper 90s to low 100s, thus a tad below advisory criteria.

Temperatures trend cooler by Sunday as heights fall over the Eastern Seaboard in the wake of weak shortwaves, bringing a weak cold front southward through the area. The cooler airmass, combined with increased cloud cover near the front, actually bring temperatures below normal Monday.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: SCT SHRA/TSRA will continue this afternoon mainly across western NC, with focus mainly over the mountains but a weak frontal boundary north of KHKY also enhancing development. TEMPO at KAVL/KHKY and PROB30 at KCLT. Saw fit to include PROB30 at KGSP/KGMU due to proximity to the mountains. Can't rule out a stray storm at KAND though chance too low to mention. Activity should diminish as usual around sunset. Winds mainly from the SW except NW at KAVL this afternoon. With the expectation of light winds tonight and similar pattern to Tue morning, some fog/stratus could develop at KAVL or KHKY. Mentioning only as MVFR vsby and SCT IFR level clouds for now, but if confidence increases some IFR restrictions could be added at a later issuance. Light SW winds pick up again late Wed morning; convective coverage expected to be slightly lesser Wed aftn so no mention is warranted at KCLT at this time.

Outlook: The North Carolina terminals will have the best potential to see TSRA chances return each afternoon and evening Wednesday through Friday. More widespread SHRA/TSRA expected across all terminals Saturday to Monday. 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.

GSP WATCHES/WARNINGS/ADVISORIES

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