textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Increased the PoPs this evening to reflect latest radar trends across the southern parts of the forecast area.

Updated the aviation discussion for the 00Z TAF issuance.

KEY MESSAGES

1. Hot and humid conditions continue today through much of the weekend with at least scattered showers and storms, especially tomorrow. 2. A backdoor cold front pushes through the area on Monday bringing drier but still warm weather through much of next week, excluding daily mountain showers/storms.

DISCUSSION

Key message 1: Hot and humid conditions continue today through much of the weekend with at least scattered showers and storms, especially tomorrow.

Scattered to numerous showers and thunderstorms have been oozing northward into our NE GA and southern Upstate counties so far this evening. Outflows from this activity are advecting higher dewpts and more unstable air, but with the sun setting and much drier air to the north, convection is expected to struggle developing along the outflows as they reach the I-85 corridor. There will be some lingering convection thru late evening, but should wane by around midnight. A couple more strong to severe storms will be possible, and isolated excessive rainfall cannot be ruled out.

A weak split flow regime remains draped across the CONUS today with strong northern stream flow extending from the Northern Rockies across the Northern Plains and into southeast Canada. Farther south, a weaker branch of southern stream flow is situated from the Desert Southwest across the Southern Plains and into the southeast states. Within the southern stream, a weak shortwave trough with a closed upper low is sliding across the Mississippi Valley and into western Tennessee. This feature will cross the Southern Appalachians tomorrow with general troughing lingering into Sunday. An unseasonably warm and humid airmass is entrenched across the region with afternoon highs today soaring well above average into the upper 80s to low 90s. Weak height falls ahead of the approaching trough in concert with a late summer airmass will help instigate at least isolated to widely scattered showers and thunderstorms this afternoon, especially south of I-85. A few of these storms may become strong to marginally severe with the main hazard being locally damaging winds.

The passage of the upper trough tomorrow will provide more notable forcing with the 12z suite of CAMs in generally good agreement on scattered to numerous afternoon/evening showers and thunderstorms across much of the area. Near-storm forecast soundings reveal tall skinny CAPE profiles with modest DCAPE. While not overly impressive, a few isolated strong to severe storms with locally damaging winds cannot be ruled out again on Saturday. Associated cloud cover will help to keep temperatures a few degrees cooler compared to today, but still above climo for mid September. The upper trough axis gradually pivots across the region on Sunday and another bout of isolated to widely scattered showers and storms can be expected, but of lesser coverage compared to Saturday.

Key message 2: A backdoor cold front pushes through the area on Monday bringing drier but still warm weather through much of next week, excluding daily mountain showers/storms.

Heading into the start of next week, a notable northern stream trough is progged to swing across the Great Lakes and into New England Sunday night into Monday. A sprawling surface high quickly translates into the Mid-Atlantic and New England and in turn drives a back door cold front down the Appalachians through Virginia and across the Carolinas. A few isolated showers and/or a storm may accompany the front, but it will mainly be a dry frontal passage for most. Temperatures ahead of the front will once again climb back into the upper 80s to low 90s. For the remainder of the week, stout upper ridging builds back over the Southern Appalachians with guidance indicating 500mb heights nearing 598dam. Gross. Despite persistent dry northeast flow afternoon highs will rebound through the week with upper 80s to near 90 common again by Thursday and Friday, which remains well above climo for mid September. Dry air will also shunt deeper moisture to over the mountains which is where the best chances for a few diurnal showers and storms will reside.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: A slow-moving line of showers and thunderstorms is slowly drifting north across the Upstate and NE GA, with some convection possibly affecting the Upstate sites over the next 2-3 hours. Confidence remains on the low side, as convection is expected to gradually weaken after sunset. Confidence is even lower at KCLT, but will leave a PROB30 there, as an outflow boundary has moved in. Overnight, expect mostly VFR mid and high clouds to linger, but an MVFR to possibly IFR stratus deck is expected to form around daybreak Saturday across the Piedmont, in addition to the typical mountain valley fog. These clouds may hold on thru midday to early aftn, before scattering out to VFR. Guidance still shows scattered to numerous convection across the area Saturday aftn. Will continue with PROB30 for TS during peak heating time frame. Given such high coverage expected, will go with prevailing SHRA at all sites and TEMPO for TS at KCLT Saturday aftn thru 00z. Winds will be light, mainly out of the S, except VRB around TS.

Outlook: Scattered afternoon and evening storms will once again be possible Sunday. Storm chances shift to the mountains Monday through much of next week. Mountain valley fog/stratus will be possible any given morning, especially in prone/traditional locations.

GSP WATCHES/WARNINGS/ADVISORIES

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