textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Tweaked rain/thunder chances and sky cover based on the very latest trends this evening.

The aviation forecast was updated to reflect the impending 00Z TAF issuance.

KEY MESSAGES

1. A cold front dropping down slowly from the north may trigger a few severe thunderstorms during the afternoon and evening, and again over the western half of the forecast area on Saturday afternoon and evening. Otherwise, this afternoon should be the last really hot day for the next week. 2. A shift in the upper pattern will finally bring some cool weather right on schedule for the start of autumn.

DISCUSSION

Key message 1: A cold front dropping down slowly from the north may trigger a few severe thunderstorms during the afternoon and evening, and again over the western half of the forecast area on Saturday afternoon and evening. Otherwise, this afternoon should be the last really hot day for the next week.

A cold front is still located to our north, but drawing closer as it pushes into Virginia. Scattered to numerous thunderstorms developed across the area this afternoon as expected, and continues across the northern Blue Ridge and I-40 corridor. Remnants of earlier activity across I-77 have moved south and east, but has generally weaked as it does so. Confidence is limited on how the activity over the Blue Ridge and I-40 will evolve, with recent CAMs depicting its quick demise. That's not especially convincing, however, given how much guidance has struggled with convection so far today. At any rate, although we're likely past the peak of activity, can't rule out another strong to severe storm or two over the next couple of hours...with damaging winds still the main threat courtesy of 1000-1200 J/kg DCAPE across the Upstate zones and southern NC mountains, where convection is headed.

Otherwise...the cold front should be located somewhere draped across the forecast area on Saturday, making the temp fcst tricky with a gradient between lingering heat and lower-90s over the Lakelands (potentially) and a cooler northeasterly flow and low/mid 80s over the NW Piedmont. More storms are likely to develop in the afternoon along/ahead of this boundary as it continues to ooze south and southwest through Saturday evening. The Day 2 Outlook shows another Marginal Risk for the western zones and the convective environment might be better than today. People with outdoor plans over the mountains, northeast GA, and the western Upstate should keep an eye out storms.

The rest of the weekend and into Monday still look unsettled with better coverage of showers and storms. We will no longer have the suppressive influence of the upper ridge and moisture will increase with a weak upper low over the northeast Gulf. By this time, what small threat for severe weather will switch to excessive rain given the deeper moisture. A marginal risk is noted on the excessive rainfall outlooks starting Monday. Temps will fall back to normal.

Key message 2: A shift in the upper pattern will finally bring some cool weather right on schedule for the start of autumn.

Forecast uncertainty increases once we get into the first half of next week and the upper pattern becomes blocky, particularly over eastern North America. It might not look especially coherent, but it also doesn't look much like summer any longer, which is appropriate given the date. The operational models indicate a rex block pattern over the Great Lakes and Ontario starting on Tuesday and continuing into Wednesday, before evolving into the more typical omega pattern by the end of the week. If this pattern develops as shown, a corresponding cold frontal passage Tuesday night or early Wednesday would usher in the first cool air mass right on schedule for the start of autumn, with temps dropping to 5-10 degrees on the cool side of normal for a change on Wednesday. The change would be remarkable. Either way, Wednesday looks cool with extensive cloud cover and precip chances. For the rest of the week, we woud remain in the cool side of the pattern with a slow modification of the air mass. Drier air would limit precip chances to the higher terrain.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Scattered convection has developed as expected across the terminal forecast area, mainly across the NC zones. For CLT, still think the peak of the action is over, but it's unclear whether activity currently resurging over the I-40 corridor will lead to a second round of TS or SHRA along and south of I-40 this evening. Hi-res guidance generally suggests it will not, but confidence is limited since the hi-res guidance has generally struggled with the evolution of today's convection.

In general, think that by 03Z or so, convection will ramp down everywhere and give way to a cloudy, but mostly VFR night. Some patchy, intermittent fog may develop in locations that received decent convection this afternoon (which currently includes KHKY and KCLT, but may eventually also include KGSP and KGMU)...and despite last night's bust, still think a round of mountain valley fog and low stratus may result in at least MVFR to IFR restrictions at KAVL before dawn on Saturday. Thereafter, a complex, low-predicability pattern should set up for the daytime hours Saturday, with a frontal boundary likely draped somewhere across the forecast area (perhaps along and near I-26, but again, confidence is low). Locations east of the frontal boundary may wind up socked into MVFR stratocu for much of the day, with NE winds and only very limited potential for afternoon convection. Locations west of the boundary, however, will likely see continued S or SW winds, VFR conditions during the daytime hours, and greater potential for another round of scattered TS during the afternoon and evening hours. PROB30s for TSRA have been added where applicable.

Outlook: Shower and thunderstorm chances continue each afternoon through Tuesday. Mountain valley fog and low stratus possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

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