textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to the public forecast.

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

KEY MESSAGES

1. A few thunderstorms with a threat for excessive rain are possible this evening, mainly over the Smokies, with rain elsewhere through Thursday. 2. Temperatures will remain cool through the weekend with dry air returning, then a gradual return to normal temperatures is expected into the middle of next week.

DISCUSSION

Key message 1: A few thunderstorms with a threat for excessive rain are possible this evening, mainly over the Smokies, with rain elsewhere through Thursday.

If there is a main concern with this forecast, it would be excessive rain through the evening hours, mainly over southwest NC and the Smokies. Cold air damming has taken over most of the forecast area, but the wedge boundary curls around northeast and north GA and over the mtns, leaving the southwestern counties exposed to some afternoon heating. The latest SPC mesoanalysis shows sfc-based CAPE of roughly 1000-1500 J/kg and storms are making the most of it. The setup is not the best for severe weather as lapse rates are somewhat poor and dCAPE is marginal, but an isolated severe storm can never be ruled out. Excessive rain is the greater threat as cell movement is slow and moisture is plentiful, but fortunately that area can tolerate the rain for the time being, so actual hydro issues remain quite isolated so far. Elsewhere, the cold air damming wedge should generally preclude convection deep enough for thunderstorms or excessive rain. Later tonight there could be some interaction between the easterly flow against the Blue Ridge Escarpment and the overall westerly flow with the upper low to our north. However, the potential for heavy rain has gone down in recent guidance, especially away from the Escarpment. Temps will remain cool.

The cold air damming regime should persist well into the day on Thursday with precip chances slowly diminishing into the afternoon. The passage of the upper system in the afternoon should act to start draining the cool pool east of the mountains by the end of the day. It could easily remain overcast through sunset as some of the guidance doesn't bring the low level flow around to a downslope direction until later Thursday evening. Dry air comes in aloft first, which could set us up for a period of dense fog late Thursday night or early Friday morning.

Key message 2: Temperatures will remain cool through the weekend with dry air returning, then a gradual return to normal temperatures is expected into the middle of next week.

In short, the weather looks spectacular for the first weekend of autumn. What a difference a week makes. Once the upper low wobbles off the coast of the Carolinas and Mid-Atlantic, dry high pressure will return on a northerly flow aloft ahead of an upper ridge axis that stays to our west through the weekend. There could be some residual cloudiness on Friday morning, but not likely, otherwise mostly clear sky looks like a good bet for several days. High temps are expected to stay in the 70s Friday, on the order of 5-10 degrees below normal, but then a gradual warm-up should take place as the air mass modifies. By Monday, highs should be right around normal, which is upper 70s to lower 80s, generally speaking. The low temperatures will be more remarkable, mostly in the upper 40s and lower 50s Friday through Sunday. The high elevations will be quite cool, possibly down into the 30s at elevations above 5000 feet Friday night. By the early part of the week, lows will return to the low to mid 50s. It won't last, of course, because the pattern looks like it will evolve again in the middle of next week toward something more damp, though details are very sketchy.

AVIATION /00Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: Most areas remain socked into a cold air damming wedge this evening, with the ob sites reporting a mix of IFR and low MVFR ceilings and patchy low visibility in areas where it's actively raining. Guidance depicts another round of rainfall...currently located over the southwest mountains...making tracks eastward through the overnight hours and producing at least brief -RA at all the TAF sites leading up to dawn. Scattered RA or DZ may continue through the first half of Thursday, and restrictions are unlikely to lift for most of the afternoon. Winds will remain out of the NE, with gusts in the mid-teens possible most of the day.

A dramatic change is in store by Thursday evening, when most guidance depicts a slug of dry air wiping out much of the moisture and resulting in improving restrictions Thursday evening into Thursday night. Can't rule out some patchy dense fog Thursday night, as any lingering low-level moisture responds to increasingly clear skies overnight. Confidence is limited, however, and as of yet haven't added much detail about this possibility to the 00Z TAFs.

Outlook: VFR should return everywhere by Friday. VFR expected through the weekend, though mountain/river valley fog/stratus could still develop before daybreak each day.

GSP WATCHES/WARNINGS/ADVISORIES

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