textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Severe thunderstorm threat has decreased for the rest of today. Otherwise, no significant changes to the forecast with the afternoon package.

The aviation discussion has been updated for the 18Z TAFs.

KEY MESSAGES

1. Widespread showers and isolated to scattered thunderstorms expected across the area through this evening. A Flash Flood Watch remains in effect for portions of the northern NC mountains and northern foothills until 8 PM today. Localized flash flooding will be possible across the remainder of the area as well. Otherwise, well-below normal high temperatures are forecast through Tuesday. 2. A return to seasonably hot and drier conditions is expected from Wednesday through the rest of week. A cold front will approach from the north and bring higher rain chances over the weekend.

DISCUSSION

Key message 1: Widespread showers and isolated to scattered thunderstorms expected across the area through this evening. A Flash Flood Watch remains in effect for portions of the northern NC mountains and northern foothills until 8 PM today. Localized flash flooding will be possible across the remainder of the area as well. Otherwise, well-below normal high temperatures are forecast through Tuesday.

A weak stacked low continues to spin across eastern TN, with an axis of 1.5-2.0" PWATs across region. The deep-layer moisture and weak lapse rates are resulting in modestly unstable air, with sbCAPE 500-1500 J/kg across most of the CWFA, and around 2000 J/kg in far southern parts of the area. Widespread convection has developed with the subtle cyclonic spin apparent on the radar mosaic. The vast majority of the activity is too shallow to produce lightning, with just a few general thunderstorms across the CWFA attm. Not surprising that the severe threat looks to be trending down. The main concern will be heavy rain and localized flash flooding. Showers are producing efficient rain rates, with some advisory-level flooding possible. Where cells can train over the same areas, flash flooding may occur. The 12z CAMs have trended toward the heaviest rainfall across the southern NC foothills to the NW Piedmont, mostly on the south side of the current Flash Flood Watch. With that said, flooding is expected to be too localized to warrant expanding the watch. Further south across the Upstate and NE GA, instability is higher and a weak front has stalled out. So isolated flooding cannot be ruled out in these areas either. Temps will remain about 10-15 deg below normal the rest of the aftn.

Convection is expected to decrease this evening after sunset, but scattered showers will continue develop near the Escarpment and adjacent foothills overnight, as easterly upslope flow will linger with a little MUCAPE. There is not much of a signal for heavy rain, and thus, expect the FFA will be allowed to expire at 8 pm this evening. Lows will be near to slightly below normal under mostly cloudy skies.

The stacked low will begin to retrograde west, as high pressure builds toward the SE from the Upper Midwest. Low-level flow will begin to turn out of the ENE and advect in some drier air to the CWFA. The global models suggest the air mass will still become modestly unstable, despite lingering cloud cover slow to scatter out. The 12z CAMs are not very excited about diurnal convection, showing almost no activity in the forecast area. The latest NBM is in between, with PoPs ranging from sub-15 north of I-40 to around 60% in NE GA. A non-zero heavy rain threat may linger in our southwest zones, as PWATs remain near 2 inches. Temps will be about 5-10 deg below normal.

Key message 2: A return to seasonably hot and drier conditions is expected from Wednesday through the rest of week. A cold front will approach from the north and bring higher rain chances over the weekend.

A large upper ridge will build southeastward from the Northern Plains Wednesday, settling into a NW-SE-axis from the Upper Midwest to the Carolinas Thursday and Friday. This pattern should allow a drier air to rotate in from the NE and result in below-climo PoPs for the latter half of the workweek (mainly confined to the high terrain and Piedmont west of I-26). Temps will warm to near normal Wednesday, then a few degrees above normal Thursday and beyond. Muggy dewpts will support heat indices creeping back up near 100 to 105 across the southern and eastern portions of the CWFA. There is still no clear indication that Heat Advisories will be needed, but it will feel hot nevertheless.

The 12z guidance is coming into better agreement on a cold front dropping south toward the forecast area Saturday and possibly reaching hte area Sunday. This brings back near to above-climo diurnal convective PoPs.

AVIATION /18Z MONDAY THROUGH SATURDAY/

At KCLT and elsewhere: Numerous to widespread showers are blossoming across the area this afternoon and will continue into early evening. There are a few embedded general thunderstorms, and the threat seems high enough to keep PROB30 for TSRA in all the TAF sites (except HKY, which is under thicker cloud cover). Clouds have scattered into several low layers, but cigs are mostly MVFR with occasional VFR. Expect shower coverage to decrease this evening, but lingering light rain and drizzle will be possible thru most of the overnight, especially near the Blue Ridge Escarpment. Cigs are expected to lower to IFR, with LIFR possible in spots, mainly near the Escarpment. Cigs should start to lift to MVFR by midday Tuesday, but not likely to reach VFR before 18z. Winds will mostly be ENE at 5-10 kt thru the period, except ESE at KAVL.

Outlook: Drier conditions should develop Tuesday into Wednesday. Low stratus and/or fog will be possible each morning in the mtn valleys and in locations that receive appreciable rainfall from the previous day.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Flood Watch until 8 PM EDT this evening for NCZ033-035-049-050- 501>506. SC...None.