textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00Z TAFs.

KEY MESSAGES

1. Numerous showers and thunderstorms this evening into the early overnight hours will pose a localized threat of flash flooding, especially where soils are saturated from rainfall over the holiday weekend. 2. Daily chances for showers and thunderstorms continue Thursday into the weekend as a front settles through the area. Drier conditions may briefly return on Friday, but there is not a signal for more robust drying until at least Sunday.

DISCUSSION

Key message 1: Numerous showers and thunderstorms this evening into the early overnight hours will pose a localized threat of flash flooding, especially where soils are saturated from rainfall over the holiday weekend.

Our CWA remains under a deep southerly flow regime between the Bermuda High and an upper low over the ArkLaTex region. Moisture is robust, exemplified by PWATs at/above 1.8" on morning RAOBs. This moisture plume feeds into an almost stationary front to our north, over the lower OH Valley and Mid-Atlantic. Shower and thunderstorm activity continue producing impressive rainfall rates this evening. Southerly, nearly unidirectional shear profiles are roughly parallel to the flow over the region and so convection has grown upscale into narrow bands, with training along said bands. Thus, the potential for isolated flash flooding will continue through the early overnight hours.

By tonight the upper low to our west is progged to lift north and fill. Flow aloft veers to a more westerly direction in response, and low-level convergence appears to be weaker Wednesday compared to Tuesday. PWATs also decline slightly in model output, reflecting the reduced moisture flux; prog soundings appear slightly drier in the mid to upper levels. In general model QPF response is lower and CAMs favor more typical diurnal convective development over the NC mountains which moves eastward into the foothills. Certainly cannot rule out development elsewhere, though lacking a clear lifting or focusing mechanism, isolated or widely scattered PoPs (20-30%) appear reasonable for the areas not in the path of the terrain- induced storms. Modest 0-6km shear of around 30 kt is seen on prog soundings in our northern CWA where coverage looks greatest, and with slightly better sfc-midlevel delta-theta-e values than today. It is for this reason SPC extended a Marginal Risk into our northern CWA, which is justifiable.

Key message 2: Daily chances for showers and thunderstorms continue Thursday into the weekend as a front settles through the area. Drier conditions may briefly return on Friday, but there is not a signal for more robust drying until at least Sunday.

A shortwave digging across the Midwest looks to impinge on the stationary front Wednesday night, pushing it southward as a cold front. Low-level flow turns more northwesterly along the TN/NC border and light precip may continue there into early Thursday. The front itself pushes east of the mountains during the day Thursday. Post-frontal drying may actually inhibit convection in the northern mountains and northwest NC Piedmont, but to the south of those areas, appears to be timed to enhance CI. Shear may be slightly better than Wednesday, but LREF probabilities of CAPE > 1000 are lower, so severe threat is no higher, possibly lower, compared to Wednesday. Temps should trend a little warmer Thu afternoon, back into the mid 80s for most of the Piedmont, owing to downslope and possibly compressional warming.

Guidance largely continues to depict the front stalling over the far southern and/or southwestern portions of the CWA by early Friday morning, leading to cooler temps and marginally lower dewpoints on Friday. Southerly flow potentially will be reestablished on Friday with a another baggy upper trough being present over the southern Plains or Mid-South and producing upglide over the boundary, similar to the pattern earlier in the week. PoPs persist over the SW half of the area, increasing and expanding Friday night into Saturday with that trough opening up and shifting east over the area, along with a reinforcing wedgelike sfc high coming in from the north. Temps remain below normal through Monday, particularly the daily max temps. Confidence remains low whether the wedgelike high is able to induce appreciable drying; small PoPs persist in most locations until the end of the period.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Showers and thunderstorms continue streaming across the western Carolinas as of 00Z. With activity expected to linger through at least the early overnight hours, maintained TEMPOs and PROB30s for SHRA through 07Z-08Z for most terminals, although have activity ending around 04Z for KAND. Drier conditions should return after this time, outside of some scattered SHRA/TSRA Wednesday afternoon/evening. Should see cigs and vsbys tank again overnight, ranging most from IFR to LIFR. Cigs and vsbys should gradually lift to VFR by mid-morning Wednesday. Confidence is highest at KAVL, KAND, and KHKY for diurnal thunderstorm activity on Wednesday so only these terminals have PROB30s towards the end of the TAF period. Winds will generally be S/SW through the period but may go light and VRB at times through daybreak Wednesday. Low-end gusts may develop east of the mountains Wednesday afternoon, with confidence highest at KCLT.

Outlook: The pattern remains unsettled through the rest of the week with at least scattered diurnal convection expected each day. Vsby and cig restrictions will remain possible each night.

GSP WATCHES/WARNINGS/ADVISORIES

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