textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Isolated severe threat possible Wednesday mainly in the northern part of the area.

KEY MESSAGES

1. Numerous showers and thunderstorms this afternoon and evening will pose a localized threat of flash flooding, especially where soils are saturated following the rainfall over the holiday weekend. Though showers and storms will not be as widespread Wednesday, an isolated threat of flash flooding and severe weather will return. 2. Daily chances for showers and thunderstorms continue into the weekend. 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 afternoon and evening will pose a localized threat of flash flooding, especially where soils are saturated following the rainfall over the holiday weekend. Though showers and storms will not be as widespread Wednesday, an isolated threat of flash flooding and severe weather will return.

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. Diurnally driven showers are spread somewhat uniformly across the area this afternoon, producing respectable rainfall rates but so far producing only isolated thunder. Southerly, nearly unidirectional shear profiles are roughly parallel to the flow over the region and so convection has potential to grow upscale into narrow bands, with training along said bands. This suggests potential for isolated flash flooding to develop. Not much dry air aloft to enhance development of downdrafts, so the severe threat appears relatively low today.

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 /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Moist airmass associated with lower than typical cumulus bases this afternoon. Scattered SHRA will continue through the evening, posing mainly a risk of lower MVFR to IFR vsby/cigs in very heavy rainfall. Isolated TSRA may also occur. Generally used a combo of TEMPO and PROB30 to reflect the continuing potential for convection through late evening. With little potential for moisture to mix out and also chances for rainfall after sunset, widespread low MVFR to IFR cigs are expected to develop overnight, with LIFR likely at all sites near daybreak Wed. Slow improvement back to VFR by 15-17z. Afternoon precip chances too low to mention at KCLT.

Outlook: The pattern remains unsettled through the workweek with at least scattered diurnal convection expected each day (possibly persisting into the overnight hours) and lowered visibility and ceilings forecast each night.

GSP WATCHES/WARNINGS/ADVISORIES

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