textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to thinking, but redefined the periods covered by each Key Message.

KEY MESSAGES

1. Above-average rain chances and below normal temperatures continue through Wednesday. 2. A more typical late summer pattern returns Thursday, though temperatures remain slightly below normal and daily chances for showers and storms will remain elevated through Saturday. Temperatures and precipitation chances return to around normal Sunday and Monday.

DISCUSSION

Key message 1: Above-average rain chances and below normal temperatures continue through Wednesday.

An upper low is centered over northern Georgia as of 2 PM Tue will drift slowly northward through Wednesday, promoting easterly flow in the low- to mid-levels, which has brought a plume of enhanced PWATs from the Atlantic into western NC and the eastern Upstate, though the remainder of the CWA remains moist at the surface despite dry air being present aloft within the cyclone. LCLs are low, and 700-500mb lapse rates are a little better than usual, so with diurnal heating already having produced 2000+ J/kg SBCAPE, a broad area of showers with embedded storms has already become established, centered on the Piedmont. Storm motion has been slow and a number of obs sites have received a quick 1 to 2 inches of rain. We will be closely monitoring for potential localized flash flooding. DCAPE is strongest near the core of the low, over the western Upstate and NE GA, and an isolated severe microburst threat does exist particularly in that area. What steering flow there is should generally push the convection toward the north or northwest. CAMs largely depict the activity propagating in that direction but vary as to whether a second round develops in the wake of the current activity. While outflows have led to some pockets of stabilization, the early start, soupy dewpoints in the lower 70s, and strong anvil-level flow suggesting a relatively low chance that cirrus will fill in and preclude redevelopment. Thus we retain at least 20-40% PoPs until midnight over much of the Piedmont. Easterly near-surface flow suggests stratus may advect in from the Atlantic toward daybreak, but radiation fog/stratus could also occur with so much of the area getting soaked this afternoon and sfc winds being light tonight.

Wednesday, the upper low fills and is reabsorbed into the westerlies. Surface winds favor southerly again; PWATs increase across the board with the drying effect of the upper low no longer being a factor. In general this distributes PoPs more climatologically, with the greatest values being over the mountains/foothills. Lapse rates just above the boundary layer are slightly better, and with the higher PWATs above-climo chances are warranted (50-80% in much of the CWA). 500mb winds will be stronger which may result in a little better 0-6km bulk shear, but DCAPE should be lower, so that will help mitigate any damaging wind threat. Storms may just have a slightly better chance of forming into clusters.

In the southerly flow regime, temps Wednesday inch back into the mid-80s over most of the Piedmont, particularly south of I-85. However all values should remain a bit below normal.

Key message 2: A more typical late summer pattern returns Thursday, though temperatures remain slightly below normal and daily chances for showers and storms will remain elevated through Saturday. Temperatures and precipitation chances return to around normal Sunday and Monday.

Following the dissolution of the upper low, our weather pattern returns to being more typical for this time of year; that is to say, becoming more ill-defined. PWAT values from the GFS and ECMWF ensembles remain just on the wet side of climo. Diurnal instability will remain the primary driver of shower/thunderstorm chances, and an upward trend in SBCAPE generally looks to occur Thursday and Friday, with values remaining similar or marginally higher on Saturday. A longwave trough slowly moves across the Mississippi Valley into the Great Lakes region while the Bermuda high remains strong in the western Atlantic. This will reinforce a southerly low-level flow regime and above-climo PoPs across the area, with 70-80% chances each day over the mountains and 30-50% for the Piedmont. Shear will be weak, and a marginally severe microburst can't be ruled out any day, though the damaging wind threat does not look particularly notable. By Sunday the aforementioned trough looks to have flattened out and/or shifted east such that height rises occur in the Ohio Valley, and low-level flow turns more westerly again. Hence, PoPs fall back to a more typical 30-60% for the mountains and 10-30% for the Piedmont that day and Monday. Temps trend back to near normal, with lower Piedmont locations flirting with 90 Sunday and most Piedmont areas rising into the lower 90s Monday.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: SCT SHRA/TSRA already ongoing around the terminal area as of 1730z Tue, which prompted inclusion of TEMPOs in all TAFs to begin the period. High moisture and slow storm motion suggests IFR vsby likely with any TSRA, and gusts up to 25 kt are a reasonable expectation. LCLs will remain low and VFR may not prevail until very late afternoon. Expecting the area to be worked over to some degree by 22z but can't rule out at least isolated redevelopment through evening. Thus VCSH used liberally after 22z. Easterly onshore flow looks to bring daybreak stratus over KCLT/KHKY if not the SC sites, but the SC sites and KAVL appear to be at risk of radiation fog/stratus also. Thus have prevailing IFR or LIFR developing between 04-05z and daybreak at all sites. Winds remain largely NE when not VRB or calm through tonight, except KAVL which remains SE thru the period. After morning stratus/fog dissipate Wed, winds should pick up from the SE to S. LCLs not expected to be quite as low though diurnal cu may form at MVFR level and so restrictions are slow to erode. KAVL gets a PROB30 for diurnal TSRA by 16z Wed and KCLT gets VCSH for Wed aftn, with lower confidence there.

Outlook: Scattered, mainly afternoon showers and thunderstorms will continue through the remainder of the week, with associated brief flight restrictions. Patchy fog and low stratus will be possible each morning, especially in the mountain valleys and anywhere that heavier rainfall has developed the previous evening.

GSP WATCHES/WARNINGS/ADVISORIES

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