textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant forecast changes for this update.

KEY MESSAGES

1. The threat for heavy rainfall and isolated flash flooding peaks this afternoon/evening, but some level of locally excessive rainfall potential continues through Saturday. A Flood Watch for flash flooding remains in effect through the afternoon for much of western NC. 2. A return to typical mid-summer weather begins Sunday and continues through at least Thursday, with hot temperatures and mostly isolated diurnal convection expected.

DISCUSSION

Key message 1: The threat for heavy rainfall and isolated flash flooding peaks this afternoon/evening, but some level of locally excessive rainfall potential continues through Saturday. A Flood Watch for flash flooding remains in effect through the afternoon for much of western NC.

Outflow-reinforced frontal boundary is currently located near the NC/SC border and will continue to move very slowly SSW through the afternoon/evening. Moderate-to-heavy rain showers and isolated thunderstorms continue to train along/just north of the boundary across mainly the southern tier of NC counties and northern tier of SC counties. This training activity will continue to drift toward the southeast through the afternoon, with rainfall rates likely increasing as the boundary interacts with moderately unstable air across the Upstate. Meanwhile, with the western part of the boundary hung up somewhere over the Blue Ridge mountains, destabilization has also occurred across the TN Valley and far western portions of the southern Appalachians. Convection is expected to increase and coverage and intensity this afternoon across all but the northern NC foothills and northwest Piedmont, with a continued potential for heavy, and locally excessive rainfall. In terms of the Flood Watch that covers much of western NC, a few counties across the far north were removed earlier. However, in light of the above thinking and signals in the latest convection-allowing guidance, we currently see no need to make further adjustments, as excessive rainfall remains possible within the Watch area. Localized flash flooding is also possible south of the Watch area...especially across the eastern half of our SC counties. However, in light of trends in observational and model guidance, the threat appears to be too localized for Watch consideration.

With the atmosphere across much of the Upstate/northeast GA west of I-26 having destabilized to the tune of 2000+ J/kg of sbCAPE, there is some potential for an isolated severe storm or two. Meanwhile, surface analysis indicates a quite stable air mass filtering into the NE corner of the area, where temps are currently in the mid/upper 60s. With 1020+ surface high settling into the Northeast, inverted surface ridge will continue to sharpen east of the Appalachians, and this should allow the front to pick up a little speed this evening into the overnight, with stable air expected to filter through the remainder of the CWA during the overnight. This will result in diminishing coverage and intensity of convection this evening, and only scattered, mostly light showers are anticipated during the overnight.

Although surface-based stable air is expected across much of the area on Friday, some elevated instability is expected across much of the southwest half...while some ripples aloft are expected to pass over the area throughout the day. Scattered showers and some storms are expected to redevelop by afternoon, with likely PoPs advertised across much of this area. While the potential for isolated excessive rainfall will be lower, a marginal risk will exist...with perhaps a slightly higher risk across the southwest fringe, where the highest muCAPE is expected. Slightly more unstable conditions are possible Saturday, albeit with generally lower moisture content. Nevertheless, scattered showers and some TS are again expected during the afternoon, with a continued low end heavy rainfall threat.

Forecast high temps are around 10 degrees below climo across much of the CWA Friday, except for warmer conditions across the southwest fringe. Saturday will be warmer, but still below normal across most of the area.

Key message 2: A return to typical mid-summer weather begins Sunday and continues through at least Thursday, with hot temperatures and mostly isolated diurnal convection expected.

A drier and hotter pattern is forecast to evolve by the end of the weekend, as the axis of a long wave trough pushes just off the East Coast, with rising heights expected across our area through much of the week as a ridge builds strongly across the Rockies and Great Plains. Temperatures warm to normal by Monday, and perhaps a little above climo by mid-week. A return to a diurnal convective cycle is expected, with said cycle currently expected to be relatively inactive, as (at most) isolated activity is forecast each afternoon/evening.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: Very messy aviation weather will occur over this forecast period, as an E=>W oriented cold front continues to drift slowly south across the Terminal Forecast Area through the afternoon/evening. With most TAF sites (except KHKY) being near or south of the boundary, widespread showers and at least scattered TS are expected near the upstate SC terminals and KCLT/KAVL this afternoon, with tempos for TSRA and categorical -SHRA (or VCSH) warranted at these sites. IFR/low MVFR cigs will persist north of the boundary, while 1-2SM visby will occur with heavier bursts of rain/TSRA. Ad the front continues to sag south this evening, cigs should lower to low MVFR or IFR at all sites by late evening or so. Shower/TS chances will linger well into the evening, but with diminishing coverage and intensity. Nevertheless, spotty showers will likely linger into the overnight, especially across the southern half of the area. Otherwise, cigs may lower to LIFR overnight, while reduced visby in BR will become more likely as well. Cigs will be slow to improve during the daylight hours Friday, while coverage of showers and perhaps some storms is expected to begin flaring up by early afternoon.

Outlook: Restrictions continue through Saturday as showers and thunderstorms remain. Much lower chances of convection and associated restrictions Sunday and Monday. Low stratus and/or fog are possible each morning in the mountain valleys and locations which receive heavy rainfall.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Flood Watch until 8 PM EDT this evening for NCZ036-037-048>053- 056>059-062>065-068>072-503>510. SC...None.