textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes to the public forecast. The Aviation section has been updated.

KEY MESSAGES

1. Seasonally hot and humid east of the mountains through Saturday, then a slight cool-down for Sunday and Monday. Though a Heat Advisory is not likely to be issued, those with outdoor plans should stay hydrated and take frequent breaks in air-conditioned or shaded areas. Never leave children or pets in unattended vehicles. 2. Diurnal showers and thunderstorms are expected across mainly western North Carolina through Friday before a cold front brings better shower and thunderstorm chances area-wide this weekend into Monday. A few strong to possibly severe storms are possible during peak heating hours each day through Friday, with damaging wind gusts being the main hazard with any severe storms that manage to develop.

DISCUSSION

Key message 1: Seasonally hot and humid east of the mountains through Saturday, then a slight cool-down for Sunday and Monday. Though a Heat Advisory is not likely to be issued, those with outdoor plans should stay hydrated and take frequent breaks in air-conditioned or shaded areas. Never leave children or pets in unattended vehicles.

The Bermuda High slowly breaks down through the end of the week and will be replaced by a weak upper trof that will support increasing storm coverage, yet temps will remain seasonally hot through Saturday...generally five degrees or so above normal. Typical summertime humidity will also continue to be a factor, but dewpoint is expected to mix out enough to keep us from reaching Heat Advisory criteria each afternoon. Model guidance continues to show a bit more breezy than usual in the afternoons, which might feel like a bit of a relief, but more likely the relief will come from showers and storms. By the weekend, a weak front will drop down into the region courtesy of a short wave in the nrn stream, which will eventually push the front through perhaps on Sunday. The increase in storm coverage and cloud cover may drop the temps back to normal Sunday, but more likely that will happen for Monday. There remains plenty of uncertainty as to the development of weak cold air damming early in the week because of a transitory sfc high moving across New England, but for now we will keep the below normal temps Monday. Some of the guidance suggests the cooler air will linger into Tuesday. Welcome relief...if it develops.

Key message 2: Diurnal showers and thunderstorms are expected across mainly western North Carolina through Friday before a cold front brings better shower and thunderstorm chances area-wide this weekend into Monday. A few strong to possibly severe storms are possible during peak heating hours each day through Friday, with damaging wind gusts being the main hazard with any severe storms that manage to develop.

For the afternoon, the main concern is the Marginal Risk of severe storms over the northwest Piedmont and northern part of metro Charlotte, but really anywhere east of the mountains where storms develop. The environment is characterized by sfc-based CAPE in the 2000-3000 J/kg range, but with dCAPE above 800 J/kg everywhere east of the mtns, and sfc-delta theta-e around 30K. Thus, the environment is supportive of wet microbursts outside the mtns, even if the HRRR shows the convective activity mainly over central NC.

More of the same is expected for Thursday and Friday, with improving coverage each day, above climo, and favoring the mtns the most even if the pulse-severe risk might be slightly higher east of the mtns. The main hazard with any severe storms will remain damaging wind gusts from microbursts. As the coverage improves, the flash flood risk will also increase over mainly the NC mountains as we get into this weekend and the boundary sinks down into the region. Precipitable water will climb up into the 2 inch vicinity which is approaching the top end of the climatology. Note the Day3-5 Excessive Rainfall Outlook has a slight risk in the TN valley into the west side of the mtns, so we will be watching that situation. Drier conditions should return by Tuesday as surface high pressure builds into the region behind the departing front.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

At KCLT and elsewhere: Prevailing VFR across the region through the TAF period, with the usual caveats for diurnal thunderstorms and the usual mtn valley fog potential at KAVL. A cumulus field will continue to develop over the region through the afternoon and a brief broken low cloud ceiling can't be ruled out, but the cloud bases will be 050 or higher. The main concern will be thunderstorms through early evening, and our thinking hasn't changed with regard to the NC terminals having the best chances. Will maintain the TEMPO at KAVL because of the convection already forming on the ridges, and the PROB30 over the Piedmont/foothill locations. The HRRR still doesn't develop storms over the Upstate terminals, so they are left out and will be amended if necessary. Wind should stay SW with some low end gusts over the Upstate. Fair weather should prevail across the region tonight. No indication of fog/low clouds right now at KAVL, but that will change if storms happen there. Expect a very similar day on Thursday.

Outlook: The NC terminals will have the best potential to see TSRA each afternoon and evening through Friday. SHRA/TSRA chances return for all terminals this weekend into Monday. Low stratus and/or fog will be possible each morning in the mountain valleys and in locations that see appreciable rainfall the previous afternoon/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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