textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes were made to the public forecast tonight.

The aviation discussion was updated to reflect the recent 06Z TAF issuance.

KEY MESSAGES

1. Diurnal showers and thunderstorms continue each afternoon through Monday, with increasingly sparse coverage expected each day. 2. Hot and humid conditions return for the new workweek. Dangerous heat index values arrive in the low terrain by the second half of the week.

DISCUSSION

Key message 1: Diurnal showers and thunderstorms continue each afternoon through Monday, with increasingly sparse coverage expected each day.

Not much change to the setup tonight, with an ill-defined quasi- stationary boundary sprawled across the low terrain and a better- defined warm sector across our southeastern zones. Widespread mid- and high-level cloud cover is in place east of I-26, but convection has drawn to a close across the region. Aloft, still see evidence of a defined trough axis extending from the Tennessee Valley up into the Mid-Atlantic, and beginning to actually push into the forecast area, albeit very slowly.

CAMs are in good agreement that another round of diurnal convection will initiate by early afternoon Saturday, but generally think it'll get less of a bump from the environment than the last couple days and thus remain mostly confined to the high terrain. Heights are already beginning to rise ever-so-slightly as the upper trough retreats northward, and this trend should continue through the next 48 hours. As a result, upper flow generally looks weaker, resulting in less-impressive deep shear than the last two days. Still, the NAMnest, RRFS, and ARW have joined forces to depict CI over the I-77 corridor, on the moist side of the remnant boundary, during the afternoon and early evening hours. These zones will destabilize to the tune of 1000-1500 J/kg sbCAPE...enough for a few stronger storms, perhaps even a severe one. Farther west, the influence of a postfrontal inversion and weak subsidence aloft should suppress convection.

There's a case to be made for some warm-advective showers developing late Saturday evening and persisting into the wee hours of Sunday morning. Deterministic models feature a weak shortwave sliding along and north of I-40 during this period, and in response, weak WAA should develop. Forecast profiles feature little, if any, elevated instability, so no expectations for severe weather or even any thunder...but a round of overnight rainfall is well within reason.

The environment will become increasingly inhospitable for deep moist convection through the early part of next week. As mid-level troughiness continues to retreat northward, lapse rates will become less and less impressive, while at the surface there should be no triggering mechanism to be found. Sunday and Monday, convection should remain mostly or entirely confined to the ridges of the NC mountains, and Tuesday and beyond even mountain showers should be isolated at best.

Key message 2: Hot and humid conditions return for the new workweek. Dangerous heat index values arrive in the low terrain by the second half of the week.

By Monday, long-range ensembles are in good agreement that the subtropical ridge will become re-established over the Southeastern United States. As heights build and a Bermuda High-like feature sets up over the western Atlantic, the heat and humidity should return in spades. Notwithstanding the influence of dewpoint mixing, which is difficult to capture effectively at long time ranges, we expect upper 90s to triple digit heat index values to return by Tuesday or Wednesday for much of the low terrain. By the second half of the week, we could be back in Heat Advisory territory, though confidence is limited at this point.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Convection has waned across the region, but significant low-level moisture remains in place, and that'll dictate how the rest of tonight evolves. Expect another round of mountain valley fog and low stratus - with IFR to LIFR restrictions possible at KAVL - and at least some potential for spotty and intermittent fog and low stratus through much of the night. If it's like last night - and there's no reason to think it won't be - then we can expect restrictions varying from VFR to IFR at various locations through dawn. With a weak quasi-stationary boundary in place essentially right along or just north of I-85, there's also potential for a more consistent deck of MVFR to IFR stratus to push into the area before daybreak embedded within easterly flow on the seaward side of the front. If this materializes as depicted in some recent runs of the HRRR and GLAMP, then it'd mainly affect the I-77 corridor. Confidence is limited. Light and variable winds expected overnight.

Regardless, whatever restrictions develop will scatter out quickly after daybreak, giving way to a mainly VFR day. Some ridgetop showers and storms are expected to develop again, but far less coverage is anticipated over the Piedmont and Upstate than we got on Friday. Included PROB30s at KGSP and KGMU, but based on recent CAM trends, even this may be overly aggressive. Winds will start out the day from the NE, then toggle around to the SE by early afternoon.

Outlook: Scattered diurnal showers and storms are expected through Sunday, mostly over the mountains, then entirely over the mountains by Monday. Mountain valley fog and low stratus will be possible each morning through the first part of next week.

GSP WATCHES/WARNINGS/ADVISORIES

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