textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 18Z TAFs.

KEY MESSAGES

1. Successive rounds of rainfall will develop again today. Can't rule out an isolated strong to severe storm, or isolated flash flood issues this evening. 2. Typical August weather returns for the new workweek. Isolated hydro issues cannot be ruled out, mainly the first half of the week.

DISCUSSION

Key message 1: Successive rounds of rainfall will develop again today. Can't rule out an isolated strong to severe storm, or isolated flash flood issues this evening.

Showers have begun to blossom across the low terrain, largely along an area of differential heating following this morning's thick cloud cover, and bolstered by a stagnant frontal boundary near the NC-TN border. Recent satellite imagery depicts a z500 trough axis extending from northern Mississippi and into Indiana and western Ohio, still sandwiched between broad ridging over the western Atlantic and the central CONUS heat dome. The latest RAP analysis that any lingering closed low is becoming less pronounced, favoring more of an open wave. This exposes us to some ripples of shortwave energy aloft, as the trough itself becomes essentially anchored in its current location. Mesoanalysis continues to depict PWs as high as 2.2" east of the mountains, and extremely warm, moist profiles across most of the area.

Expectations for today haven't changed much since the previous forecast; the 12Z suite of guidance came in with good consistency on the development of scattered showers again today east of a stalling frontal boundary progged to remain just over the western tier of the forecast area. The environment will be a juicy one, featuring 2-2.25" PWs, extremely warm profiles with anemic mid-level lapse rates, and tall, skinny unstable layers. sbCAPE will surge to 1500-2000 J/kg, but struggle to produce much actual buoyancy. About the only thing today's environment will have going for it is 15-25kts of deep layer shear, enough for some degree of organization and a resultant low-end damaging wind threat...possibly augmented by hydrometeor loading given the exepctation for efficient rainfall- producers today. A layer of dry air aloft also looks to push into the area later this afternoon, which should raise DCAPE values going into the evening. Guidance suggests that at least one round of rainfall will develop during the afternoon, and possibly a second round going into late evening or the first half of tonight.

The CAMs are in decent agreement that for most of the area, QPF will fall below any threshold of concern. Widespread hydro issues aren't likely. However, successive runs of the HRRR and RRFS have come in with a ribbon or two of 2-4" rainfall totals, mainly falling this evening where stronger cells train. There's no consistency in *where* this sets up, though, with some HRRR runs keeping higher QPF confined to the mountains, and a couple RRFS runs producing higher totals as far east as the I-77 corridor. If such a corridor of higher rainfall this evening were to set up across an area already saturated by previous rainfall (mainly the NC and SC Blue Ridge Escarpment) then a flash flood warning or two could be in order.

The front will struggle to make any further eastward progress going into the new workweek, as the upper pattern becomes stagnant in response to a complex blocking pattern that'll persist well into the week.

Key message 2: Typical August weather returns for the new workweek. Isolated hydro issues cannot be ruled out, mainly the first half of the week.

Most of the upcoming week still looks like it'll be driven by a persistent upper pattern consisting of an Atlantic ridge, a central CONUS ridge, and a trough trapped between them, over the Tennessee Valley and western Carolinas. The ensembles are coming into increasingly good agreement that for at least the middle of the week, we'll have a closed cutoff low sitting in place across area, as a result of which we can expect more widespread rainfall on Tuesday and Wednesday, potentially lingering past the typical diurnal max. So, especially during the first half of the week, isolated hydro/flash flood issues aren't out of the question if training cell set up in the right spots.

By Thursday, the majority of LREF members feature the upper low opening back up into the mean flow as a longwave trough slides across the northern US and Canada. We'll remain within a relative weakness in the subtropical ridge, and that will continue to drive daily showers and thunderstorms, but the deeper moisture and associated high PWs and pathetic lapse rates will have exited the area by then, so we'll be trending back toward seasonally normal weather.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: Widespread SHRA continues across most of the terminal forecast area, with showers and embedded thunder ongoing. Many sites remain MVFR after struggling to fully scatter out morning stratus decks, and increasingly widespread rainfall will result in varying conditions through the remainder of the afternoon. Expect this first round to persist into early evening before translating east...and then a second round along an advancing frontal boundary later on this evening. Confidence is low on how far east this second round will make it before dissipating. Another night of at least MVFR to IFR ceilings, and perhaps some patchy fog, will develop overnight, but guidance suggests it will scatter out a little faster than it did today and yesterday.

Outlook: Daily showers and thunderstorms, along with associated flight restrictions, are expected to linger through at least the middle of next week. Patchy fog and low stratus will be possible in the mountain valleys each morning, and wherever heavier rainfall fell the previous evening.

GSP WATCHES/WARNINGS/ADVISORIES

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