textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00Z TAFs.

KEY MESSAGES

1. Hot, humid conditions continue into the new workweek, but the worst of the heat is behind us for this particular event. Nevertheless, conditions will remain humid with higher-than-normal temperatures for the next couple of days. Those with outdoor plans should take steps to remain hydrated and take breaks indoors when possible. Never leave children or pets unattended in hot vehicles. 2. Scattered showers and thunderstorms are expected each afternoon through Tuesday, with a low-end threat of severe weather, mainly due to damaging winds. 3. The pattern becomes increasingly unsettled the latter half of the week, resulting in more showers and thunderstorms, but not necessarily confined to the afternoons.

DISCUSSION

Key message 1: Hot, humid conditions continue into the new workweek, but the worst of the heat is behind us for this particular event. Nevertheless, conditions will remain humid with higher-than-normal temperatures for the next couple of days. Those with outdoor plans should take steps to remain hydrated and take breaks indoors when possible. Never leave children or pets unattended in hot vehicles.

Heat and humidity will continue each day this week, but it generally looks like the highest-impact temperatures are over with. Forecast temperatures look to remain above normal by a category or two through the week (so, mid-90s outside the mountains) but with heat index values struggling to reach into the triple digits.

Key message 2: Scattered showers and thunderstorms are expected each afternoon through Tuesday, with a low-end threat of severe weather, mainly due to damaging winds.

Persistent subtropical ridging is expected to remain in place for much of this week. Guidance broadly agrees this will keep upper troughing and any associated synoptic-scale forcing shunted well to our northwest today through Tuesday, before the pattern gets a little more progressive the latter half of this week (see third key message). Through at least Tuesday, we'll remain within a moisture- rich air mass directly exposed to the Gulf, with a frontal zone stalled well to our north. Mainly diurnal convection can be expected each afternoon/evening. Each day, initiation can generally be expected over the NC mountains, but confidence on convective evolution remains limited as is typical on these pulse days.

Scattered convection has become confined to being focused along a series of outflow boundaries across mainly the southern tier of forecast zones this evening. With waning instability, the severe storm risk appears to have largely abated. However, slow cell movement will continue to produce locally heavy rainfall, and can't rule out a very isolated excessive rainfall event if cell(s) become anchored along colliding outflow boundaries.

The convective setup looks a little more scrambled on Monday. We won't have the strong cross-boundary component to winds over the mountains, and without the presence of a lee trough / convergence zone, the CAMs depict more ridge-driven initiation and look less organized overall. Deep layer shear may become marginally higher by Tuesday afternoon, as a cold front settles across Tennessee and Virginia, and upper flow becomes elevated by an approaching shortwave. Values of 15-20kts as depicted in the GFS, GDPS, and their ensembles would support at least a modicum of organization, even though any frontal passage will likely be delayed until at least early Wednesday morning.

Key message 3: The pattern becomes increasingly unsettled the latter half of the week, resulting in more showers and thunderstorms, but not necessarily confined to the afternoons.

The pattern should become more active the second half of next week. Synoptic guidance agrees on the gradual retreat of subtropical ridging Wednesday and Thursday, which should allow us more opportunity for some shortwaves to make it into the area. Indeed, the operational models all feature a few low-amplitude trough axes crossing the area during the Thursday-Saturday timeframe. Confidence remains quite low on timing and details of any such features, but in general, would expect the potential for more synoptically well-forced convection late in the week, should upper features align well with the thermodynamics. The environment during this period looks like it'll feature better shear as well, so there's potential for more organized convection as opposed to the typical summertime pulse convection we've been dealing with recently.

AVIATION /00Z MONDAY THROUGH FRIDAY/

At KCLT and elsewhere: Isolated to widely scattered convection continues across the Piedmont this evening. One last cell appears poised to move over KCLT, and a tempo for -TSRA is included there through 01Z. Can't rule out some convection near KAND through mid-evening, but convection should otherwise steer clear of the other TAF sites this evening. Increasing moisture levels will create improved potential for fog/low stratus overnight into early Monday. For now, the signal for restrictions is strongest at KAVL, and IFR visby has been included there from 08-13Z. While KHKY saw some reduced visby Sat night/early Sun, the signals are actually weaker for restrictions there tonight. Nevertheless, MVFR visby will be featured there from 09-13Z. MVFR is also forecast at KGSP during the pre-dawn hours. VFR will be featured elsewhere, but certainly can't rule out low cigs and/or reduced visby at any site.

Thunderstorm outflows are playing some havoc with the winds this evening, but they should become light/vrbl or calm by late evening. SW winds of 5-10 kts are expected to develop late Mon morning into the afternoon. Another round of scattered/perhaps numerous convective coverage is expected during the afternoon...warranting Prob30s for -TSRA at all sites.

Outlook: Mainly afternoon/evening shower and thunderstorm chances will continue through at least Tuesday. The pattern remains unsettled Wednesday and beyond, but confidence is lower on how things will unfold, and what the convective potential will be. 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.