textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The risk for severe thunderstorms on Saturday has increased.

The aviation forecast was updated to reflect the 18Z TAF issuance.

KEY MESSAGES

1. Hot and humid conditions persist through Saturday, with expected heat index values in the lower 100s most places east of the mountains each afternoon. A heat advisory remains in effect for Union County NC this afternoon. 2. Severe thunderstorm risk will increase on Saturday afternoon as a strong cold front approaches.

DISCUSSION

Key message 1: Hot and humid conditions persist through Saturday, with expected heat index values in the lower 100s most places east of the mountains each afternoon. A heat advisory remains in effect for Union County NC this afternoon.

A typical hot summertime pattern will persist through Friday, featuring a large upper anticyclone initially centered over the Carolinas this afternoon. The anticyclone should stick around through tonight, but on Friday changes will be underway as an upper level blocking pattern amplifies over North America. From Friday into Friday night, the pattern change should move/relocate the anticyclone to the Plains, but not fast enough to prevent one more potentially hot day on Saturday, depending on the timing of a strong cold front. More on that later.

Fcst high temps for Friday look very similar to today, meaning a continuation of highs on the order of 10-15 degrees above normal, while the dewpoint should remain in the upper 60s and lower 70s. The dewpoint is the tricky part of the fcst as it has mixed out lower than fcst for the past few days. No reason to think that it won't mix out again on Friday afternoon, so some of the drier raw model guidance was blended with the NBM to drop the dewpoint down to probably more reasonable levels. This has the effect of lowering the Heat Index down a few degrees and keeps it below Heat Advisory criteria for all but a few spots around metro Charlotte. The general idea here is that confidence is low at this time that we would need an Advisory for Friday afternoon. Saturday is more problematic because the temps will be more likely to be affected by outflow boundaries and the timing of convection, but in the absence of those, the air mass remains hot and capable of making 10 degrees above normal. The dewpoints should be higher, though, so Heat Advisory criteria would be more attainable.

At any rate, temps should cool back down to around normal for the first half of next week as we come under the influence of the cyclonic upper flow around the upper low off the Canadian Maritimes. Alas, the blocking pattern comes undone and progressive flow will bring the heat back later next week.

Key message 2: Severe thunderstorm risk will increase on Saturday afternoon as a strong cold front approaches.

Deep convection is expected to remain relatively isolated and confined to the higher terrain this afternoon and most likely again Friday afternoon. For today, it would appear that just enough of a weak cap remains in place to keep the convection shallow, owing probably to the lack of a good trigger. The mesoanalysis shows sufficient buoyancy across the region, with roughly 3000 J/kg of most-unstable CAPE to work with, and yet the radar remains fairly quiet and the HRRR only develops a few cells over some ridgetops where upslope flow converges enough to overcome any capping. The chances of any strong storms is very low, but non-zero. Friday looks similar to today with perhaps a little stronger capping, so once again the deep convection will be confined mainly to the mtns. Will add a slight chance to the northwest Piedmont in deference to the Day 2 Convective Outlook which brushes the Marginal Risk across that area. Thunderstorm chances are certainly much higher farther to the north, but there is some small chance that something could arrive late in the day on an outflow boundary from the convection moving past well to our north.

Which brings us to Saturday. Seems like a lot of heat waves (if that's what you want to call the last several days) end with a bang. The Day 3 Convective Outlook from the SPC shows a Slight Risk across much of the fcst area, and that is certainly warranted. As we come more under the cyclonic influence of the upper low over the Canadian Maritimes, a cold front should drop down from the northwest, possibly with ideal timing during peak heating. The 15Z RAP looks compelling at 18Z Saturday, showing an environment with upwards of 3500 J/kg of muCAPE and 20-30 kt of effective/deep layer sheer to go along with an excellent trigger. Convection moving past to the north may also leave behind an old outflow or cloudy/clear boundary to be a focus for afternoon storms. The event remains tantalizingly just outside the window of the CAMs, so confidence is kept low, but we eagerly await later runs to provide more clarity. Should be quiet after the front passes Saturday night.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR through this evening with scattered high based cu. Wind will be light and variable in many locations. If there is a problem, it will be mtn/valley fog and low stratus at KAVL. It didn't happen this past morning and there are no guarantees for the upcoming night, but IFR will be included after 09Z. Apart from that, not much going on for Friday. Diurnal convection will develop over the mtns, but confidence is too low for including in the TAF.

Outlook: More widespread showers and storms are expected on Saturday as a cold front crosses the area. Suppressed weather returns Sunday and beyond.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Heat Advisory until 8 PM EDT this evening for NCZ082. SC...None.