textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

A Heat Advisory was issued for the area from the Lakelands/lower Piedmont to parts of the Charlotte metro area from 11 AM to 8 PM EDT Saturday.

SPC expanded the Marginal Risk for severe weather for Saturday to include all of northeast GA.

KEY MESSAGES

1. Hot and humid conditions persist through Saturday, with heat index ranging from the upper 90s to mid 100s outside of the mountains this afternoon and again Saturday afternoon. Cooler and less humid conditions return behind a cold front starting Sunday. 2. Severe thunderstorms will be possible across the area Saturday afternoon and evening ahead of a strong cold front. Drier conditions should return for much of next week.

DISCUSSION

Key message 1: Hot and humid conditions persist through Saturday, with heat index ranging from the upper 90s to mid 100s outside of the mountains this afternoon and again Saturday afternoon. Cooler and less humid conditions return behind a cold front starting Sunday.

A Heat Advisory remains in effect through early evening for parts of the Charlotte metro area. A quick pattern shift should already be underway through tonight as the center of a mid/upper anticyclone moves from the western Carolinas to the central Plains, underneath the amplifying upper ridge over the middle of the country. This will more or less complete the omega blocking pattern. Meanwhile, short wave axis rotating around a deepening upper low centered over the Canadian Maritimes will eventually drive a strong cold front southward and across the fcst area, though not in time to save Saturday from being another hot and humid one. Unlike the past few days, with the front approaching from the north, the dewpoint is not expected to mix out as much. The CAMs suggest that convective initiation won't occur until 18Z at the earliest, which won't prevent us from heating up considerably. The result will be heat index values climbing a few degrees higher than today. The way the forecast is presently constructed, an area of max heat index above 105 wraps around from the Lakelands and lower Piedmont of SC to the Charlotte metro area.

There are a number of ways that our forecast could be wrong, but in particular it depends on the later convection. Should convective debris clouds move down from the north early in the day or if an active outflow boundary drifts down from overnight convection, the temps could stay a bit cooler and thus keep the heat index in check. This is most likely to affect the Charlotte metro area. That being said, it being a holiday weekend and all with potentially lots of outdoor plans, it seems prudent to issue a Heat Advisory in this situation. Fortunately, Saturday should be the last really hot day. Temps cool down behind the front starting on Sunday and should be around normal for Monday and Tuesday. Alas, a warmup is expected from mid-week onward.

Key message 2: Severe thunderstorms will be possible across the area Saturday afternoon and evening ahead of a strong cold front. Drier conditions should return for much of next week.

For the rest of the afternoon, expect it to play out similar to yesterday given a similar environment, which resulted in isolated convection over a larger area than expected. The mesoanalysis shows 2000-3000 J/kg of mixed-layer CAPE with a minimal cap, we just lack a good trigger other than differential heating over the mtns. We have isolated showers already and expect this to continue through the afternoon. The entire area has dCAPE greater than 1000 J/kg, so if a storm can make the most of what it has in its surroundings, an isolated severe wind gust will be possible, just like yesterday.

Saturday should be more active with a decent chance for severe storms, but lots of questions remain. On the one hand, the environment should have plenty going for it in terms of ingredients. The RAP depicts mlCAPE above 3000 J/kg and deep layer shear around 25 kt in the afternoon/evening and the expectation is for a boundary to move down from the north. That's usually enough to support several severe storms and the SPC expanded the severe risk over the fcst area a bit in the updated Day 2 Outlook. On the other hand, the CAMs were somewhat disappointing in their depiction of coverage and strength through 00Z Sunday, though the presence of a boundary could be inferred. In this case, I would trust the forcing/environment in the model more than the perceived lack of response in the model. Precip probs were increased above what the model blend offered, but might still be too low. Experience tells me that stretches of hot and humid weather often end with a round of severe weather triggered by a strong cold front, so that raises expectations in line with a Slight Risk.

Storms will be possible again on Sunday, though without the severe risk. The rest of the period looks below normal for precip coverage.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR for all terminals through the evening hours with only a few high-based cu. Cannot rule out a thunderstorm at KAVL and thus the PROB30 and potential brief restriction. Wind direction might vary between SW and NW, but speeds will be light. The ever-present fog/low stratus in the mtn valleys will be a concern overnight. The past few nights have remaind VFR at KAVL, but will include some LIFR to stay in agreement with the shower chances this afternoon. For Saturday, the main concern will be afternoon/evening storms, but convective initiation happens too late to affect anyone but KCLT and its longer TAF period. A PROB30 should suffice for now. Wind will be greatly affected by outflow boundaries, but should still be relatively light.

Outlook: Shower and thunderstorm activity should gradually increase through the holiday weekend as a cold front crosses the area. More suppressed weather returns by early next week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from 11 AM to 8 PM EDT Saturday for GAZ026-028- 029. NC...Heat Advisory from 11 AM to 8 PM EDT Saturday for NCZ071-072- 082. Heat Advisory until 8 PM EDT this evening for NCZ071-072-082. SC...Heat Advisory from 11 AM to 8 PM EDT Saturday for SCZ009>014- 019.