textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Increased precip/thunderstorm chances across the region and moved up the timing. Threat of severe storms is increasing along/N of I-40 into early afternoon.

KEY MESSAGES

1. Slight risk of severe thunderstorms across most of the forecast area this afternoon and evening. The risk for severe storms is less on Sunday, but non-zero. Make sure you have multiple ways to receive warnings, especially if you have outdoor plans. 2. Hot and humid again today with triple digit heat index values returning east of the mountains. A Heat Advisory remains in effect today from 11 AM to 8 PM for the southern zones. Make sure to stay hydrated and take frequent breaks in the shade or air conditioning. Cooler and less humid conditions finally return behind a cold front starting Sunday.

DISCUSSION

Key message 1: Slight risk of severe thunderstorms across most of the forecast area this afternoon and evening. The risk for severe storms is less on Sunday, but non-zero. Make sure you have multiple ways to receive warnings, especially if you have outdoor plans.

A strong cold front will approach from the north through this evening before tracking across the forecast area overnight. Most of western North Carolina and the northern Upstate are in the Slight Risk on the updated Day 1 Convective Outlook from SPC. A Marginal Risk is in place across the rest of the fcst area.

A line of thunderstorms was developing late this morning over southwest VA and moving into northeast TN, on track to reach the nrn mountains by 17Z at the latest. The latest mesoanalysis shows the morning cap has lifted and we have roughly 2000-3000 J/kg of sfc-based CAPE to work with, though the storms appear to be moving out ahead of the better effective bulk shear. We also have greater than 1000 dCAPE out ahead of the developing line. Suffice to say, this line of storms should probably survive the diurnal minimum and keep trucking to the east-southeast. Precip probs have been updated already to raise chances over the area mainly along/N of I-40. There is a distinct possibility that we end up with a SVR Watch before early afternoon. It remains to be seen how this activity, or an outflow boundary, will maintain itself or trigger new storms east of the mtns, but chances are going up.

The main change for Sunday (at least compared to this time yesterday) is that the Day 2 SPC Severe Weather Outlook has a good chunk of the GSP forecast area highlighted in a severe risk. Most of the GSP forecast area is in a Marginal Risk, with the exception of the northern NC mountains, northern NC foothills, and northern NC Piedmont which are not highlighted for a severe risk. The Slight Risk extends into portions of Greenwood and Abbeville counties in SC. With the front expected to be south of the area on Sunday, the severe threat would appear to be lower compared to today. Guidance has deep layer shear decreasing to 15-20 kts on Sunday and generally 2,000 J/kg or less of MLCAPE. The best instability would appear to be across the western and southern zones per the NAMNest and RAP. The main hazard with any severe storms that develop Sunday afternoon and evening will be damaging wind gusts but isolated hail up to one inch in diameter cannot be ruled out.

Drier conditions should return for much of next week as an upper anticylcone builds across the region. Another cold front may bring rain chances back Friday.

Key message 2: Hot and humid again today with triple digit heat index values returning east of the mountains. A Heat Advisory remains in effect today from 11 AM to 8 PM for the southern zones. Make sure to stay hydrated and take frequent breaks in the shade or air conditioning. Cooler and less humid conditions finally return behind a cold front starting Sunday.

No changes to the Heat Advisory this forecast update as the product appears well placed based on the 00Z CAMs timing of convection ahead of a strong cold front this afternoon/early evening. The southern zones are currently expected to see a later arrival time for storms compared to the northern zones, so thinking is that the southern zones east of the mountains are more likely to reach Heat Advisory criteria compared to the northern zones east of the mountains. However, if the line of storms ends up developing earlier in the day, this would act to limit some, if not all, of the southern zones from reaching Heat Advisory criteria (will have to wait and see what the 06Z CAM trends are later this morning). Regardless, with a myriad of outdoor plans expected over the holiday weekend, a more "impacted-based" Heat Advisory appears warranted.

Cooler and less humid conditions finally return behind the front starting Sunday, with Monday still expected to be the coolest day in the forecast period. A warming trend can be expected Tuesday through the at least Thursday as another upper anticyclone builds over the southern and eastern United States. Luckily, this dome of high pressure looks less hot and humid overall. Another cold front looks to bring back slightly cooler temperatures by late week.

AVIATION /15Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Dry and VFR through the morning hours, possibly even the early afternoon hours for some terminals, outside of mountain valley fog and stratus that developed overnight. Fog and stratus will lift shortly after sunrise. A cold front will bring a broken line of TSRA and associated restrictions across the terminals this afternoon into early this evening so maintained PROB30s at KAVL and KAND as confidence is lower on these terminals seeing storms. Introduced TEMPOs across the rest of the terminals due to higher confidence on storm development. Confidence on timing for both PROB30s and TEMPOs is low as guidance has not been consistent run to run regarding TSRA activity. Dry and VFR conditions should return by early or late evening behind the front. Winds be NW at KAVL through the period. Winds east of the mountains will start out WSW/SW before toggling more WNW/NW by late morning or early afternoon. VRB winds can then be expected by mid to late afternoon into the late evening hours. Overnight, winds east of the mountains will toggle more N/NE.

Outlook: Another round of showers and thunderstorms develops Sunday, possibly lingering into Monday. Drier conditions return by Monday night, lingering through mid-week.

GSP WATCHES/WARNINGS/ADVISORIES

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