textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Aviation discussion updated.

KEY MESSAGES

1. Mainly diurnal convection is forecast through the weekend, with isolated severe storms possible. 2. Hot and humid conditions continue through the weekend.

DISCUSSION

Key message 1: Mainly diurnal convection is forecast through the weekend, with isolated severe storms possible.

Under the influence of a weakening upper ridge, the atmosphere has become hot and unstable across the forecast area this afternoon...with sbCAPE of 2500-3000 J/kg analyzed across the entire area. Isolated deep convection is beginning to initiate along the ridge tops of the southern Appalachians, and coverage is expected to gradually expand across the mountains through mid-afternoon, with isolated activity eventually expected elsewhere. The robust instability will support the potential for a few strong-to-severe pulse storms, especially during late afternoon. Convection is expected to largely dissipate by late evening, giving way to a very warm and muggy night.

The axis of the upper ridge is forecast to be over the area Friday, resulting in even hotter conditions. Having said that, forecast soundings are more suppressed and a little drier (especially in the boundary layer) with high LCLs indicated through the afternoon. As such, initiate of diurnal deep convection is expected to be limited in our forecast are. However, a region of modest height falls is expected to approach the Appalachians from the west late in the day...likely resulting in at least scattered convection developing along an attendant weak boundary across east TN during the late afternoon. At least some of this activity should move into far western NC during the evening. While deep layer shear is forecast to remain unimpressive, low level shear may be sufficient to promote cell clustering along outflows, which along with typical pulse activity will support an isolated severe storm threat. Considering the timing with respect to the diurnal cooling cycle, the potential for convection to develop east of the mountains during the overnight is very uncertain, but the severe storm threat is nevertheless expected to wane past mid-evening.

The upper ridge is forecast to progress off the East Coast by Saturday, with more in the way of a typical diurnal convective cycle expected Saturday afternoon/evening, with mostly scattered activity forecast over the mtns and isolated-at-most coverage across the remainder of the area. The next round of (more substantial) height falls is forecast to impact the East by the end of the day Sunday, enhancing the potential for deep convection across mainly the mountains by Sunday evening. Improving shear parameters may support another severe storm threat during this time.

The global models agree in depicting establishment and gradual strengthening of a long wave trough across the East during the first half of next week. Attendant baroclinic zone will set up over the Southeast, potentially providing a focus for rounds of convection. However, this activity may...and in fact is most likely to (per current guidance) impact locations just south and east of the CWA, and our forecast currently carries only token chance PoPs for mostly diurnal convection Monday through Thursday. Cooler temperatures are also forecast under the anomalously low heights, with generally below normal temps forecast from Tuesday onward.

Key message 2: Hot and humid conditions continue through the weekend.

Under the influence of the upper ridge, conditions are expected to be quite hot through the weekend, with temps forecast to be at least 5 degrees above climo through Sunday. With the axis of the ridge overhead, Friday is expected to be the hottest day, with highs in the mid-90s forecast for all areas outside the mountains, and lower 90s in the mountain valleys. The increasingly subsident regime should make for a little drier air mass, and surface dewpoints are forecast to mix out a bit during the afternoon...taking a bit of an edge off the Heat Index potential. Nevertheless, peak HI values of 99-102 are expected across much of the lower elevations. This is several degrees below Heat Advisory criteria of 105, but being the first truly hot day of the year, we'll issue a Heat Advisory for areas that are just shy of criteria...namely Charlotte proper and areas south and east of there.

With the ridge axis shifting east, Saturday is expected to be a little "cooler," but still above normal, with some potential for near-Heat Advisory criteria being met across the Lakelands, especially Saturday afternoon. Temps may warm slightly again Sunday in advance of approaching frontal zone, with again some potential for near-Heat Advisory criteria. Temperatures begin a cooling trend at the beginning of the new work week.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: With showers and thunderstorms diminishing, expect VFR conditions overnight at most terminals. KAVL and KHKY could see vsby restrictions into MVFR and briefly IFR for BR and low stratus between 09-13z. Mostly light to calm SW winds overnight and pick up again Friday morning/afternoon out of the S/SW. KAND and KGMU could see low-end gusts of 15-20kts during the afternoon. Another round of TSRA could occur as a weak front approaches from the NW. Will go with a PROB30 at KCLT and mountains sites with VCSH elsewhere. Expect winds to turn more NW after 00z at KCLT.

Outlook: Diurnal convection and associated restrictions are expected into early next week. Valley fog and low stratus is possible each morning, primarily in the mountain valleys.

CLIMATE

RECORDS FOR 06-12

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 91 2016 61 1903 69 1981 40 1988 1894 1964 1972 KCLT 98 1956 64 1913 74 1998 45 1972 1926 1986 1902 1943 KGSP 99 1914 61 1913 74 1981 47 1972 1920

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Heat Advisory from 11 AM to 8 PM EDT Friday for NCZ071-072-082. SC...None.