textproduct: Greenville-Spartanburg

SYNOPSIS

A cold front moves across the area tonight and to the Carolina coast on Wednesday, producing widespread showers and a few thunderstorms. Much colder temperatures arrive behind the front and continue into the weekend. Another cold front arrives Sunday and Sunday night.

NEAR TERM /THROUGH WEDNESDAY/

As of 630 PM EST Tuesday: A warm conveyor belt associated with an approaching cold front can be seen on IR and the radar mosaic entering the forecast area this evening. So far, the precip has been just light to moderate showers, with deeper convection still well southwest of the area over east-central AL. There seems to be just enough isentropic lift and precip to support an in-situ cold air damming wedge near the Blue Ridge, extending east toward the I-85 corridor. This wedge may hold on and keep the majority of the CWFA free of sfc-based CAPE. However, a plume of 200-750 J/kg of MUCAPE is expected to cross the area as the WCB crosses the area this evening into the overnight. The latest CAMs are in good agreement on convection staying disorganized and relatively weak as it crosses the area. Shear will support some rotating cells, but storms will likely stay elevated. Thus, severe threat will remain very low. The one exception would be along the southeast edge of the CWFA, where the wedge front may lift north and bring in 100-250 J/kg of SBCAPE around or just after midnight. Along this boundary a non-zero wind and tornado threat may materialize.

Otherwise... Height falls will accompany a northern stream trough as it takes on a negative tilt and surface cyclogenesis will be underway underneath the core of the trough near the Upper Great Lakes this evening into the overnight hours. A warm sector becomes better established later this evening with the second round of convection. Better forcing for ascent is expected with height falls and jet dynamics in place, while deeper moisture transport along and just ahead of the incoming cold front sets up a favorable environment for convective development. In this case, robust organized convection later this evening through a good portion of the overnight as the activity moves in from west to east. Elevated instability will be present to go along with 55-65 kts of deep layer shear. Can't rule out a few stronger storms capable of producing localized damaging wind gusts. However, latest 12Z CAMs suggest 100-250 J/kg of SBCAPE will be present despite the nocturnal time frame. A warm front will be present across the Piedmont zones, so any surface instability that develops and interacts with the boundary could form an isolated tornado even though low-level wind shear isn't that impressive (15- 25 kts). Expect the line of convection to move east of the area just before daybreak Wednesday. Overnight lows will run 15-20 degrees above normal thanks to the warm sector regime.

The cold front gradually pushes across the CWFA during the daytime period Wednesday with a full fropa expected just before sunset. Stout CAA behind the front will bring in very gusty winds across the mountains, especially at elevations above 3500' by Wednesday evening. Very tight pressure gradient (5-8mb) becomes established as a result. Confidence is fairly high that criteria Advisory gusts will be met by the very end of the forecast period and into the short term. The onset of CAA will be delayed, especially outside of the mountains where a northwesterly downslope component settles in. With dry conditions, weak downsloping, and better insolation with clearing skies behind the front, expect afternoon highs on Wednesday to remain 6-12 degrees above normal.

SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/

As of 110 PM EST Tuesday: Key Message 1: Windy and much colder conditions return behind the cold front Wednesday night

A strong cold front will continue to push southeast across the Southern Piedmont of NC, the GA Piedmont, and much of Upstate SC Wednesday evening. Rapid pressure rises will follow the front, with six hour rises of 6 to 7 mb forecast across the mountains by 00Z Thursday and 5 to 6 mb across the remainder of the area by 03Z Thursday. This will support a brief but notable surge of post-frontal winds.

NBM ensemble probabilities show a high likelihood of impactful winds across the higher elevations Wednesday night. The probability of maximum gusts exceeding 45 mph approaches ninety percent along the ridge tops of the Blue Ridge and Smokies. Coordination with surrounding offices supports holding off on a third period Wind Advisory at this time, but confidence is high that advisory-level gusts will ultimately be realized.

Cold air advection behind the front will maintain gusty winds through the night as the nocturnal boundary layer remains partially mixed. Winds will gradually diminish toward daybreak as the low level jet weakens. Temperatures will fall below freezing in the higher elevations of the NC mountains, with lows mainly in the mid 30s elsewhere.

Key Message 2: Cold and dry conditions for Thanksgiving and Friday

Winds will weaken Thursday night into Friday as high pressure builds overhead. A much colder air mass will be in place for the holiday period. Highs on Thanksgiving will be 10 to 15 degrees below normal in the mountains and 5 to 10 degrees below normal elsewhere. Even colder conditions arrive Friday, with temperatures 15 to 20 degrees below normal in the mountains and 10 to 15 degrees below normal across the Piedmont.

Model guidance shows limited agreement on how quickly winds diminish across the mountains. Ensemble spread for peak wind gusts remains significant, ranging from 10 to 15 mph between the 25th and 75th percentiles in the broader mountain region and 15 to 25 mph at the highest elevations. Despite this spread, dry weather is expected through Friday with abundant subsidence under the high.

LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/

As of 115 PM EST Tuesday:

Key Message 1: Cold and dry conditions continue through Saturday

High pressure will remain the primary feature Friday night into Saturday, maintaining cold and dry weather across the region. Although southerly return flow begins to develop on Saturday as the high shifts toward the Mid Atlantic coast, warm air advection does not arrive in earnest until after peak heating. As a result, temperatures on Saturday will remain similar to Friday, with highs mainly in the mid to upper 40s despite gradual warming aloft.

Key Message 2: Wet weather returns late this weekend into early next week

A broad upper trough will deepen across the north central CONUS this weekend while several southern stream disturbances track eastward around its base. These waves will move across the southern states and bring widespread precipitation to the Carolinas late in the weekend and early next week.

Ensemble guidance shows considerable spread in onset timing. A few fast members bring precipitation into the area late Saturday night, while most early arriving solutions favor early to mid Sunday morning. The median and mean cluster mid to late Sunday, and slower members delay onset until Sunday evening. This results in a roughly twelve to eighteen hour window across the forecast area. Confidence is highest that precipitation begins at some point on Sunday, but exact timing remains uncertain, especially across the Piedmont.

Precipitation will fall mainly as rain. If onset occurs on the early end of the ensemble envelope late Saturday night, a brief mix is possible across the highest elevations of the NC mountains and perhaps parts of the northern foothills before temperatures warm aloft early Sunday morning. Any wintry component is expected to be brief and limited to the high terrain.

A hybrid cold air damming pattern will likely develop as the surface high shifts offshore and initial precipitation falls into a dry boundary layer. This wedge will be difficult to erode without a full airmass change, so temperatures will likely remain in the 40s and lower 50s from Sunday through Tuesday. NBM ensemble spread is sizable, with high temperature spreads of 5 to 10 degrees on Sunday, 8 to 12 degrees on Monday, and 12 to 18 degrees on Tuesday, reflecting continued global model difficulty resolving the strength of the wedge.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Widespread showers and a few embedded thunderstorms will move across the area this evening into the early part of the overnight, and warrants prevailing SHRA, with TEMPO for TSRA. Ahead of the activity, LIFR cigs have formed in the foothills, including KGSP and KGMU. Reduced visibility and cigs are possible with showers and storms that pass near or over the terminals, but may help mix out some of the LIFR cigs. The showers and thunderstorms should push out of the area by daybreak Wednesday. Winds are mostly variable and will continue this way through the evening and overnight, especially with showers and storms in the area. Behind the line of showers and storms overnight, an LIFR/IFR low stratus deck and MVFR/IFR vsbys are expected to develop, while winds pick up out of the SW, limiting dense fog potential. Model guidance scatters out cig/vsby restrictions rather quickly later in the morning Wednesday as winds pick up out of the southwest (KAVL will take on a northwest component). Gusty winds expected from late morning thru the aftn across the entire area, toggling to NW at KCLT Wednesday evening.

Outlook: Drier and predominantly VFR conditions should linger through the end of the week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.