textproduct: Greenville-Spartanburg

SYNOPSIS

Widespread rain lingers through this evening ahead of a cold front, with some isolated thunder embedded at times this afternoon. Drier conditions return behind the front tonight into early next week as high pressure builds into the region. Another cold front may bring precipitation chances back Wednesday into Thursday.

NEAR TERM /THROUGH SUNDAY/

As of 230 PM Saturday:

Key Message 1: Locally heavy rainfall will continue to produce isolated flooding in the southern mountains this afternoon. The threat for strong to severe thunderstorms is diminishing as convection struggles to organize farther south into the Piedmont.

As of early afternoon, a cold front was positioned across the western North Carolina mountains and is progressing slowly eastward. The front is expected to reach the foothills near sunset and move into the southern and eastern Piedmont later this evening. A warm front remains draped near the GA, SC, and NC foothills, placing much of the southern Piedmont within the warm sector.

North of the warm front, widespread moderate rainfall continues as warm, moist air overruns the boundary. Orographic enhancement along the southern Blue Ridge Escarpment combined with modest elevated instability around 200 to 300 J/kg has focused convection and enhanced rainfall rates across the southern mountains. This has resulted in a narrow corridor of 3 to 5 inches of rainfall across Rabun County, GA and portions of the western NC and SC border region, producing localized flooding. Recent HREF and REFS guidance highlighted this corridor well, and probabilistic guidance does not support a significant eastward expansion of the heavy rainfall threat through the remainder of the afternoon.

Farther south within the warm sector across portions of northeast Georgia, Upstate South Carolina, and possibly the southern North Carolina Piedmont, instability is surface based and supports a marginal risk for a few strong to severe thunderstorms through roughly the 2 to 4 PM time frame. However, convection has struggled to organize upstream, and this has tempered confidence in storm intensity as activity moves eastward. While isolated stronger storms remain possible, the overall severe threat is trending lower and should remain brief and localized, with damaging wind gusts as the primary hazard.

Key Message 2: Gusty winds this afternoon and again on Sunday may lead to isolated power outages and difficult travel, especially at higher elevations.

Strong southerly flow aloft has been able to mix down to the surface within the warm sector this afternoon. As the low level jet peaks through mid afternoon, occasional gusts of 25 to 35 mph are expected across northeast Georgia, the Upstate of South Carolina, and the North Carolina Piedmont.

Winds shift to northwest behind the front tonight, then strengthen again on Sunday as the pressure gradient tightens between the departing low and high pressure building in from the west. Forecast soundings support widespread gusts of 30 to 40 mph Sunday afternoon across much of the area. The strongest winds will occur at higher elevations in the North Carolina mountains, where gusts near 50 mph are expected above 3500 feet. These winds may result in downed tree limbs, isolated power outages, and hazardous travel for high profile vehicles. A Wind Advisory has been issued for the higher elevations.

Northwest flow behind the front will also support a brief period of upslope snow along the Tennessee border in western North Carolina. Accumulations will generally remain under one inch, though locally up to two inches is possible in the Smokies above 5000 feet.

Key Message 3: A conditional fire weather concern exists Saturday across the southern and eastern Piedmont if rainfall fails to materialize this afternoon.

Fire weather concerns for Saturday hinge on how far south and east rainfall associated with the front progresses later this afternoon. Some CAM guidance depicts a sharp rainfall cutoff near or just south of I-85, while other solutions support a more organized line moving eastward and producing a brief period of measurable rainfall across far southeastern zones.

If rainfall remains limited, a conditional fire weather risk could develop Saturday across the southern and eastern Carolina Piedmont as gusty northwest winds combine with rapidly falling relative humidity behind the front. Ensemble guidance used in the NBM often struggles with post frontal mixing regimes, leading to a high dewpoint and RH bias. As a result, probabilistic guidance is more informative in this case. Experimental NBM joint probabilities of RH at or below 25 percent combined with wind gusts of at least 30 mph remain very low, generally 5 percent or less, suggesting that Red Flag headline criteria are unlikely to be met.

SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/

As of 1200 PM EST Saturday...

Key Message #1: Dry with a warming trend expected through the short term.

Surface high pressure builds in from the west Sunday evening into Monday, lingering over the region through Tuesday night. This will allow dry conditions to continue. Low relative humidity is expected each afternoon but the return of lighter winds will act to limit fire weather concerns. Mostly sunny/mostly clear skies can be expected through Tuesday before cloud cover gradually increases Tuesday night. Highs on Monday will end up near normal to just below normal for most locations, becoming 5-10 degrees above normal Tuesday. Lows Sunday night will end up ~3-5 degrees below normal. Lows Monday night will end up a few degrees above normal across the mountains and a few degrees below normal east of the mountains. Lows Tuesday night will end up ~5-10 degrees above normal thanks to increasing cloud cover.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

As of 130 PM EST Saturday...

Key Message #1: A more active pattern can be expected through the long term with precipitation chances returning.

A large upper trough will swing across the eastern CONUS Wednesday into Thursday. This will drag a pair of surface cold fronts across the forecast area on Wednesday before the fronts push east early Thursday. The latest GFS appears to be the most bullish regarding moisture, showing precip chances area-wide during the middle of the week as the model depicts a cut off low developing right over the Carolinas. Meanwhile, the Canadian and ECMWF keep moisture confined to mainly the North Carolina mountains as these models do not depict a cut off low developing. With models still not in agreement regarding how much moisture will be available with this system, confidence on PoPs remains low for now. Temperatures should be cold enough to allow northwest snow to develop behind the front across the North Carolina mountains Wednesday night into early Thursday. Light snow accumulations look plausible for most counties along the NC/TN border but elevations above 4,000 feet could see a few inches of snow develop. Areas along and north of I-40 could also see a brief shot of light snow early Thursday morning but confidence is very low and will be highly dependent on how far east moisture develops. Elsewhere, temperatures should end up warm enough for rain if precip develops. Another front pushes across the area late Friday into early Saturday, possibly bringing some light precipitation to the spine of the Appalachians. Above normal temperatures linger through Wednesday night before below normal temperatures return the rest of the period.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: A cold front is currently move thru the fcst area as sct light showers continue to make their way eastward. Most of this shower activity should move east of our area over the next few hrs, with TEMPOs for -SHRA and MVFR cigs carried at KCLT and the Upstate terminals until 03z. At KHKY and KAVL, showers are less likely, so only a VCSH is mentioned. Otherwise, any lingering low-end VFR cigs should scatter out overnight with VFR conditions expected for the rest of the taf period. The main concern will be the gusty winds. Winds will veer from S/SW to W tonight/overnight, and eventually north of west by Sunday morning. Low-end gusts are likely to continue at most terminals thru the overnight and into the morning. By late Sun morning, winds are expected to strengthen again as the pressure gradient tightens. Gusts of 25 to 35 kt are anticipated during the afternoon and into the evening, with gusts weakening by the end of the taf period.

Outlook: Expect dry, VFR conditions for the first half of the week with sfc high pressure over the region. Another frontal system will approach on Wednesday and may bring showers and associated restrictions.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Wind Advisory from 6 AM to 6 PM EST Sunday for NCZ033-048>053- 058-059-062-063. SC...None.