textproduct: Greenville-Spartanburg

SYNOPSIS

Above normal temperatures continue through Saturday, with near- record warmth possible. Rain chances increase Friday through Saturday ahead of a cold front. Temperatures return to near-normal Sunday into early next week under building high pressure.

NEAR TERM /THROUGH FRIDAY/

As of 1:15 PM Thursday:

Key Message 1: Increasing clouds this afternoon and tonight with a few light showers developing in the mountains overnight. Impacts remain minimal.

A shallow CAD setup is in place this afternoon as high pressure over the Northeast ridges southwestward along the eastern slopes of the Appalachians. Mid and high level moisture is streaming northeastward ahead of the Plains trough, while dry low level air remains in place across much of the forecast area. That dry layer is slowly eroding from south to north as the surface high shifts offshore.

Low clouds will overspread the region tonight as isentropic lift and low level moisture increase. Forecast soundings indicate lift is shallow, largely confined below 800 mb, which should limit both shower coverage and intensity. As a result, slight chance PoPs remain focused over the mountains and adjacent foothills late tonight.

From an impacts standpoint, rainfall amounts will be light and localized with no hydrologic concerns. Widespread fog is not expected due to increasing cloud cover, with any visibility restrictions mainly confined to ridge tops embedded in the stratus deck.

Key Message 2: Shower chances increase on Friday, with steadier rain limited mainly to the southwestern mountains. Temperature uncertainty remains elevated, especially where CAD may hold.

On Friday, surface low pressure lifting through southeast Ontario and Quebec will drive a cold front southeastward toward the Ohio and Tennessee Valleys. The front will slow as it approaches the area due to downstream blocking and deep layer flow aligned parallel to the boundary. This will delay eastward progression of precipitation, keeping the best forcing and heaviest rainfall west of the forecast area for much of the day.

Most of the Piedmont, especially the lower and southeastern sections, may remain dry through peak heating. The main exception will be the southwestern North Carolina mountains, where steadier rain is expected to arrive during the afternoon. Ensemble guidance shows a 30 to 40 percent chance of exceeding one inch of rainfall there during the day, with probabilities of two inches or more remaining below 10 percent. Given recent dry conditions, impacts from rainfall are expected to be minimal.

Temperature confidence is lower than usual, particularly across the foothills and northern Piedmont, where ensemble spreads reach 8 to 10 degrees. This reflects uncertainty tied to a hybrid or in situ CAD setup and the timing of precipitation relative to daytime heating. Given typical model biases in these scenarios, high temperatures were nudged below NBM guidance in areas where the wedge is most likely to persist and where cloud cover or rain may arrive near peak heating.

SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/

As of 1215 PM Thu:

Key message 1: Rain chances continue through Saturday, with heavy rain likely in the southwestern mountains Friday night into Saturday. Severe storms possible Saturday.

The area remains in the warm sector ahead of a cold front as a wave of low pressure develops along the front to our southwest. This stalls the front to our west Friday night before the low moves north and drags the front across the area late Saturday. Very high PW values remain across the area ahead of the front. This combined with strong forcing will lead to the potential for heavy rainfall, mainly across the mountains where showers can train before the front begins moving on Saturday. Still, the lack of strong convection may limit the rainfall rates and the very dry antecedent conditions will limit the flood threat. That said, conditions will need to be monitored as the event unfolds.

There remains a concern for severe thunderstorms Saturday as the front tracks across the area. Very strong bulk shear will accompany the front, but instability looks to be limited. The Ops models, even the NAM which usually has higher CAPE values, show almost no sbCAPE and very low muCAPE. The SREF shows only a low potential for muCAPE above 100 J/kg. The latest LREF ensemble though shows a very high potential for 100 J/kg and moderate potential for 250 J/kg. Therefore, this will need to be monitored as well. The current Marginal risk for severe storms looks reasonable.

Precip chances taper off quickly Saturday night for most. However, guidance now showing the potential for a secondary short wave and short burst of low level moisture rotating through the trough and across the area on Sunday. For now, have kept Sunday dry, but this will also need to be monitored for high elevation snowfall potential.

Key message 2: Strong winds possible throughout the area Saturday and Sunday along and behind cold front.

The LLJ preceding the front will bring strong southerly winds over the SW mountain ridges early Sat. A fine line of shallow convection could serve to bring down strong wind gusts. Have gone with wind gusts above the NBM to advertise peak values around 30-35 mph in many Piedmont locations.

Cold advection behind the front will usher in strong gusts over the mountains Saturday night, potentially warranting at least a high-elevation Wind Advisory. Gradient at the surface and 850mb peaks Sunday afternoon as base of shortwave crosses the CWA; with downsloping offsetting CAA and mostly sunny skies, good mixing should occur, reaching the 30-40 kt winds at 850 mb and resulting in 25-30 mph gusts across the area; any Wind Advisory for the mountains probably would continue thru Sunday or even Sunday night, as gusts probably won't taper off until high pressure centers west of the area early Monday. Lows Sun night 5-10 below normal; any wind chill issues would be confined to very high elevations.

LONG TERM /MONDAY THROUGH WEDNESDAY/

As of 1240 PM Thu:

Key message: Generally quiet weather with typical January temperatures for the first half of next week. Next trough bringing chances for precip Wednesday into Thursday.

Mostly clear with winds becoming light Monday as the center of surface high pressure moves by to our south. Highs Monday end up a few degrees below normal. Warmer but still below normal Monday night. Southwesterly flow develops Tue as next trough digs into the Midwest. The trough deepens with an upper low diving to the Gulf Coast by Thursday. Some guidance wants to move a southern stream shortwave to our south during this time as well. The guidance keeps the initial cyclogenesis to our south and east keeping any precip mainly to our south and east as well. Secondary cyclogenesis is also to our south and east but closer to our area. They do agree on increasing low level moisture across the mountains though. This leads to quite a bit of uncertainty and low confidence to the forecast for Wednesday and Thursday. For now, the model blend limits chance PoP to the mountains with slight chance elsewhere. Any snow is limited to the mountains where temps may be cool enough to support it with rain elsewhere.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR conditions this afternoon with overcast cirrus canopy overspreading the area. Light E to NE winds (except SE at AVL due to channeling up French Broad Valley) under 10 kt have become established as high pressure retreats off the Mid-Atlantic Coast. Strengthening southerly flow just above the surface will help saturate the low-levels tonight. Stratus deck should form this evening and eventually lower to MVFR across the area from SW to NE between 06Z and 12Z Friday. There is a potential for IFR CIGs to develop in the morning. IFR conditions were added to the TAFs for AND, GMU, and GSP where probabilities of CIGS below 1 kft are above 50-60 percent and highest across the region. Elsewhere where the probability of IFR CIGs is lower, a SCT deck below 1 kft was advertised after sunrise. Used NBM probabilistic guidance to hone in on timing of lowering CIGs. This conservative approach yielded a slightly slower onset than the various deterministic LAMP guidance. Rain chances increase from west to east after 18Z on Friday. Although the LAMP guidance is advertising a strengthening 5-10 kt southerly wind during the day, it will most likely be lighter and more variable, especially as you get closer to the foothills terminals. Farther east in the Piedmont like CLT, the southerly wind may eventually become established, but it may take a bit longer for that to happen.

Outlook: A cold front slowly moves into the region Friday night and Saturday. Flight restrictions will be likely during this time with elevated chances for IFR restrictions for the Foothills and mountain terminals and MVFR restrictions for the Piedmont terminals. Conditions improve late Saturday into Sunday behind the front.

GSP WATCHES/WARNINGS/ADVISORIES

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