textproduct: Greenville-Spartanburg

SYNOPSIS

Dry but mild high pressure remains over the area through Monday. A moist cold front crosses the area Tuesday into Wednesday, producing widespread showers and a few thunderstorms. Much colder temperatures arrive behind it.

NEAR TERM /THROUGH MONDAY/

As of 100 PM Sunday:

Key Message 1: Dry Weather High pressure centered over the Mid Mississippi Valley this afternoon will continue to build eastward through tonight and Monday. A weak cold front will slide across or just north of the area this evening, but with little available moisture, no precipitation is expected. The high will move from the Ohio Valley this evening to the Central Appalachians overnight, reaching the Mid Atlantic states Monday morning and the coast by late in the day. This pattern will maintain quiet and dry conditions across the region through Monday. Confidence: High

Key Message 2: Increasing Clouds Monday The low-level ridge axis will shift east of the area on Monday, allowing southerly return flow to develop on its backside. This will initiate weak isentropic lift, favoring the formation of stratocumulus during the day. Uncertainty remains in both the timing and areal coverage of this cloud development, with ensembles showing increasing spread (NBM 25th75th percentile range of 40 to 50 percent) from late morning through afternoon. The most likely scenario supports clouds developing first across the western zones (west of I-26) and gradually expanding east toward the I-77 corridor by late afternoon. Forecast trends lean slightly more pessimistic on cloud cover, blending toward the NBM 75th percentile, with upslope flow enhancing clouds across the GA/SC Foothills and adjacent mountains. Confidence: Moderate due to timing and coverage uncertainty.

Key Message 3: Mild Temperatures with Gradual Cooling The current warm spell will persist through this afternoon before moderating slightly behind tonights weak cold front. Highs today will range from the lower to mid 70s across the Piedmont, around 10 to 15 degrees above normal and more typical of mid to late October. Cooler air will filter in overnight, leading to highs on Monday mainly in the mid to upper 60s, still 5 to 10 degrees above normal.

NBM ensemble spread remains small for highs both today and Monday (less than 4 degrees between the 25th and 75th percentiles), indicating high confidence. For lows tonight, a modest 4 to 7 degree spread appears across the NC/SC Foothills and NC Piedmont. Probabilities of temperatures falling below 37 degrees remain only 10 to 15 percent in these areas, and the REFS indicates probabilities of temperatures below 38 degrees with winds under 4 mph (within a 10 km radius) around 20 to 35 percent. Frost development is therefore unlikely, and any occurrence would have minimal impact given the end of the growing season. Confidence: High in temperature trends, Moderate in low temperature distribution.

SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/

As of 1023 AM EST Sunday...

Key Messages:

1) A cold front arrives on Tuesday, with at least scattered showers expected through the day Tuesday and continuing through most of Wednesday.

2) Embedded thunder is expected with this system, mainly over the eastern Piedmont and Upstate. Severe risk is limited but nonzero, with the main threat being damaging straight-line winds.

Deep layer SW flow will be in place over the Carolinas by Monday night, with our region sandwiched between potent ridging over the western Atlantic and an amplifying longwave trough across the west/central CONUS. A cold front positioned out over the Tennessee Valley will struggle to make much eastward progress overnight a deamplifying shortwave more or less merges with the mean flow, leaving the upper flow basically parallel to the front. In the low-levels, a ~1025 mb surface high will push off the Mid-Atlantic coast, maintaining a CAD-like regime through the overnight hours...featuring all the hallmarks of a solid wedge...with low-level WAA and moist upglide...a strong inversion below 950mb...and a weak inverted ridge east of the Appalachians. Given this, although mentionable PoPs aren't warranted for most of the CWA until daybreak, some patchy light drizzle can't be ruled out across much of the area before dawn, particularly across the Savannah River Valley where a few models are keying on a surge in moist isentropic ascent after midnight.

Either way, the front will finally push into the area after daybreak, spurring widespread showers and a categorical PoP across the western 2/3 of the CWA. Despite some timing differences in ensemble solutions still, the general trend in the last 2 model cycles or so is toward a slower FROPA, with the front stalling across the area well into Tuesday night. No surface-based instability looks to develop on Tuesday, but by Wednesday, with the front still draped across the low terrain, model consensus is that a tongue of 100-300 J/kg sbCAPE will develop, mainly along and south of I-85. The latest LREF cycle supports a 40-50% chance of at least 300 J/kg across our southeastern-most tier of counties. Given this, it's important to consider wind profiles...which feature some 40-50kts of bulk shear but generally unidirectional hodographs. So, a tornadic HSLC event continues to look unlikely, but the chances are increasing for at least isolated severe storms to produce damaging wind Wednesday afternoon and evening. QPF response, meanwhile, has increased somewhat over the Savannah River Valley, while remaining more or less unchanged over the Smokies and Balsams. The thinking remains that dry antecedent conditions will preclude much if any hydro threat.

Otherwise...expect highs a little cooler on Tuesday, near- or only slightly-above normal across the Foothills and parts of the NC Piedmont which will be inside the cold wedge...but still well above normal over most of the Upstate and eastern Piedmont, which will be outside the wedge. Widespread above-normal temperatures are then expected on Wednesday, before the front picks up and begins to usher in cooler air Wednesday evening.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

As of 1103 AM EST Sunday...

Key Messages:

1) Dry and cold weather persists through at least Saturday, with chilly highs and frigid lows.

2) A cold front approaches from the west on Sunday.

Wednesday night will be our coldest night in a while, as strong post-frontal CAA ushers in a much colder and drier air mass. Lows will fall into the 30s across the low terrain...and may drop into at least the mid 20s, maybe colder, at higher elevations. Continental high pressure will dominate the forecast through at least Saturday, keeping highs in the low 50s each afternoon, and frigid lows in the low 30s or 20s each night, even at lower elevations. By Sunday, broad troughing over the western CONUS will eject a shortwave into the southern Great Plains, spurring surface cyclogenesis and driving another cold front toward our area. It likely won't make it to us by the end of the 7-day forecast period Sunday evening, but by all accounts the pattern looks to be amplifying by then.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: VFR conditions will persist at all terminals through the period. The region remains cloud free early this afternoon, and this will continue into the evening hours. High clouds will begin to arrive from the west shortly after daybreak Monday. Stratocumulus is expected to develop across the southwestern terminals, including AND, GMU, and GSP, during the mid to late morning. Bases should remain around 4 to 5 kft with a brief BKN deck possible, but impacts are not anticipated. This cloud deck may spread northeast toward AVL, HKY, and CLT during the afternoon, although there is greater uncertainty in timing and coverage for those sites.

Winds have become light and variable at most locations following a weakened pressure gradient this afternoon. The exception is AVL, where a north to northwest wind around 5 to 10 kt continues due to terrain channeling in the French Broad Valley. Winds will remain light and variable tonight, with a slight preference for a northerly or northeasterly component as high pressure passes to the north. On Monday, light winds of 3 to 6 kt will gradually veer to the east and southeast as the high shifts toward the Mid Atlantic coast.

Outlook: VFR conditions are expected to continue Monday night, although mountain valley fog or low stratus cannot be ruled out early Tuesday morning. Shower and thunderstorm chances increase Tuesday into Tuesday night ahead of an approaching cold front, along with a greater potential for restrictions. The front will pass through on Wednesday, followed by a return to drier and predominantly VFR conditions late in the week.

GSP WATCHES/WARNINGS/ADVISORIES

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