textproduct: Greenville-Spartanburg

SYNOPSIS

High pressure will result in chilly and dry conditions today and likely Thursday. Conditions turn even colder Friday as precipitation returns to the area, with a wintry mix likely in portions of western North Carolina. Chances for precipitation linger through the weekend, with warmer but still below-normal temperatures.

NEAR TERM /THROUGH TONIGHT/

As of 1214 AM EST Wednesday...

Key Messages:

1) SPS remains in place through 9 AM today for icy road conditions along the NC-TN border.

2) Dry and chilly weather continues Wednesday.

Still seeing some lingering low stratus across the extreme southern tier of the forecast area, as well as some valley stratocu across the NC mountains. Radar suggests light NW flow snow is still underway across some locations along the NC-TN border - particularly the Smokies - but with little to no accumulation expected. An elongated 1020mb surface high is presently analyzed extending from the eastern Ozark Plateau southward into the Mississippi delta region, and slowly filtering into the Carolinas. Expect light winds the remainder of the overnight, allowing excellent radiative cooling to bring temperatures across most of the region, with the exception of those zones across the Savannah River Valley and Midlands which may see lingering cloud cover for much of the night, into the mid-20s by daybreak. Some patchy mountain valley fog may manage to develop, particularly across the Little Tennessee Valley.

By day, skies will clear out and high pressure will migrate eastward...becoming centered over the Carolinas by evening. High temperatures will only climb into the upper 40s despite ample sunshine. A light southerly wind should develop by evening and continue into the overnight hours, maintaining lows up to a category warmer on Thursday morning than they will be this morning.

SHORT TERM /THURSDAY THROUGH FRIDAY/

As of midnight Wednesday morning:

Key message: Precipitation chances increase Thursday night and peak Friday, with temperatures supporting a chance of light snow and/or freezing rain in a portion of the area.

Cold front will be associated with shortwave moving across the eastern Great Lakes and St Lawrence Valley Thursday. The shortwave appears too weak to budge the flat ridge over the Caribbean and subtropical Atlantic, and the front practically stalls over NC by afternoon. High altitude cloud cover is shown to spread over the area. Midlevel speed max will promote stronger downslope flow. With that and some airmass modification temperatures should rebound a few degrees east of the mountains, despite mid to high altitude clouds brought by the front. Slight CAA and earlier arrival of clouds suggests slightly lower temps over the mountains compared to Wed.

The sfc high behind the front expands east out of the mid-MS Valley which should lead to the front oozing southward across GA/SC Thu night. A previously existing baroclinic zone in the lower MS Valley will activate ahead of another southern-stream shortwave. Moist WAA associated with that development is responsible for light QPF response over GA/SC which translates to slight-chance (around 20%) PoPs in our southern zones Thu afternoon. Surface temp and wet-bulb support that activity being all rain at that time, if it materializes that early.

Models have come into better agreement that the sfc high will shift eastward and be in position for in-situ CAD as the baroclinic zone amplifies Thu night and PoPs increase from SW to NE after midnight Friday morning. Prog soundings are largely supportive of snow at onset over high elevations of the SW NC mountains and almost all mountain/Escarpment locations northeast of the French Broad. That could be a result of guidance consensus bringing the parent high in faster and more strongly than earlier runs. Surface temps/wet-bulbs look to be the main factor in p-type in this early stage of the event, with wet-bulb profiles indicating a weaker or even nonexistent warm nose compared to the past two CAD events we just had. Though NBM bias corrections seem to have picked up on the wedge (values being close to the 75th-90th NBM percentile), blended in raw model temps/dewpoints which performed well in last weekend's CAD to try to nail the p-type trends more closely. Wet-bulb temp looks likely to be slightly above freezing in the NW NC Piedmont (areas near/north of I-40) and in some of the FB Valley. Much of those areas are forecast to have a rain-snow mix with accumulation thus limited to only a few tenths of an inch. Some sleet and/or freezing rain may develop Friday morning as warm nose builds with increasing WAA over the wedge; ice accums mainly result along the usual areas of the eastern Escarpment. Most areas that see wintry precip in the morning change over to all rain by noon or so, but ice could accumulate in the coldest northern Escarpment areas into late day.

Friday should be an exceptionally cold day with the eastern NC foothills and I-40 corridor not likely making it out of the 30s, and maxes in the lower 40s even in the warmest areas such as the Little TN Valley and south of I-85.

LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/

As of 110 AM Wed:

Key message 1: Confidence remains low for Friday night through Sunday due to model differences in the handling of the stalled front.

Still expect that a Miller A-type coastal low will spin up Friday afternoon along the stalled front and push out into the Atlantic overnight, following the primary jet streak. Models largely show a secondary speed max developing at 250mb over the Ozarks and lower OH Valley Fri night which reactivates the inland portion of the front, producing more QPF response mainly south of I-20. The GFS and NAM solutions prolong precip in our CWA into Friday night as a result of this process. Such occurrence likely would keep a shallow wedge in place even as the parent high becomes increasingly distant. 02/12z EC and 03/00z GDPS begin CAD erosion. A few members of the various global ensembles do support the wetter Friday night solution over some or all of the CWA. Hence low PoPs persist thru Saturday morning and since temps dip below freezing in some of our northern zones, some of that precip falls out of the process as freezing rain. More likely, by the time temps fall below freezing, precip will have tapered off. Most models support a lull then and some or all of the daytime hours Saturday; temps will bounce back to the upper 40s to near 50 but that is still several below normal.

Height falls are progged to continue Saturday in the southern Plains, and with a boundary still stalled north of the Gulf Coast, no surprise the GFS/EC/GDPS all depict another activation and PoPs spreading in from the south Saturday night or Sunday. Still seeing spread among their ensemble members as to whether precip results this far north, and also onset timing is basically all over the place in that period. Surface high pressure is not supportive of another CAD wedge, but it is not terribly likely that the first wedge will erode on Saturday. So, a few colder areas in our north could see freezing rain or sleet with the second round, but with more limited extent than the earlier events.

Key message 2: Precip could redevelop late Sunday thru early Monday as a shortwave passes. Light snow is possible in portions of the mountains.

Most models depict the Plains shortwave finally reaching the southern Appalachians Sunday night. Multiple models show coastal cyclogenesis occuring yet again along the stalled front, though this time probably distant enough that we would not see precip via the low, only NW flow activity near the TN border. The GFS however is depicting a deeper wave which spins up a low over AL/GA and produces widespread precip over the CWA Monday, which turns to snow over almost all the CWA before ending Mon evening. PoPs increase to a mentionable 20-30% value over all zones early Monday as a nod to a GFS-like solution, though with the current temp trends snow only results in the mountains and northern foothills. A clipper follows Tue night which bears a slight chance of actual NW flow snow to the mountains at that time. Temps remain below normal thru Day 7 with the coldest night likely Monday night, when lows are mostly in the mid-upper 20s.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Most sites are now VFR, and should mostly remain so through the 06z TAF period. Some intermittent MVFR ceilings could appear at KAND over the next few hours as some trapped stratocu continues to clear out south of I-85, and there's a chance that some MVFR valley fog could develop across the NC mountains. Still carrying a TEMPO for that at KAVL. Otherwise...expect light N winds through the period and mostly clear conditions. By Wednesday night, an increase in cirrus appears likely, but will be of little operational significance.

Outlook: VFR conditions persist through at least the first part of Thursday. Rain chances return Thursday night and into the weekend, along with possible associated flight restrictions.

GSP WATCHES/WARNINGS/ADVISORIES

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