textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for 12z TAF issuance.

KEY MESSAGES

1. A cold front will arrive this morning possibly producing a few mountain showers. Low humidity and gusty winds this afternoon will increase fire danger. Much cooler Tuesday but still with low humidity, so fire danger may remain elevated. 2. Cold-air damming will maintain below-normal temperatures and potentially light precipitation for southern portions of the area, before temperatures trend warmer again on Thursday. 3. A backdoor cold front crosses the area on Friday, possibly producing some showers and storms, and definitely ushering in a cooler air mass for next weekend.

DISCUSSION

Key message 1: A cold front will arrive this morning possibly producing a few mountain showers. Low humidity and gusty winds this afternoon will increase fire danger. Much cooler Tuesday but still with low humidity, so fire danger may remain elevated.

Surface cold front will cross the Appalachian spine around 12z this morning; convection in the Ohio Valley is expected to wane nocturnally by that time. A shallow moist layer should accompany the front into the mountains, so small PoPs are warranted along the TN border for upslope showers beginning around daybreak and lasting thru mid-morning until the moisture is exhausted. Profiles don't look to saturate east of the mountains, but a few shallow cumulus may break out at the top of the mixed layer. Higher elevations in the mountains should see temps fall slowly from daybreak onward with post-frontal CAA. Downsloping will offset the CAA east of the mountains, but the foothills and far NW NC Piedmont will see temps peak around midday before falling. Other Piedmont areas should still see max near the climatological time, with temps still reaching the mid 70s to near 80. Think mixing may prove a bit deeper than what most models expect so raised gust potential a few kt compared to the operational NBM, so occasional gusts of 25-30 kt appear likely this afternoon. Though dewpoints look to fall thru the afternoon in all areas, the onset of substantially drier air aloft appears to come after peak heating/mixing. Thus RH still most likely will fall "only" to the 25-30% range in the NC/SC Piedmont, although recovery during the evening probably will be slower than usual. Combined with the wind, we will retain the previously coordinated Increased Fire Danger Statement; still a few points off Red Flag criteria.

Temps tonight actually fall a few degrees below normal, with a freeze likely in higher mountain elevations. Surface high will remain centered in the upper OH Valley late tonight. Winds taper off slowly through the evening; northeasterly CAA however will keep breezy conditions across the Piedmont. Tuesday, the high builds east toward the Mid-Atlantic coast in a configuration that eventually will foster hybrid CAD, though without precip it technically doesn't qualify as CAD during the day. Winds and gusts taper off during the afternoon. Temps remain a few below normal. Exceptionally dry air is shown above the PBL; again mixing will be limited by subsidence but already low dewpoints should dip during the afternoon. RH is expected to fall closer to 20 percent, but with lighter winds the objective IFD criteria may not be met in NC/SC. Fuel moistures should trend lower by then, and in some setups like this land management agencies ask to waive the criteria. Additional coordination may be undertaken today.

Key message 2: Cold-air damming will maintain below-normal temperatures and potentially light precipitation for southern portions of the area, before temperatures trend warmer again on Thursday.

By early Wednesday morning, models depict the center of a broad surface high extending from the Chesapeake Bay into the western Atlantic and driving S/SE flow across the western Carolinas. Weak cold-air damming will likely already be in place owing to this configuraion...but mostly dry and suppressed on the eastern periphery of a broad blocking ridge over the west-central CONUS.

A number of members of the 12z GEPS members as well as a handful of ENS members depict a decaying mid-level vorticity max drifting across the Tennessee Valley and prompting some weak moist upglide on Wednesday afternoon...enough to justify introducing a slight chance PoP back into the Upstate and Savannah River Valley on Wednesday aftenroon. It still seems more likely that any precipitation will be patchy and very light, but there's a potential scenario where more appreciable drizzle will develop if this solution were to verify. Whatever rainfall develops should fall apart through late evening, leaving us dry again on Wednesday night and Thursday.

Temperatures, meanwhile, which without the CAD would quickly be modifying at this point and trending warmer, will instead be held in check by weak surface CAA as well as any precipitation that occurs. At least on Wednesday, they'll likely remain a category or so below normal east of the mountains. If more appreciable rain develops as described above, temperatures could conditionally be even cooler than currently forecast over the Upstate. By Thursday, however, as the surface high retreats into the Atlantic, and SW low-level flow intensifies ahead of an approaching baroclinic zone, CAD should steadily erode from the bottom up, allowing temperatures to climb back into the 70s - above normal - for the low terrain.

Key message 3: A backdoor cold front crosses the area on Friday, possibly producing some showers and storms, and definitely ushering in a cooler air mass for next weekend.

By Friday, broad, low-amplitude z500 troughing will drive a well- defined cold front out of the Mid-Atlantic and into the Carolinas. As described in previous forecasts, nailing down timing for this frontal passage remains challenging...but the latest 12z LREF cycle strongly favors a slower/later solution, allowing at least a few hundred joules of sbCAPE to develop over parts of the forecast area, and supporting a band of showers and embedded thunder crossing the region sometime Friday. Those members which are more unstable also appear to favor weaker bulk shear, by virtue of keeping the upper wave farther north during peak heating...such that some degree of organization remains a possibility, but confidence in any severe risk remains quite limited.

Behind the front, temperatures will plummet as the cooler air mass settles in over the weekend. High temperatures could drop 20 degrees or more from Friday to Saturday, putting them back a few degrees below normal on Saturday...while lows Saturday night should fall as low as the mid-30s across the Piedmont and Upstate...and below freezing for the NC mountains.

AVIATION /12Z MONDAY THROUGH FRIDAY/

At KCLT and elsewhere: Breezy conditions persist around the terminal area with temperatures having been elevated overnight and starting out already somewhat mixed. Cold front will pass the terminals this morning, bringing rapid onset of gusts with sfc warming; upslope moisture along the Appalachians near the front will maintain low VFR stratocu near KAVL. Mixing should bring down NW winds and invoke wind shift 14-17z outside the mountains. Low VFR cumulus could develop along the front briefly at the other sites, around midday, but rapid drying post fropa suggests they will be short lived. Gust potential peaks in mid to late afternoon by same token, at 25-30 kt. Gusts slowly diminish thru the evening as winds turn NE. Too dry for fog tonight.

Outlook: VFR through Tuesday. Some restrictions may develop Tue night with moist frontal zone forming over the area. Diurnal convection possible later in the week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...INCREASED FIRE DANGER from 3 PM EDT this afternoon through this evening for NCZ035>037-056-057-068>072-082-502-504- 506-508-510. SC...INCREASED FIRE DANGER from 3 PM EDT this afternoon through this evening for SCZ008>014-019-101>109.