textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Slightly higher snowfall totals through Monday across portions of the NC mountains, but no change to hazards.

Relative humidity values lowered to below 20% across much of the area Tue afternoon...enhancing the fire danger.

PoPs increased to 80-90% across the entire forecast area on Thursday in association with a cold front.

KEY MESSAGES

1. Accumulating snow with gusty NW winds across portions of the mountains near the TN border through Monday, with below normal temperatures through Tuesday. The snow will cause mainly travel problems along the TN border. 2. Near-critical fire weather conditions are expected each afternoon through Tuesday. 3. Temperatures warm during the latter half of the week, with a cold front likely bringing a band of showers/possibly embedded thunderstorms Thursday into Friday.

DISCUSSION

Key message 1: Accumulating snow with gusty NW winds across portions of the mountains near the TN border through Monday, with below normal temperatures through Tuesday. The snow will cause mainly travel problems along the TN border.

Upper low will dig across the Appalachians today, with cyclogenesis off the NC coast rapidly deepening the surface low. This of course has implications further north, but the increasing surface pressure gradient between the low and the arctic high building into the Northern Plains will result in very gusty NW winds across the area today and tonight, continuing into Monday, especially across the mountains where gusts especially >3500ft will be well into Advisory range, possibly peaking out >60mph at the very highest elevations. Will continue to wrap this wording into the WSW.

Deep moisture associated with the upper low will continue to batter against the mountains, leading to a prolonged period of NW flow snow. Some disagreement in QPF among operational and ensemble guidance, but certainly could see some hefty amounts given the extremely cold temps and high snow ratios. NBM accums were quite a bit higher than last night with some areas in the smokies and near Roan Mtn approaching 20". Pulled back on those amounts given the shallow moisture but considering 50kt winds at 850mb tonight plus multiple rounds of DPVA spinning around the larger vort lobe, would not be surprised if some areas outperform the current forecast. Operational GFS keeps 700mb fairly saturated through 15-18z Monday at temps -15 to -12C, so plenty of dendritic snow growth potential. Highest amounts will as usual be at the highest elevations so will continue products as they are (WSW for Avery and >3500ft Mitchell, WSY for most of the rest of the border counties except >3500ft Swain). Certainly expect some snow showers at the lower elevations but minimal if any accumulations.

With the very strong CAA, temperatures will be well below normal through Monday night, improving slightly on Tuesday. Wind chills tonight at some of the highest elevations will meet Cold Weather Advisory criteria, so per coordination with surrounding offices will wrap that wording into the WSW as well. Cold and blustery on Monday, more than 15 degrees below seasonal normals. High pressure will slide toward the Deep South Monday night and the pressure gradient will weaken, and expect even colder temperatures though wind chills will be slightly improved.

Key message 2: Near-critical fire weather conditions are expected each afternoon through Tuesday.

Sustained winds out of the NW of 15 to 25 mph, with gusts generally in the 40-50 mph range are expected across the mountains late this afternoon through much of Monday before diminishing Monday evening. Much of upstate South Carolina, northeast Georgia, and the NC Piedmont is expected to see near-critical RH this afternoon, with better chances for critical RH for all areas outside the mountains expected Mon afternoon. These conditions will elevate the fire danger, especially across the foothills, where a wetting rainfall failed to occur Sat/Sat night.

Widespread very low/critical RH...15-20% in most areas...is expected Tue afternoon, although winds are expected to be considerably weaker...out of the SW mostly at 5-10 mph Tue afternoon. Nevertheless, the fire danger will again be elevated, mainly in locations east of the NC counties bordering TN, where at least some snowpack is expected to linger into Tue. Though the overall fire danger will be elevated, below-normal temperatures could be a limiting factor.

Key message 3: Temperatures warm during the latter half of the week, with a cold front likely bringing a band of showers/possibly embedded thunderstorms Thursday into Friday.

A short wave trough and attendant cyclone will progress across the Great Lakes and New England during mid-week, with a baroclinic zone forecast to stretch from the central Plains through the Ohio Valley into New England. A warm sector with return SW low level flow will become established across the Southeast from Wed onward, as large scale height falls overspread the central Conus. Conditions will quickly warm as a result, with temperatures generally forecast to be 5-10 degrees above climo from Wed through the end of the forecast period. Cyclogenesis associated with aforementioned height falls will activate a warm frontal zone across the region Wed night into early Thu, with mostly chance PoPs warranted across much of our forecast area. As a cyclone lifts toward the northeast Conus by late Thursday, global models remain in excellent agreement in dragging a well-forced cold frontal boundary through the southern Appalachians and vicinity, with more than adequate moisture flux forecast to support 80-90 PoPs in all areas by Thu evening. Global models continue to hint at the potential for a narrow ribbon of surface-based instability to accompany the front through the CWA, with shear likely being more than adequate for a low-end severe storm threat...IF any positive buoyancy is realized.

Showers are expected to clear out Thu night/early Fri. However, small PoPs are maintained through much of the weekend as global models indicate some degree of an active upstream pattern, with at least some global models depicting light precip development along the remnant baroclinic zone draped across the region.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: VFR cigs beginning to push out of the Piedmont at TAF time, but with increasing moisture tonight expect restrictions to remain across a chunk of the mountains, and cannot rule out brief restrictions with -SHSN at KAVL in the overall northwest flow pattern, with PROB30 remaining in the TAF to cover this during the early overnight hours. Otherwise mainly a wind forecast with gusty NW winds across the area. Winds will weaken slightly overnight, and it's possible gusts may drop off across Piedmont TAFs, but this would be brief and would pick up again Monday with increased mixing. Cannot rule out FEW-SCT VFR Cu Monday afternoon but no restrictions expected at terminals.

Outlook: Quiet weather returns to region for Tuesday and Wednesday. Another system will approach the area Thursday, bringing showers and associated restrictions.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Storm Warning until midnight EST Monday night for NCZ033-050. Winter Weather Advisory until midnight EST Monday night for NCZ048>052. SC...None.