textproduct: Greenville-Spartanburg

SYNOPSIS

Cool high pressure will control our weather into the weekend. Another cold front is set to sweep across the area early Sunday, bringing a very cold and dry airmass for the start of next week.

NEAR TERM /THROUGH FRIDAY/

As of 630 PM EST Thursday...

Sat pix show high clouds streaming over the area with mid clouds poised to enter the mountains ahead of an approaching clipper low. Winds are generally light and should remain so for all but the higher elevations. Lows look on track, along with the small PoP and sprinkles/mountain flurries already in the forecast as the clipper moves by late tonight/early Friday.

Key Message #1: Quiet weather continues into Friday.

As the axis of a deamplifying z500 shortwave pivots east of the forecast area early Thursday afternoon, the upper pattern will flatten out while remaining somewhat constrained overnight into early Friday. In the lower levels, CAA should quickly abate through mid-afternoon Thursday, allowing winds to slacken considerably. Some guidance depicts a weak lee trough developing this afternoon, resulting in a period of S/SW winds before becoming light and variable overnight into Friday. For most of the forecast area, conditions should remain dry through late Friday. Temperatures are still expected to peak below climo this afternoon, but should climb back toward normal on Friday amid weak low-level WAA and a slight downslope component to daytime winds.

Key Message #2: Snow flurries briefly develop tonight over the northern Blue Ridge and Foothills.

Another area of z500 vorticity is expected to dig out of the Cumberland Plateau into the northern NC Appalachians early Friday morning, in conjunction with an 850mb wave lifting out of the Tennessee Valley and northern Georgia during the day. In advance of this feature, guidance depicts at least light snow developing in the predawn hours across parts of Avery and Mitchell Counties, as well as some points farther east into the Foothills and northern Piedmont. Interestingly enough, this takes place during a period of low-level WAA, and forecast profiles depict a subtle warm nose developing. However, that "warm nose" appears to stay below freezing, rendering it moot for the p-type forecast. Rather, expect any light precip that develops to be snow, and with essentially no accumulation, even in the mountains. For much of the NC mountains and even some portions of the Foothills and Piedmont, occasional flurries will persist into Friday evening, becoming increasingly isolated through midnight or so Friday night and abating entirely thereafter.

SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/

As of 1130 AM Thu:

Key message 1: Still anticipating briefly warmer temperatures Saturday.

Weak high pressure in place over the coastal Southeast early Saturday. Though flow at 500 mb will be slightly cyclonic in advance of trough digging across the Midwest, 850 mb flow turns WSW to SW and promotes WAA over previous stalled baroclinic zone south of the CWA. 500 mb heights actually rise slightly. Under mostly sunny skies, max temps reach the mid to upper 50s across most lower elevations, with a few SW NC mountain valleys and the Savannah Valley areas of GA/SC passing 60.

Key message 2: Small precip chances return to the area Saturday night and early Sunday morning in advance of Arctic front.

The low-level SW flow should induce some moistening below 800 mb Saturday night over the Piedmont. However, overnight, it looks more likely that the approach of the trough axis and Arctic front out of the OH Valley will neutralize thermal advection and keep the baroclinic zone centered south of the area. NAM and GDPS are two models that are on the northern side of the envelope, in this regard. All the models do depict upper divergence east of the Apps associated with right entrance region of 250mb jet streak centered over the NE CONUS, and where that divergence rides near/over the baroclinic zone, they respond with bands of QPF. The NAM/GDPS thus produce more precip in our CWA than the GFS/ECMWF do, those models showing the frontogenesis farther south. PoPs remain no better than 20-30% for the Piedmont owing to the low confidence in where (if) the precip bands form. Winds will turn more westerly overnight and begin CAA; similarly to the spread in the precip positioning, models vary as to when the cooling trend begins. NAM for one would suggest a nondiurnal trend for parts of the Piedmont as WAA hangs on longer in its solution and it seems to pick up on downsloping downwind of the Escarpment also. Saw fit to base overnight temps on the NBM hourly trends which allows colder air to reach the mountains before daybreak Sunday morning, but east of the mountains delays the cooling, allowing the forecast to reflect precip most likely ending before temps support a change to anything other than rain in the Piedmont. Shallow moisture does not lend itself to snow/sleet there anyway.

Based on the above, westerly upslope PoPs do increase Saturday night and an early morning change to snow is reflected near the TN border. The moist layer looks short-lived and shallow so accumulation still is forecast to remain below an inch in that area, with probably nothing more than flurries after 12z Sunday.

Key message 3: Below-normal temps and gusty winds for Sunday.

Max temp for Sunday along the TN border probably will be at midnight that morning (i.e. 05z Sun), and very early in the day otherwise over the mountains, with CAA ongoing. The Piedmont may warm slightly through late morning or very early afternoon via downslope, but temps tumble thereafter. Maxes below freezing in most of the mountain areas and lower to mid 40s for most of the Piedmont. Widespread wind gusts of 25-35 mph expected across the mountains thru early Sunday evening with higher elevations likely peaking 45-50 mph; thus can't rule out a Wind Advisory, and/or a Cold Weather Advisory for onset of subzero wind chills in some of the higher terrain Sunday afternoon, which spread to lower elevations by evening.

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/

As of 1245 PM Thursday:

Key message 1: Exceptionally cold Sunday night and Monday, and temps still well below normal Monday night.

Strong 1030+ mb Arctic high centers just west of the southern Apps Sunday night; gradient relaxes such that winds weaken during the evening, mitigating the wind chill threat in lower elevations, though temps fall enough that if a Cold Wx Advisory is put into effect late Sunday, it probably will be retained into Monday morning. Mins look to be the coldest of the season so far for most or all of the area, in the single digits or low teens over the mountains and Piedmont I-40 corridor, and mid to upper teens elsewhere, even in the Lakelands. Despite clear skies, max temps Monday will be mainly in the 30s to lower 40s throughout the area. Winds remain light Monday night and still with clear skies as the high drifts east of the Apps. Though temperatures rebound 10-15 degrees over the mountains, the Piedmont will be almost as cold. Tue morning mins in the lower 20s for most areas; wind chill not suggestive of Cold Wx Advisory at that time, though average temps over the 36-hour cold period are near our criteria for a Cold Wave.

Key message 2: Moderating temps Tue-Wed with next chance of precip being Wed night or Thu.

Shortwave ridge riding over the area Tue will allow temps to climb back to the upper 40s and lower 50s, 1-5 degrees below normal for most. Quasi-zonal flow across the Southern US allowing temps to moderate back a little above normal for Wed. Models still vary in their handling of the nearly cut-off southern-stream shortwave and in the timing of a northern-stream wave variously shown in the northern Plains or Great Lakes, with a few guidance members showing one or both of those features inciting precip in our area by early Thursday, and responsible for small PoPs at that time, probably rain.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Increasing mid and high clouds through the evening and into the overnight, with some low VFR cigs possible at KAVL overnight. Light SW wind, NNW at KAVL, will become light N or light and variable overnight, remaining NNW at KAVL. As a clipper low passes to our north, there could be some sprinkles, or flurries across the mountains, but no significant precip expected. Clouds scatter out through the day Friday with winds picking up from the south then southwest. KAVL sees an initial south wind becoming NNW during the afternoon.

Outlook: Another clipper system could bring lower clouds and precip into the area Saturday night into Sunday, along with gusty winds. Robust, dry high pressure should settle into the area late Sunday into Monday then slide east on Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

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