textproduct: Greenville-Spartanburg

SYNOPSIS

Below normal high temperatures stick around through next week. Dry and cold high pressure remains in control through today before a cold front brings precipitation chances Sunday. Drier conditions develop briefly Monday before a low pressure system from the Gulf brings better precipitation chances Monday night and Tuesday. Dry high pressure returns Wednesday into Thursday.

NEAR TERM /THROUGH TONIGHT/

As of 603 AM EST Saturday...

Key Messages:

1) Quiet but cold weather tonight and Saturday, with frigid lows in the 20s this morning, and highs only climbing into the 40s this afternoon.

2) Increasing cloud cover Saturday and Saturday night as in situ cold-air damming develops.

3) Wintry mix develops over the Blue Ridge Saturday night, with isolated light ice accumulations possible. Winter Weather Advisory now in effect for Avery County beginning 1 AM Sunday.

Still seeing widespread cirrus streaming across the CWA this morning, albeit with a little less coverage over the NC Piedmont than originally anticipated. Winds over much of the area have remained a little steadier than expected, which has resulted in temperatures running 1-3 degrees warmer than forecast. Still, seeing widespread temps in the 20s already, and even the warmest of sites are approaching 30 degrees with at least another 1-2 hours of cooling before hitting their nadir.

Otherwise...surface high pressure will quickly migrate eastward across the Mid-Atlantic today, reaching the Atlantic coast by late afternoon or evening, and continuing northeastward into the night. High temperatures will only climb into the mid-40s this afternoon. By tonight, we'll have a brief window during which there's a signal for some synoptically-supported cold-air damming development...but with the high continuing eastward and surface-CAA weak to begin with, that window will be *very* brief. Nonetheless, as low-level isentropic ascent begins after sunset, there will already be a somewhat cool surface layer beneath it.

The real action should take place after midnight tonight, as continued isentropic ascent results in the development of a warm nose, and a strong LLJ centered around the 850mb level passes to our northwest...ushering in better moisture content and moistening profiles from the top down. Precipitation may not start in earnest over the mountains until a couple hours before daybreak, but some CAMs as well as general pattern recognition would support patchy WAA drizzle as early as 1 AM. It thus appears that despite the lack of synoptically-supported cooling, some locations along the Savannah River Valley and across the Appalachians could see some diabatic cooling, which for the mountains at least, should bring valleys to near or below freezing a few hours before daybreak. Therefore, light ice accumulations are expected to develop, particularly on elevated surfaces, across parts of the mountains. Confidence and coverage are highest in the northern Blue Ridge, especially Avery County, where a Winter Weather Advisory will be issued beginning at 1 AM Sunday morning and continuing through 9 AM, by which time virtually the entire county should be above freezing, precluding further icing. Temperatures should continue to rise after daybreak Sunday, so for any zones where freezing rain is occurring, a quick changeover to liquid rain is expected that will continue into the early afternoon Sunday ahead of an advancing cold front.

SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/

As of midnight Saturday morning:

Key message 1: Precip will taper off Sunday afternoon, most likely ending as rain even in the mountains.

Sunday afternoon, prefrontal shower activity will progress across the Piedmont. An low-level inversion looks to persist, attributable to the weak CAD, with a stronger subsidence inversion developing during the afternoon keeping moisture shallow. Very shallow convection may occur above the wedge but overall QPF is small. PoPs diminish as CAMs and global models largely depict the remaining precip fizzling out as it passes east of the mountains. Most models depict SW winds during the afternoon, but with clouds not looking to break until Sunday night, max temps are limited to the upper 40s to near 50 in the upper Piedmont, and lower 50s SE of I-85. Temps will be warm enough for all rain in the mountains after 18z. The mountain valleys and foothills may be particularly warm with downslope flow developing. The moisture in the NW flow layer behind the front is shallow so PoPs are too small to mention near the TN border, but flurries or some light rime icing could occur as temps fall in the late aftn and evening. Temps trend colder in the mountains and I-40 corridor Sun night, but end up slightly warmer elsewhere, though mins still look several below normal for Mon morning.

Key message 2: A light wintry mix appears likely in portions of the mountains and northern foothills Monday night or early Tuesday morning.

The forecast evolution of the next system has not changed appreciably from that discussed 24 hours ago. High pressure will migrate from the Midwest/OH Valley to the NY/PA/NJ area over the course of Monday. Cold front will stall south of the high, and deep shortwave crossing the Ozarks and lower MS Valley Monday night and inducing Miller-A cyclogenesis. Surface high is in a somewhat more favorable position for CAD with this event; temps will trend slightly cooler for Monday with winds remaining NE'ly over most of the area thru the day. Precip chances develop from SW to NE as the front activates Monday evening, steadily increasing thru daybreak Tuesday, remaining in the categorical 80-95% range until the incipient low pulls away Tuesday afternoon and chances diminish from the west. Confidence remains fairly high that south and east of the Blue Ridge Escarpment, this will be an all-rain event. Temps should be mostly or entirely above freezing in the mountains if PoPs begin as soon as some models suggest Monday evening, but diabatic cooling looks to bring them below freezing for a time after precip begins. Those models resolving p-type suggest mainly a rain or freezing rain split based on sfc temp, given the strength and saturation within the warm nose aloft. How effective the diabatic cooling is at onset to lock in subfreezing temps thus looks the primary determinant of forecast impacts. Have chosen to advertise sleet along with freezing rain where the latter was favored by our algorithm, in order to reflect "mixy" impacts, given that we often can't account for enough sleet, and with some question as to how much QPF will translate to ice accumulation. Areas that receive a few hours of freezing rain--the most likely area being the mountains and foothills northeast of the French Broad--look likely to need a Winter Wx Advisory as the event nears. High elevations of the Balsams might see some freezing rain also, although current forecast accums may reflect too early of an onset there and thus may be slightly inflated. Moistening and warming temps in the mountains on the back side of the system look to bring p-type over to all rain by 8-9 AM in most areas of the mountains, though a few of the highest peaks may quickly transition to snow as CAA sets in by noon or so. Light accumulation of NW flow snow near the TN border could occur thru evening but all PoPs fall below slight-chance by midnight. Several degrees colder Tue night versus the previous night.

LONG TERM /WEDNESDAY THROUGH FRIDAY/

As of 100 AM Fri:

Key message 1: Dry weather and moderating temperatures Wednesday and Thursday.

Quasi-zonal upper pattern returns by early Wed with high pressure migrating across the Southeast, comparatively weak to the previous two highs. Winds will be light Wednesday but skies trend clear, and temps rebound slightly. Mid to upper level flow turns slightly cyclonic Wed night and a dry cold front looks to push across the area. Some high altitude cloud cover may return, but skies remain partly cloudy. Westerly downslope flow develops Thu with the incoming, stronger high, which will allow temps to increase further despite the clouds, especially east of the mountains. Temps will remain below normal, although back to within 2-3 degrees thereof in the Piedmont.

Key message 2: Another brief freezing rain event could occur in the mountains Thursday night into Friday morning, with chances for rain elsewhere by Friday.

Low pressure developing along coastal front in/near the western Gulf potentially will activate the front and bring precip chances up from the south, most likely not until Friday afternoon or night, but some ensemble members depict precip reaching the area as soon as Thursday night. If that occurs light wintry precip, currently looking most likely to be freezing rain, could develop in a few spots of the SW NC mountains. Confidence is very low with wide spread among guidance members as to onset time or even the existence of precip, so the freezing rain is mentioned only as a slight-chance for now. Temps remaining a few degrees below normal thru Friday night.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR conditions expected to prevail through most of tonight. Widespread cirrus is visible on satellite imagery and should continue streaming into the western Carolinas this morning, thinning somewhat by mid-morning. Winds, mostly out of the north at 2-4kts, should pick up somewhat, climbing to 8-10kts during the afternoon. Some periods of SE winds are expected during the daylight hours at all the non-mountain terminals. An increase in upper-level cloud cover will begin late this afternoon, and some mid-level cloud cover aorund FL060-FL090 will work into the area from the west after sunset, before precipitation associated with an advancing frontal system picks up during the latter half of tonight...initially across Appalachia, but eventually spreading eastward across the entire terminal forecast area by 12z Sunday. Associated MVFR flight restrictions may develop at KAVL and other mountain sites before daybreak Sunday; elsewhere, there's not much signal in guidance for any widespread restrictions through the end of the 12z TAF period.

Outlook: Restrictions and rain chances continue Sunday before dry and VFR conditions briefly return Monday. Another round of rain and restrictions are expected Tuesday before dry and mainly VFR conditions return Wednesday into Thursday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Weather Advisory from 1 AM to 9 AM EST Sunday for NCZ033. SC...None.