textproduct: Greenville-Spartanburg

SYNOPSIS

Cold high pressure arrives for Thanksgiving and the weekend from central Canada. Another cold front arrives Sunday with a secondary low pressure system on Tuesday. This may result in lingering precipitation chances through the first half of the upcoming week.

NEAR TERM /THROUGH THURSDAY/

As of 645 PM EST Wednesday...

Key Messages:

1) Wind Advisory Remains in Effect Across the Northern North Carolina Mountains through 11 AM EST Thursday

2) Cold Night Ahead with Lows Falling Near or Below Freezing

3) Dry again Tomorrow with Much Cooler and Below Normal Highs

A digging upper trough propagates eastward from the Upper Great Lakes into western Quebec as broad cyclonic flow aloft settles over the eastern CONUS through the near term. The attendant cold front will push east of the forecast area early this evening.

Strong CAA behind the front will create a sharp pressure gradient across the mountains, leading to advisory level gusts in the northern mountains (45-55 mph above 3500 ft) through late Thursday morning. Thus, the Wind Advisory remains in effect for all of Avery County as well as Yancey and Mitchell Counties above 3500 ft through 11 AM EST Thursday. Strong CAA will also lead to cold overnight lows, with temps expected to fall near or below freezing across the forecast area. Intermittent gap winds can be expected across the usual areas of the SC Upstate this evening into tonight.

Upper cirrus will increase around/shortly after daybreak, becoming broken to overcast Thursday morning. Cirrus will to clear out briefly east of the mountains before thickening again Thursday afternoon. Clear skies return area-wide by the late afternoon Thursday. Gusty winds will linger across the mountains Thursday. Breezy winds may return east of the mountains as mixing develops Thursday afternoon. However, winds east of the mountains are not expected to be as gusty as what we saw today. Thursday's highs will be much cooler, ending up 6-8 degrees below normal east of the mountains and 10-15 degrees below normal across the mountains. Min RH values should drop below 30% east of the mountains, but recent rainfall should help limit the fire danger somewhat.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/

As of 100 PM EST Wednesday:

Key Message: Cold and dry conditions continue through the short term

A cold and very dry air mass will remain in place through Friday and Saturday. Forecast temperatures will generally run ten to fifteen degrees below normal through the period. The drop will feel especially sharp given the eight day stretch of above normal temperatures that preceded this pattern.

NBM ensemble spread remains small for daytime highs, with only a two to three degree difference between the twenty fifth and seventy fifth percentiles for both Friday and Saturday. This supports high confidence in below normal but relatively uniform temperatures during the day.

Overnight lows are slightly more uncertain due to questions regarding how efficiently winds decouple, especially across the Foothills where NBM spread for MinT ranges from three to five degrees Thursday night. Even if winds remain elevated enough to limit radiational cooling somewhat, confidence is high that temperatures will fall below freezing across nearly the entire forecast area. NBM probabilities of MinT below 28 degrees range from seventy to one hundred percent across all areas except the urban core of Charlotte and portions of the Upper Savannah River Basin.

High pressure centered overhead Friday night will bring ideal radiational cooling conditions under clear skies and calm winds. This will likely be the coldest night of the upcoming week, with lows falling into the teens across the mountains and the lower to mid twenties across the Foothills and Piedmont.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

As of 115 PM EST Wednesday:

Key Message 1: Unsettled weather returns late this weekend into early next week

High pressure will retreat quickly off the East Coast Saturday night. Upstream, an upper trough moving out of the Pacific Northwest will phase with a digging Alberta clipper as it reaches the Intermountain West Friday night. This combined system will then phase with a second shortwave digging out of central Canada. The resulting evolution favors a Great Lakes cutter scenario with the surface low tracking well to our north and west this weekend.

Although the deeper synoptic lift will pass north of the region, moisture advection from the Gulf will support a chance of light overrunning precipitation late Saturday night into Sunday ahead of the cold front. Forecast trends have shifted toward lower PoPs and QPF with this initial round. A brief wintry mix at the onset cannot be completely ruled out across the higher elevations of the NC mountains and portions of the northern Foothills north of I 40, where cold air damming and wet bulb cooling may hold temperatures near freezing. However, NBM probabilities for snow or freezing rain remain less than ten percent, and any wintry component would be brief and confined to the terrain if it occurs at all.

Key Message 2: Increasing rain chances Monday and Tuesday with potential for lingering cold air

There remains uncertainty regarding the position of the cold front on Monday, as solutions vary between a clean frontal passage and a stalled boundary lingering over the region. The NBM maintains low chance PoPs Monday, with precipitation expected to fall as rain.

Confidence is increasing in a secondary wave of low pressure developing along the Gulf Coast and tracking northeast along the boundary Monday night into Tuesday. This wave would bring a more widespread and heavier round of rainfall to the region. With high pressure positioned to the northeast during this period, the setup may reinforce a cold air damming wedge across western NC. If the wedge persists or strengthens, elevated concern would arise for freezing rain potential in climatologically favored locations such as the northern mountains and northern Foothills. Current ensemble probabilities remain low, but the pattern warrants monitoring.

In the wake of the departing wave, wrap around northwest flow snow may develop across the highest elevations late Tuesday or later, depending on the speed of the system.

AVIATION /00Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR and dry through the 00Z forecast period. Gusty NW winds will continue at KAVL through the period. Winds east of the mountains could still see intermittent gusts through late this evening (especially at KGSP and KGMU due to the gap wind) before gusts gradually diminish overnight. Wind direction east of the mountains will generally be W/NW through the period. Low-end gusts may return tomorrow afternoon, especially at KCLT and KAND. Clear skies will linger through late tonight before BKN cirrus develops around or shortly after daybreak across the terminals. Cirrus looks to clear out briefly east of the mountains before thickening again early Thursday afternoon. KAVL should see BKN to OVC cirrus stick around through late tomorrow afternoon. Clear skies return by the late afternoon hours on Thursday across the terminals.

Outlook: Dry and VFR through the rest of the week. A storm system approaches the area Sunday, bringing the next chance for flight restrictions.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Wind Advisory until 11 AM EST Thursday for NCZ033-049-050. SC...None.