textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Update for 12Z Aviation Discussion.

KEY MESSAGES

1. Dry and mild conditions persist through Saturday night. Gusty winds this afternoon. 2. Showers and thunderstorms will develop ahead of a potent cold front on Sunday. The cold front will push across the forecast area on Monday, bringing the potential for severe weather. 3. Gusty winds develop in association with the cold front Sunday through late Tuesday, peaking on Monday as the cold front tracks across the area. A Wind Advisory may be needed across portions of the North Carolina mountains Monday into Monday night. 4. A brief period of northwest flow snow should develop along the NC/TN border Monday afternoon into Monday night behind the cold front, but any snowfall accumulations should remain light. 5. A much colder air mass returns behind the cold front Monday night into Wednesday night with most areas likely falling near or below freezing each night.

DISCUSSION

Key message 1: Dry and mild conditions persist through Saturday night. Gusty winds this afternoon.

High pressure slides east of the area today as a dry cold front approaches from the west. Deep mixing and a tightening pressure gradient will lead to gusty winds during the afternoon, but not quite as gusty as Thursday. Gusts across the higher elevations will approach Wind Advisory level, but should remain below for most locations. The drier air mass and mild temps will help RH values drop to near the 25% range outside of the mountains.

Lighter winds expected Saturday as high pressure builds into the area. Highs will be warmer on Saturday with a not quite as dry air mass. RH values will drop into the 30% range, but could be lower if moisture recovery is slower. Lows will see a warming trend as well.

Key message 2: Showers and thunderstorms will develop ahead of a potent cold front on Sunday. The cold front will push across the forecast area on Monday, bringing the potential for severe weather.

Although the center of surface high pressure will push off the Northeast coast Sunday into Sunday night, the southwestern periphery of the surface ridge will remain extended over the Southeast while a strong cold front approaches out of the west. This will allow low- level flow to gradually turn SE'ly Sunday into Sunday night, leading to an influx of moisture off the Atlantic. Thus, shower and thunderstorm chances will return Sunday into Sunday night. Although 30 to 40 kts of deep layer shear will be in place on Sunday, global models depict very little surface instability developing and the NBM/LREF are in line with global models in showing less than 500 J/kg of surface instability available during peak heating Sunday. Bulk shear increases to 40 to 50 kts Sunday night with both the 00Z GFS and 18Z ECMWF showing the potential for a few hundred J/kg of MUCAPE developing. The latest LREF is bullish, showing 300-600 J/kg developing of MUCAPE developing Sunday night. Confidence on these higher LREF MUCAPE values is low at this time. So, the severe threat Sunday into Sunday night should remain low, but a few strong, sub- severe storms cannot be ruled out.

The aforementioned cold front will track across the forecast area sometime Monday. Guidance is still split on the timing of the front with the ECMWF remaining faster compared to the GFS and Canadian. Thus, confidence on the severe weather potential on Monday remains low as this will be highly dependent on whether the front tracks across the forecast area during peak heating or not. If the front pushes east prior to peak heating Monday then the severe threat will be lower. If the front remains over the forecast area during peak heating hours, then the severe threat will be higher. With deep layer shear increasing to 50 to 60 kts along the frontal boundary Monday, it will not take too much surface instability to allow severe storms to develop especially with a 40 to 50 kt LLJ overhead. For now, it appears that eastern portion of the forecast area will have the best potential to see severe storms develop on Monday, per the NBM, LREF, Canadian, and GFS.

Key message 3: Gusty winds develop in association with the cold front Sunday through late Tuesday, peaking on Monday as the cold front tracks across the area. A Wind Advisory may be needed across portions of the North Carolina mountains Monday into Monday night.

Gusty SE winds will develop well ahead of the cold front on Sunday but should remain below advisory criteria. Winds will gradually turn more S/SW ahead of the front Sunday night into Monday morning, and WNW/NW behind the front late Monday afternoon (the mountains) into late Monday evening (east of the mountains). Wind speeds will gradually increase Sunday night into Monday afternoon east of the mountains, with wind speeds gradually increasing from Sunday night into late Monday evening across the mountains. Wind speeds east of the mountains should remain below advisory criteria as both the NBM and LREF generally show less than a 20% chance of reaching advisory criteria (>45 mph). Portions of the North Carolina mountains are more likely to reach advisory criteria (> 45 mph) per the NBM which has mainly the higher elevations seeing roughly a 50% to 70% chance for advisory level gusts. The LREF shows slightly lower probabilities, but with a 40 to 50 kt LLJ tracking overhead on Monday, thinking is that the NBM is closer to reality regarding gusts. Thus, a Wind Advisory may be needed for portions of the North Carolina mountains Monday into Monday night. Gusty NW/WNW winds will linger through Tuesday before gradually diminishing Tuesday evening into Tuesday night as the pressure gradient relaxes.

Key message 4: A brief period of northwest flow snow should develop along the NC/TN border Monday afternoon into Monday night behind the cold front, but any snowfall accumulations should remain light.

Temperatures will drop below freezing Monday afternoon into Monday night behind the cold front allowing a brief period of NW flow snow showers to develop, mainly along the NC/TN border. Guidance is still split regarding how much moisture will be available so confidence on snow accumulations remains low at this time. The GFS continues to show better moisture available compared to the ECMWF. The latest NBM came in slightly higher for snow accumulations Monday afternoon into Monday night, but still shows most locations remaining below advisory criteria (<2"). Confidence is fairly high (for now...this could change) that this would be a low impact, sub-advisory event as the 24 hour snow accumulation probabilities from the both the NBM and the LREF only depict a 10% to 15% chance of reaching advisory criteria snow along the NC/TN border.

Key message 5: A much colder air mass returns behind the cold front Monday night into Wednesday night with most areas likely falling near or below freezing each night.

Surface high pressure builds into the region behind the departing cold front Monday night into Tuesday night, lingering over the region through Thursday. This will allow drier conditions and below normal temperatures to return. The coldest day should be Tuesday before a gradual warming trend develops through late week. Lows Monday night and Tuesday night will fall near to below freezing area- wide. Slightly warmer lows return Wednesday night, but areas across the mountains are likely to fall near to below freezing again, while areas east of the mountains are more likely to remain near or above freezing. Although the growing season has not officially started, the recent period of abnormally warm weather has allowed some vegetation to start blooming. Any sensitive plants that have already bloomed will be particularly vulnerable to these cold temperatures each night.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR expected through the period with mainly cirrus streaming overhead. Light winds pick up from the south this morning. Winds increase from the southwest and become gusty by afternoon. Winds diminish during the evening.

Outlook: VFR will continue through at least Saturday night. A cold front will bring a round of showers and associated flight restrictions by Sunday afternoon and continuing into Monday. VFR returns on Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

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