textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Lowered RH values this afternoon.

Increased PoP Sunday.

No major changes regarding the severe weather potential or northwest flow snow event Monday.

Cold snap still looks likely Monday night through Wednesday.

KEY MESSAGES

1. Dry and mild conditions persist through Saturday night. Increasing clouds and shower chances on Sunday. 2. A strong cold front will bring the potential for severe weather Sunday night into Monday afternoon with the main hazards being damaging winds and isolated tornadoes. We are only a few days out, so start preparing now. Make sure you have multiple ways to receive warnings and think about where you would seek safe shelter if a warning was issued for your location. 3. A brief period of northwest flow snow will develop along the NC/TN border behind the cold front on Monday, with light accumulations possible. 4. Gusty winds develop ahead of and behind the cold front Sunday night through Tuesday before finally tapering off Tuesday evening. A Wind Advisory may be needed for portions of the North Carolina mountains Monday into Monday night. 5. A much colder and drier air mass returns behind the front Monday night into Wednesday night with most areas seeing lows each night falling near or below freezing. Warmer temperatures return by late week with mostly dry conditions continuing.

DISCUSSION

Key message 1: Dry and mild conditions persist through Saturday night. Increasing clouds and shower chances on Sunday.

Weak high pressure crosses the area today with much lighter winds than the past two days. Highs increase to around 10 degrees above normal. Mixing dew points will drop RH toward the 25% range.

Low level southeasterly winds develop tonight and continue into Sunday with high pressure to our east and a cold front approaching from the west. Skies will be mostly clear tonight with lows 5 to 10 degrees above normal. Low level moisture ramps up on Sunday in the southeasterly flow. Showers are expected to develop in the increasing isentropic lift and upslope flow. An isolated thunderstorm is possible as weak instability develops. While the CAMs do show the thunderstorm potential, they don't show much in the way of severe chances, despite increasing shear, as even muCAPE remains weak during the day Sunday. Highs will be 5 to 10 degrees above normal, but could be lower if a weak in-situ CAD develops.

Key message 2: A strong cold front will bring the potential for severe weather Sunday night into Monday afternoon with the main hazards being damaging winds and isolated tornadoes. We are only a few days out, so start preparing now. Make sure you have multiple ways to receive warnings and think about where you would seek safe shelter if a warning was issued for your location.

A potent upper trough will approach out of the west Sunday evening into sunday night before swinging across the forecast area Monday afternoon into Monday night. At the surface, the associated cold front will approach out of the west Sunday evening into late Sunday night before tracking across the GSP CWA early Monday morning into Monday afternoon. This front will bring the potential for strong to severe storms late Sunday night into Monday afternoon.

Plentiful low-level moisture will stream into the region ahead of the front, with dewpoints climbing into the upper 50s to lower 60s east of the mountains by Sunday night, lingering through Monday afternoon. Plenty of forcing will be available as well with a 40-50 kt LLJ tracking over the forecast area Sunday night into Monday afternoon. Deep layer shear will start out ranging from 40-50 kts Sunday night, increasing to 50-60 kts just ahead of the frontal boundary Monday morning/afternoon. With strong forcing and shear available, it will not take much instability for severe storms to develop. Both global and high-res guidance agree that MUCAPE will steadily increase Sunday night into Monday afternoon, with a few hundred J/kg of MUCAPE developing. The LREF depicts roughly a 30% to 50% chance of 500 J/kg of MUCAPE developing east of the mountains late Sunday night into Monday afternoon. The primary hazard will be damaging wind gusts, with the secondary hazard being isolated tornadoes thanks to 200 to 300 J/kg of 0-1km SRH developing late Sunday night into Monday afternoon per the 00Z NAM. However, the tornado threat will be highly dependent on how the deep layer shear vectors align with the frontal boundary. If shear vectors end up more parallel, like the 18Z ECMWF shows, this could limit the tornado threat somewhat. If shear vectors end up less parallel and lean more perpendicular, like the 18Z GFS shows, the tornado threat would not be as limited.

As we have been messaging the last few forecast cycles, the severe threat will be dependent on the exact timing of the cold front. If the convection associated with the front pushes east of the GSP CWA prior to peak heating, this could limit the severe potential somewhat. If convection lingers over any portion of the GSP CWA during peak heating, then the severe threat will be higher. Once more of the high-res models start going out all the way through Monday, we will should get a better idea regarding the exact timing of severe storms. Regardless, now is the time to make your severe weather preparations. Make sure you have multiple ways to receive warnings such as NOAA Weather Radio, Wireless Emergency Alerts (WEA), social media, or local TV and radio. If you live in a mobile or manufactured home make plans ahead of time to stay with friends or family who live in a sturdy building as mobile/manufactured homes are not safe when it comes to tornadoes.

Key message 3: A brief period of northwest flow snow will develop along the NC/TN border behind the cold front on Monday, with light accumulations possible.

As temperatures drop below freezing behind the cold front across the North Carolina mountains Monday afternoon and evening, rain will quickly transition to snow, mainly along the NC/TN border. Depending on how much moisture is available behind the front, NW flow snow showers could linger through early Tuesday morning. This event is still shaping up to be sub-advisory, with both the NBM and LREF only showing a 15% to 30% chance of snowfall accumulations exceeding 2" in 24 hours along the NC/TN border counties. So, only minor impacts such as slippery road conditions are expected at this time.

Key message 4: Gusty winds develop ahead of and behind the cold front Sunday night through Tuesday before finally tapering off Tuesday evening. A Wind Advisory may be needed for portions of the North Carolina mountains Monday into Monday night.

Wind speeds will steadily increase ahead of an approaching cold front Sunday evening into late Monday as a 40 to 50 kt LLJ tracks overhead. Wind speeds will continue increasing across the mountains Monday evening into early Monday night as wind direction turns NW behind the front. A Wind Advisory will likely be needed for portions of the forecast area Monday into Monday night as the NBM shows a 50% to 60% chance of gusts exceeding 45 mph, mainly across the higher elevations. Gusts east of the mountains still look to remain well below 45 mph so an advisory will not be needed across these locations. Gusty winds linger through Tuesday afternoon before gradually diminishing Tuesday evening into Tuesday night.

Key message 5: A much colder and drier air mass returns behind the front Monday night into Wednesday night with most areas seeing lows each night falling near or below freezing. Warmer temperatures return by late week with mostly dry conditions continuing.

Surface high pressure builds in from the west behind the departing front Monday night into Tuesday night, allowing drier and much colder conditions to return. Temperatures will run well below normal through mid-week. Temperatures Monday night and Tuesday night are likely to fall below freezing, with the NBM showing a 70% to 100% chance of low temperatures less than 32 degrees. Although warmer low temperatures are expected to return Wednesday night as the center of the surface high pushes offshore, most of western North Carolina could see freezing temperatures return (outside of the Charlotte metro area). Although the growing season has not officially started, the recent period of abnormally warm weather has allowed some vegetation to start blooming. Sensitive plants that have already started blooming will be particularly vulnerable to these cold temperatures each night.

Warmer temperatures return Thursday into Friday while mostly dry conditions linger as the southwestern periphery of the surface ridge remains extended over the Carolinas. Highs on Thursday will be noticeable warming, but still a few degrees below normal (outside the mountain valleys) before much warmer and above normal highs return Friday.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR expected through the period. Light SW wind already beginning to taper off with the sun preparing to set. early this morning becomes light NE or light and variable after daybreak. Winds turn SE for the afternoon into the evening. Mainly cirrus expected but some stratocu could cross the area as well.

Outlook: Low level moisture brings MVFR cigs during the day Sunday. A cold front will bring a round of showers and associated flight restrictions by Sunday evening, continuing into Monday with thunderstorms possible. VFR conditions return on Tuesday into Wednesday.

GSP WATCHES/WARNINGS/ADVISORIES

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