textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Confidence is increasing for severe potential with a line of strong thunderstorms expected to cross the area Sunday night into Monday.

Chances for widespread freezing overnight temperatures increasing for Tuesday night.

KEY MESSAGES

1. Dry and mild conditions persist through Saturday night. 2. A strong cold front increases the chances for showers and thunderstorms late Sunday into Monday, with a potential for severe weather. Possible hazards include damaging winds, heavy rainfall, and a brief tornado cannot be ruled out. 3. A brief period of northwest flow snow should develop along the NC/TN border behind the cold front, with light accumulations possible. 4. Lingering gusty winds develop behind the cold front through late Tuesday, bringing much colder air to the area and dropping overnight temperatures to near or below freezing through Wednesday night.

DISCUSSION

Key message 1: Dry and mild conditions persist through Saturday night.

A rather benign forecast continues through tomorrow night as broad and flat troughing lifts into New England and height rises return to the Southern Appalachians. Gusty winds will continue through this afternoon before relaxing tonight. Otherwise, mostly sunny/clear conditions will continue with afternoon highs tomorrow rebounding into the upper 60s to low 70s and overnight lows in the low to upper 40s.

Key message 2: A strong cold front increases the chances for showers and thunderstorms late Sunday into Monday, with a potential for severe weather. Possible hazards include damaging winds, heavy rainfall, and a brief tornado cannot be ruled out.

After a few days of quiet weather, a stout trough swings into the area, returning the chances for showers and thunderstorms. Current guidance depicts a developing low over the central plains and ejecting northeast and dragging a strong cold front toward the CWA Sunday night into Monday. Ahead of the front on Sunday, pre-frontal isentropic lift looks to occur, increasing PoP chances across much of the region. This is expected to remain more showery than convective as guidance doesn't have much in the way of instability on Sunday. Additionally, southeasterly winds pick up across the area, increasing gusts across the entire area. Though 15-25 mph gusts are possible, winds should remain well below any Wind Advisory criteria. As the front approaches and provides better lift of parcels, the severe threat increases. At of this forecast, the timing of the front has slowed down from previous model runs, meaning the probability of severe weather is increasing. Since the front is looking to cross into the area during the daytime hours, this increases the chances for better instability, especially east of the mountains. Current guidance shows a 40-50% chance of sbCAPE of 500 J/kg or higher, with the eastern fringe of the NC Piedmont and portions of SC highlighted. This lines up with the current Day 4 outlook from the Storm Prediction Center. Additionally, shear with this system also packs 50-60kts at the 850mb level and plenty of moisture at the surface. So, for the time being at this current forecast, all hazards are possible. As for any tornado potential, guidance does show a decent amount of 200-250 SRH during the later afternoon period on Monday. What this indicates is the environment is supportive of spinning up the lower levels, which could lead to the development of a brief tornado along the leading edge of the convective line.

The forecast still remains a few days out and is subject to change. The main challenge to this forecast is going to be the timing. Again, if this system comes through during the daylight hours, especially afternoon, this increases the chances for damaging winds and a brief tornado or two. If the system speeds up and crosses the area overnight, this can limit the amount of instability available to the storm and decrease the chances for severe hazards. Generally, confidence is increasing that a cold front brings showers and thunderstorms, possibly severe late Sunday into Monday. Either way, severe weather season is here and now is a good time to make sure you know where to seek shelter from a storm and have your emergency kit in that safe place.

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

As the cold front moves through the area and the surface low lifts northeast on Monday, NW flow snow chances are possible at the NC/TN border. Current guidance shows a bit of lingering surface moisture behind the front, but as usual, depends on how quickly the colder air moves in. This directly impacts the amounts. For now, the chances for receiving an inch or more of snow is 15-20%. However, given the strong CAA expected behind the front in this typical situation, it's possible that very light accumulations are possible. Should remain well under any advisory criteria and taper off early Monday night.

Key message 4: Lingering gusty winds develop behind the cold front through late Tuesday, bringing much colder air to the area and dropping overnight temperatures to near or below freezing through Wednesday night.

Gusty winds are expected Monday associated with the line of convection, but behind the front, guidance keeps winds elevated with the strong CAA moving in. Current models are continuing to trend toward keeping gusty winds across the area, mainly the mountains, through Tuesday. This is predominately from the strong trough lifting out of the region. Looking at the wind gusts post-frontal, there is a higher chance (80-90%) of gusts over 45mph at the highest peaks in the mountains. Elsewhere, the chances quickly decrease to 30-50% of gusts over 45mph, which would reach Wind Advisory criteria. Given the tight 850mb gradient of the exiting trough, there is a possibility for the need of a product issuance, but this will need to be re-evaluated closer to the system as guidance changes.

Additionally, confidence is increasing for overnight freezing temperatures behind the front Monday night through Wednesday night. At this time, the chances for overnight temps of at least 32 degrees is 90%-95% across the mountains on Monday night and expands across the entire area for Tuesday night. The air mass starts to modify as winds turn more southerly and the high pressure breaks down on Wednesday. By Wednesday night, freezing temps overnight look to be more likely across the mountains, as elsewhere chances decrease into the 40-50% range.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR conditions will prevail through the TAF period at all terminals. Gusty southwest winds will continue through the afternoon before becoming light overnight. Wind direction will also become variable overnight and may gradually shift to out of the north and east.

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 with thunderstorms possible. VFR returns on Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

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