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FXUS62 KGSP 172356 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 756 PM EDT Fri May 17 2024

SYNOPSIS
A warm front and weak low pressure system will bring rounds of showers and thunderstorms today into the weekend. Somewhat drier conditions return early next week resulting in only isolated afternoon and evening showers and storms. A cold front will bring better shower and thunderstorm chances back into the forecast during the middle of next week.

NEAR TERM /THROUGH SATURDAY/
As of 755 PM Friday: Extensive cloud cover over the area has lowered cloud bases. Area of precip stretching across the NC Piedmont will continue to lift north and provide a lull in the activity. Updated forecast based on current observations and latest trends.

Otherwise, problem once again appears to be convection to the south...this time a band of rain and thunderstorms across south central AL/south central GA, to the north of a warm front stretched out north of the FL Panhandle, which was contributing much of the cloud cover that was limiting our buoyancy to 1000 J/kg or less.
Temp/Dewpt trends still look ok, but without much buoyancy or forcing, and a lack of a trigger, it's starting to look like even scattered showers might be a stretch. Precip probs will be nudged downward as a result.

The situation should change this evening as better forcing will move up from the southwest, including improved upper divergence, mid-level DPVA as a few ripples of vorticity lift northeast from the upper system over the Arklatex, and low level isentropic lift as the warm front also lifts northward. Precip probs are expected to increase thru the evening in response, perhaps with some elevated thunderstorms. However, the concentration of convection might never be more than scattered. Any precip will help to keep/maintain a cool pool across much of the region. If the precip fails to develop, we could get areas of fog
Low temps should remain mild
and generally five degrees above normal. That sets the stage for Saturday, when we have some concern for severe storms. The upper system will continue to approach from the southwest through the afternoon with improved forcing, but it remains unclear as to the severe storm potential. The CAMs look unimpressed and depict mainly garden-variety thunderstorms in the afternoon, with the environment characterized by 1500-2000 J/kg of sbCAPE and maybe 25-30 kt of effective shear, which is nothing to sneeze at, so we will have to keep an eye on what develops. However, for now, agree with the SPC Day 2 Marginal, as the better shear and instability will stay to our southeast. High temps will be right around normal.

SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/
As of 200 PM EDT Friday: Short Term Key Messages

1) Models are trending drier Sunday into Monday.

2) The severe weather potential looks minimal through the period.

3) Highs on Monday are trending warmer.

An upper trough and associated upper low will gradually push eastward across Georgia the Carolinas Saturday evening into Sunday.
12Z CAMs show shower and thunderstorm activity lingering over the forecast area, especially the northern zones, Saturday evening into late Saturday night before gradually diminishing in coverage through daybreak Sunday. Thus, have the likely PoPs (55%-65%) confined to the northern two-thirds of the CWA through midnight. Capped PoPs to chance (40%-54%) across the southern third of the CWA through midnight as coverage of convection is expected to be lower. The locally heavy rainfall threat will continue through Saturday night thanks to 90th percentile PWATs lingering over the region. 12Z CAMs are trending towards much lower coverage of showers and thunderstorms on Sunday, with any activity looking to be limited to the mountain zones. Thus, have likely PoPs (55%-63%) across the mountains with chance PoPs (40%-54%) elsewhere. PoPs may need to be lowered further on Sunday if this trend of lower coverage of convection continues. Drier air looks to filer into the forecast area from the north throughout Sunday, allowing PWATs to gradually lower. Thus, the heavy rainfall threat will be limited somewhat, especially if the lower coverage of convection holds true. With much lower deep shear values expected on Sunday, the severe threat should be very low, if any. However, global models do show 1,000-1,500 J/kg of SBCAPE over the mountains during peak daytime heating, so an isolated strong storm or two cannot be entirely ruled out. The upper trough and associated upper low will continue pushing eastward Sunday evening into early Monday morning, eventually pushing offshore. Sfc high pressure will build into the region behind the departing low, leading to drier conditions Sunday night into Monday.
However, isolated showers and thunderstorms are expected to develop Monday afternoon and evening over the mountains per the latest global models. Thus, have chance PoPs (15%-25%) in place across the mountain zones. The severe weather potential looks minimal, if any, on Monday due to even lower deep shear values and SBCAPE values generally at or below 1,000 J/kg. Lows Saturday night will end up ~5- 8 degrees above climo thanks to cloud cover and rain. Lows Sunday night will end up near climo to a few degrees above climo thanks to lower cloud cover and drier conditions. Highs on Sunday will be ~2-6 degrees below climo. Highs across the mountains on Monday will be a few degrees above climo, with highs elsewhere near climo to a few degrees below climo.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/
As of 230 PM EDT Friday: Long Term Key Messages

1) Mostly dry conditions should linger through Wednesday night.

2) Isolated diurnal convection is possible on both Tuesday and Wednesday in the North Carolina mountains.

3) A cold front will bring better shower and thunderstorm chances late next week, but timing differences remain between the global models.

Sfc high pressure will remain over Georgia and the Carolinas leading to mostly dry conditions through mid-week but isolated diurnal convection will be possible each day. With global models generally showing <1000 J/kg of SBCAPE and weak deep shear during peak heating, the severe threat looks to be low, if any. A cold front will approach out of the west Wednesday before pushing across the forecast area late Thursday into early Friday increasing shower and thunderstorm chances. The 12Z GFS shows precip pushing east of the area by Friday afternoon with dry high pressure building in behind the front. Meanwhile, the Canadian and ECMWF show a wave of low pressure tracking along the stalled frontal boundary on Friday, keeping shower and thunderstorm activity around. With models not in agreement regarding on whether precip will linger through the end of the work week, capped PoPS to chance (25%-45%) on Thursday and Friday. Deep shear will increase in association with the front towards the end of the workweek. This combined with 1,000-1,500 J/kg of SBCAPE each afternoon, may lead to strong to severe storm development. However, with this being towards the end of the long term period, confidence will be low on the severe potential. Both high and low temps through the period will remain a few to several degrees above climo as a gradual warming trend is expected through the workweek.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/
At KCLT and elsewhere: VFR/MVFR cigs continue to roam the area as rainfall became more widespread than expected and cloud bases responded by lowering. Updated TAFs to include this mention with more precip expected later this evening. Wind light S to SE.
Overnight, a surge of low level moisture is planned that brings a ceiling restriction into the area along with any precip, then locks in an IFR stratus deck and variable winds. If the precip coverage is less than expected, we may get widespread fog. Once that gets locked in, it should persist through the morning hours and not break up until midday Saturday. Beyond that, convection may develop, but that is beyond all but the period for KCLT. Added a PROB30 for thunderstorms there.

Outlook: Periods of convection, which could be widespread at times, are expected through the weekend. Late night/early morning restrictions in low stratus and/or fog will also be possible. Drier weather is expected to return early in the next work week.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...None.
SC...None.

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