textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to the forecast with this update.

Updated the aviation discussion for the 18z TAFs.

KEY MESSAGES

1. A cold front will approach the area from the north today, and push south into the Upstate on Thursday. This will bring continued diurnal showers and storms, mainly in western NC today, and the Upstate and northeast GA on Thursday. There will be a low- end risk of isolated flooding and severe storms producing damaging wind gusts. 2. A front stalls just south of the area, keeping mainly diurnal rain chances through the weekend. Drier air may finally work into the entire area from the north by the middle of next week.

DISCUSSION

Key message 1: A cold front will approach the area from the north today, and push south into the Upstate on Thursday. This will bring continued diurnal showers and storms, mainly in western NC today, and the Upstate and northeast GA on Thursday. There will be a low- end risk of isolated flooding and severe storms producing damaging wind gusts.

Height falls over the Ohio Valley to the Mid-Atlantic will work to flatten the upper flow atop the forecast area tonight thru Thursday. Deep-layer westerly to west-southwesterly flow will advect slightly drier air and help reduce the excessive rain threat. With that said, PWATs are still 1.5-1.75" and decent instability across the area will support convective/heavy rain rates. Steering flow should take convection that is currently blossoming across the mountains into the Piedmont, where MLCAPE is around 1500-2000 J/kg and effective shear is 30-35 kt. This is just enough to keep a non-zero severe threat, mainly across the NC Piedmont, where the CAMs are in decent agreement on some multicell clusters pushing off the Blue Ridge Escarpment and tracking east this aftn into early evening. This is still in line with the SPC's marginal risk of damaging wind gusts in the Day 1 Convective Outlook. Coverage of storms looks to be lowest in the southern and eastern SC Piedmont due to lack of low-level convergence. Convection should largely wane late evening, but a few showers may linger near the TN border overnight, within weak westerly upslope flow.

On Thursday, as the upper trough deepens along the East Coast, a weak cold front will sag south into the forecast area. The latest guidance has trended drier with the front, pushing in dry air from the north and showing a NWLY downslope low-level flow across most of the forecast area by early aftn. Still expect scattered showers and storms to develop along and south of the front. Up to 2000 J/kg of sbCAPE across the Upstate and NE GA may be enough for a storm or two to become severe. But overall, the threat will remain low. There will also be a marginal risk of excessive rainfall/flash flooding, but trends in the guidance have been trending down. Otherwise, temps will be a few degrees above normal with elevated humidity.

Key message 2: A front stalls just south of the area, keeping mainly diurnal rain chances through the weekend. Drier air may finally work into the entire area from the north by the middle of next week.

The 12z deterministic guidance is in pretty good agreement on a series of shortwaves reinforcing an upper trough along the East Coast, eventually closing off into a large upper low over the Northeast US by the middle of next week. A couple of impulses will eject out of the Southern Plains, as an upper ridge gets pinched off over the Midwest to the TN Valley. The first wave will help activate a stalled front just to our south Friday, bringing a round of showers and possibly a few storms into the southwestern part of the CWFA Friday evening. A CAD-like pattern will be in place, with more stable air across most of the CWFA. So confidence remains low on how much PoPs will really be able to expand NE overnight. A lack of better forcing suggests the heavy rain threat should remain low. But a marginal risk of excessive rainfall has been expanded into the SW third of the CWFA. The front lifts further north into the area Saturday, resulting in another active convective day that likely will persist into Sunday.

The final shortwave that rotates thru the mean East Coast trough will cross the area Sunday night, keeping rain chances thru the night. Models have come into better agreement on a deepening trough and a good push of dry air from the north Monday into Tuesday. This may finally give us a break from the above normal rainfall most of the region has been seeing. The latest NBM has a gradual decreasing PoP trend Monday thru Wednesday. Temps will also trend down a little below normal, with dewpts possibly into the 40s and 50s, making it feel pleasant for early June.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

At KCLT and elsewhere: Widespread showers will continue to blossom across the mountains this aftn, with a few storms expected to form within this activity and drift east into the Piedmont. Confidence remains highest for storms to track thru the NW NC Piedmont this aftn into early evening, so will go with a TEMPO for KHKY. Most of this activity may stay north of KCLT, so VCSH and a PROB30 for some TS will be carried in the 23-03z time frame. Similar situation for the Upstate sites, with enough of a chance to warrant some PROB30 for TSRA late aftn to early evening. Convection should wane late evening, leaving some mid and high clouds around overnight. Given the moist environment, may see patchy fog and stratus around, especially in the mountain valleys. A front will push into the area Thursday morning, toggling winds out of the N/NW and remaining light. There is some uncertainty on how far the front will get by peak heating Thursday aftn. For now, will go with a PROB30 at KCLT after 18z, but this may get trimmed back if the drier model trends hold.

Outlook: The pattern remains unsettled through the upcoming weekend, with at least scattered diurnal convection expected each day. Vsby and cig restrictions will remain possible each night.

GSP WATCHES/WARNINGS/ADVISORIES

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