textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Small uptick in severe storm and flood chance through this evening then again on Saturday.

KEY MESSAGES

1. Cold air damming will maintain dreary conditions for most. Wedge boundary across the area could become a focus for severe thunderstorms. Heavy rainfall expected tonight which may lead to flooding. Cold air damming lingers into Saturday. 2. Unsettled weather continues through most of the next week as the large-scale pattern ushers Gulf moisture into the Carolinas and northeast Georgia.

DISCUSSION

Key message 1: Cold air damming will maintain dreary conditions for most. Wedge boundary across the area could become a focus for severe thunderstorms. Heavy rainfall expected tonight which may lead to flooding. Cold air damming lingers into Saturday.

The wedge boundary is quite obvious in the surface obs with warm and moist conditions over the Upper Savannah River Valley and the Lakelands. Much cooler and cloudy with stronger NE winds north of the boundary. sbCAPE also shows this boundary with a moderately unstable air mass to the south diminishing to non across much of the NC foothills and Piedmont. That said, even in the wedge there is muCAPE. Expect convection spread across the area through the evening as a short wave rotates into the area. Even in the wedge, thunder will occur given the muCAPE. SPC has moved the Marginal severe risk into the Upstate, which makes sense given the wedge boundary and the increased effective shear due to the instability. Damaging wind is the main threat, but storms near the boundary will have to be monitored for a weak tornado spin up.

The severe threat diminishes through the evening but heavy rain and the flood threat will increase. High PW values, upslope flow, and an increasing low level jet will lead to some heavy rainfall, especially over the Upper Savannah River valley across the border into the NC southern mountains. Despite the ongoing drought conditions, rainfall rates in these areas could be enough for some isolated flash flooding to develop. The flood threat tapers off overnight as the convection moves east.

The cold air damming continues into Saturday with cloudy and cool conditions along with another round of showers and thunderstorms. Weak instability with some effective shear continues, especially along and south of the wedge boundary. This could lead to some strong to severe storms. Rainfall rates, while not quite as high as today, could be high enough for heavy rainfall and isolated flooding. The convection moves east through the evening ending the severe and flood threat.

Key message 2: Unsettled weather continues through most of the next week as the large-scale pattern ushers Gulf moisture into the Carolinas and northeast Georgia.

Synoptic guidance continues to indicate that broad troughing will remain in place across the central CONUS while upper ridging will stay centered over the western Atlantic from Sunday through much of next week. This configuration is favorable for steady moisture flux off the Gulf, meaning mainly diurnal periods of generally widespread convection should continue.

With the unsettled weather and above-normal rainfall chances, it is possible that a low end heavy rain/flood threat will continue, so we'll have to keep an eye on that.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Cold air damming is fully ensconced across all the sites except KAND which is on the edge given the variable MVFR and low VFR cigs. Expect IFR to LIFR cigs to continue for all but KAND where it will be generally MVFR. Convection will steadily increase through the afternoon, so have TEMPOs with generally MVFR vsby. Can't rule out brief IFR vsby in heavier showers. Convection continues into the evening with IFR to LIFR cigs and MVFR to IFR vsby. Showers with MVFR vsby and LIFR cigs expected overnight. Periods of showers with MVFR and IFR vsby and IFR to LIFR cigs expected after daybreak. NE wind with low end gusts through the period. KAVL will be SSE. KAND may go ESE briefly until the CAD settles in there.

Outlook: Cold-air damming persists thru Saturday night. The CAD will likely produce widespread MVFR to IFR ceilings into Saturday night. Periods of mainly diurnal convection will also continue atop the CAD wedge and into next week, even as the CAD erodes. The potential for overnight fog/low clouds will increase through the period as well. Active diurnal convection expected to continue thru the first half of next week.

GSP WATCHES/WARNINGS/ADVISORIES

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