textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The wet and cool forecast continues through the Memorial Day holiday weekend. Wet and near normal through the rest of the period.

KEY MESSAGES

1. Developing cold air damming will maintain dreary conditions for Friday and Friday night, with widespread cloudy and rainy conditions. The risk of severe thunderstorms appears quite low. The risk of flooding rain appears low also, but nonzero, with extremely isolated issues possible going into Friday night. 2. Unsettled weather continues through most of the week as the large-scale pattern ushers Gulf moisture into the Carolinas.

DISCUSSION

Key message 1: Developing cold air damming will maintain dreary conditions for Friday and Friday night, with widespread cloudy and rainy conditions. The risk of severe thunderstorms appears quite low. The risk of flooding rain appears low also, but nonzero, with extremely isolated issues possible going into Friday night.

Increasing coverage of shallow convection, including some isolated thunder, was observed across the area early this morning (circa 05 UT) in response to strengthening upglide over a slowing backdoor cold front and associated shallow cold air damming. Instability generally remains limited (and capped) with extremely limited effective shear and DCAPE...which should continue to inhibit any severe prospects the rest of the night. That said, seeing a resurgence in 1-3 in/hr rain rates as updrafts start to get some traction again, so expect at least some isolated downpours with any stronger cells that get going.

For Friday, the forecast remains largely unchanged: periods of rain and embedded thunder are expected as low-level isentropic ascent picks up steam over the southern periphery of surface high pressure centered over Ontario, and a bonafide CAD wedge takes shape east of the mountains. 00z hi-res guidance doesn't have a great handle on exactly how rain will evolve on Friday, but generally fall into two camps: one scenario in which coverage largely hits a lull during the morning hours (recent HRRR runs and the 00z FV3), and another in which a round of stronger upglide-induced rainfall picks up steam through the morning hours (00z NAMnest, ARW, and RRFS). Don't have a great feel for which camp will ultimately be correct. Either way, would expect rain throughout the day as well as the steady development of hybrid CAD to have a significant impact on Friday afternoon highs; the NBM still appears to be 1-3 degrees too warm...and so modified the forecast accordingly.

Regardless, the potential for more widespread rain and embedded (mainly elevated) thunderstorms should increase through Friday afternoon, as a shortwave axis migrates from the lower Mississippi Valley northeast into Georgia. The resulting increase in DPVA should improve elevated instability, and so although operational guidance remains in good agreement that the CAD wedge will remain in place all day, the case for wedge-top convection looks better and better going into Friday night. Tonight's 00z HREF paints a pretty convincing picture of heavy rainfall developing after 00Z Saturday, particularly across the upper Savannah River Valley and western Upstate, where hi-res guidance is in good agreement that the combined potential for convective rates and training cells will produce low-end hydro issues. To wit, both the latest HREF and REFS output indicate potential for a corridor of 3hr QPF of 1.5-2.5" across northeast Georgia and the western Upstate, and extremely isolated pockets of 2.5-3", mainly along the SC Blue Ridge Escarpment. Despite ongoing D2-D3 drought conditions across the region, such totals are enough for at least isolated nuisance flooding.

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

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 at least mid-week. This configuration is favorable for steady moisture flux off the Gulf, meaning periods of widespread rain should continue on Sunday and the first several days of next week. Operational models depict another low-amplitude shortwave lifting across the Carolinas on Sunday night, and accordingly another increase in QPF during that period. Confidence is low on any impacts, but at this time Sunday night appears to be the next point of interest for more widespread significant rainfall.

Unsettled weather and above-normal rainfall chances look to continue through at least midweek. Depending on how the next few days pan out, it's still within reason that we could be looking at a nonzero heavy rain / flash flood threat by midweek...and it's somewhat more likely that we should put a serious dent in ongoing drought conditions. Still...best not to count our eggs before they hatch. There's a lot that can go right (or wrong) for things to proceed as anticipated.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: As we approach 06Z, convection seems to have gained a second wind, with scattered showers redeveloping across much of the NC Piedmont and, at the time of the this writing, to some extent across the southern Appalachians as well. Generally think that this will be mainly showery, with instability fairly limited at this point, and the atmosphere not quite "juiced up" enough for widespread TSRA. Still, could certainly see a rumble of thunder here and there. During the wee hours of Friday morning, still expect a lull in coverage, supported by most of the 00z guidance. The latter half of tonight will instead be marked by a quick expansion of MVFR to IFR ceilings - already in place across parts of the I-77 corridor - across much or all of the area. At least widely scattered showers will continue across the terminal forecast area through early afternoon Friday, after which an increase in elevated instability should result in more widespread SHRA and a greater chance for TSRA, especially during the late afternoon and evening hours. Cold air damming is still expected to persist for most or all of Friday, with IFR restrictions unlikely to reliably lift all day. More intense SHRA should redevelop after 00Z Saturday, particularly across the western SC Upstate.

Outlook: Cold-air damming persists thru Saturday. The CAD will likely produce widespread MVFR to IFR ceilings into Saturday. 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.