textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated to reflect the impending 12Z TAF update.

Minor changes were made to today's QPF, but overall expectations for widespread heavy rain through this afternoon have not changed.

KEY MESSAGES

1. Widespread rainfall continues tonight and Saturday, with some periods of heavier rainfall. There is a conditional threat of severe weather, mainly across the SC Upstate and northeast Georgia. 2. Unsettled weather continues through most of next week as the large-scale pattern ushers Gulf moisture into the Carolinas and northeast Georgia.

DISCUSSION

Key message 1: Widespread rainfall continues tonight and Saturday, with some periods of heavier rainfall. There is a conditional threat of severe weather, mainly across the SC Upstate and northeast Georgia.

As expected, coverage of upglide-induced showers has blossomed again this morning in response to the arrival of a weak shortwave, now visible on satellite imagery over northeast Georgia and the southwestern Appalachians (if perhaps washed out by cloud tops). Widespread low stratus remains in place owing to sustained hybrid CAD, though surface analysis suggests the parent high is now centered off the Massachusetts coastline, meaning synoptic support should begin to lessen over the coming hours.

Otherwise...through this morning, the full effects of the shortwave should be felt, and mid-level lapse rates should begin to steepen. As a result, a transition from solely upglide-induced showers to more convectively-enhanced rain rates looks likely by afternoon. Most of the CAMs depict an axis of heavier QPF crossing the area from mid-morning through the afternoon, producing anywhere from 1 to 3 additional inches, with isolated higher amounts possible. A secondary issue will be the resurgence of severe weather potential...mainly in northeast and the southern and eastern most portions of our NC and SC zones...as any daytime erosion of the wedge opens those zones up to surface-based instability. Locations where the wedge mixes out will be exposed to a convective environment featuring some 1500-2500 J/kg sbCAPE. Despite modest shear profiles, a couple of strong to severe storms cannot be ruled out...highly conditional upon the erosion of the wedge. The main threat with any severe storms that develop would be wind.

Guidance generally over-estimated how well the wedge would erode on Friday. Whether that's a useful barometer for today is questionable. On the one hand, synoptic support will have begun to wane by this afternoon, a point toward more erosion today, but there's also a signal for heavier, more widespread rainfall today that would lend itself to better diabatic maintenance of the wedge. All told, the HREF is tending toward a stronger, more persistent wedge today, and recent runs of both the HRRR and RRFS are tending toward keeping surface-based instability entirely shunted to our south and east. Confidence is limited, but if, and only if, this currently-favored scenario pans out, convection would remain elevated and severe prospects would be very, very limited.

Shower coverage should trend downward after sunset Saturday. Though at least widely scattered rainfall will likely continue across the region, QPF trends downward Saturday night and Sunday as we hit a lull in synoptic forcing.

Key message 2: Unsettled weather continues through most of 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. Ensembles still struggle to consistently place the remnant frontal boundary, but by pattern recognition alone, periods of in-situ cold air damming would appear possible.

QPF has trended downward for Sunday night compared with previous forecasts...as guidance has largely pushed the shortwave that'll cross the area during that period farther north, such that it'll have less impact on our area. Comparatively, QPF has instead trended upward on Monday night, but confidence remains pretty low here since long-range ensembles are split on both the strength and timing of yet another weak shortwave progged to cross the Carolinas and lower Mid-Atlantic Monday night. One thing that's more certain is this: the dreary weather isn't going anywhere anytime soon.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Cold air damming continues this morning across the terminal forecast area. Scattered showers continue across the Upstate, with lighter intermittent drizzle elsewhere. IFR to LIFR ceilings reported at most locations will persist for much of today as shower coverage increases steadily through mid-afternoon. Some embedded TS will be possible during the afternoon hours. Brief improvement to MVFR ceilings will be possible in the afternoon, but confidence is limited. A decrease in shower coverage is expected after sunset. Winds should remain NE throughout the period, but may occasionally toggle to the SE, especially this morning. Conditions should crash to IFR/LIFR again tonight. Restrictions may persist into the first part of Sunday.

Outlook: Periods of mainly diurnal convection will 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.