textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for new Aviation discussion.

KEY MESSAGES

1. Rain chances increase through morning ahead of an approaching cold front, then decline from north to south this afternoon as the front passes through the area. Rain could redevelop tonight or Friday. Flooding is not expected and rainfall should be beneficial. 2. Drier conditions and warmer temperatures for the weekend before rain chances and cooler temps return early next week.

DISCUSSION

Key message 1: Rain chances increase through morning ahead of an approaching cold front, then decline from north to south this afternoon as the front passes through the area. Rain could redevelop tonight or Friday. Flooding is not expected and rainfall should be beneficial.

A weak shortwave is embedded in the broad upper trough, which will swing east across KY/TN later this morning and help propel the front southeastward across our CWA. GFS/NAM/ECMWF are in better agreement than they were 24 hours ago in depicting a wave forming along the baroclinic zone (south of the main cold front) developing in response to the shortwave. The wave looks to be near the TN/NC border around 15z and east of the CWA in central NC by 21z. Stratiform precip may taper off or shift toward the north during the morning hours in advance of that feature. Most guidance does develop at least small elevated CAPE as lapse rates increase in the warm sector of this wave. Accordingly most models' QPF output has a convective look across GA and the Upstate from late morning onward, and PoPs finally increase to categorical values (> 75%) in those areas for a time. The mountains, foothills and NW NC Piedmont probably will see a lull in precip behind this wave, but the synoptic cold front should settle through in the late afternoon or evening and some CAM output suggests shallow convection remains possible along that as well. These CAPE values are too small to support severe risk even with decent effective shear of 30-40 kt; highly isolated thunder is possible. Gusty, sub-advisory winds likely Thu afternoon in the Piedmont. Though PWATs peak near +2 SD above climo, perhaps on account of the mediocre forcing and limited convective response, probability of 2 inch total QPF is very low. SW-facing ridges, especially west of the French Broad Valley, do look ideally oriented for orographic lift, so those areas bear the greatest chance of exceeding 1.5" total. 3-hr HREF PMM QPF is on the order of 0.5" to 0.75" this afternoon, for a fair estimate of what is possible if the Piedmont convection materializes as expected. So, most likely beneficial rainfall helping to ease the drought conditions.

There will be a second embedded shortwave tracking across MS/AL/GA Thu night into early Friday, which still leads some models to redevelop precip along the front near our southern CWA border. Guidance has trended further south with that development, but at this time we still feature 20-30% PoPs lingering into early Friday afternoon southeast of I-85. We now feature a dry forecast Friday night and Saturday.

Key message 2: Drier conditions and warmer temperatures for the weekend before rain chances and cooler temps return early next week.

Surface high pressure builds across the region Friday into the weekend allowing drier conditions and warmer temps to return. these warmer temperatures and drier air mass should allow dewpoints to dip into the 30s and 40s each afternoon over the weekend, leading to lower RH. Current trends show less than a 25% chance of RH values reaching or dropping below critical thresholds east of the mountains this weekend. This does depend on how quickly the drier air mass moves in and the amount of mixing each afternoon. Additionally, with the much needed rainfall that did occur, this provides a bit of relief in terms of fuel moisture. For now, any fire weather concerns remain very low as winds should remain light for the majority of the area.

By the end of the weekend, another round of high pressure amplifies from central Canada and into the central CONUS, before reaching the east coast to start the week. Current guidance has a weak cold front moving across the forecast area on Sunday into Monday. This boundary looks to stall to the south of the CWA and allows rain chances to return Monday into Tuesday. For the rain chances, confidence remains low as the location of the front directly impacts rain chances. For this, keeping slight chance (25-35%) PoP outside the mountains and slightly higher for the higher elevations. Regardless of rain chances, the overall QPF response for this next potential system is low with a 30-40% chance for the mountains receiving at least 0.25" of rain. It drops off even lower east of the mountains with around a 20% chance. All in all, this next system does not look to make a major dent in the needed rainfall totals for the area. A cooling trend can be expected Monday into Tuesday as cold air damming develops behind the stalled front. This filters in below normal highs by Tuesday.

AVIATION /12Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: We are in the painful transition downward thru flight categories at issuance time, with the main area of light/moderate rain moving east across the mtns and out over the Upstate/Piedmont thru mid-morning. In the very near term, this will translate to most locations dropping down from MVFR to IFR, with some indication of LIFR potential during the early afternoon. The HRRR in particular brings the heaviest rain eastward across the region during the early afternoon. Once established, the flight restrictions will be with us through the better part of the daylight hours, and probably variable between LIFR and MVFR. As the light rain shifts southward late in the day, some VFR holes will open up, particularly over the NC foothills, but with sunset the low clouds are expected to fill back in with more MVFR and IFR ceilings into the early morning hours on Friday. As for winds, SW thru the day until a cold front moves thru in the late afternoon and early evening hours, bringing a shift to NW and N bu 01Z at the latest.

Outlook: Cannot rule out return to IFR Thursday night, and restrictions could return Friday with front remaining near the region. Dry and VFR conditions should return by Saturday. Another front may bring showers and associated restrictions on Monday.

GSP WATCHES/WARNINGS/ADVISORIES

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