textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 12z TAF issuance.

KEY MESSAGES

1. Light rain tapers off this morning. Another round of showers/storms may develop ahead of a cold front this afternoon as it progresses across the area, though this is not expected to result in either severe weather or a flash flood risk. Cool and mostly clear conditions over the mountains tonight could lead to patchy frost in a few valley locations. 2. Drier conditions return Friday before rain chances increase Sunday and into the start of next week.

DISCUSSION

Key message 1: Light rain tapers off this morning. Another round of showers/storms may develop ahead of a cold front this afternoon as it progresses across the area, though this is not expected to result in either severe weather or a flash flood risk. Cool and mostly clear conditions over the mountains tonight could lead to patchy frost in a few valley locations.

Cold front now appears to be aligned from near the NC/TN border southwest to AL, continuing to slowly sag south and east. Broad area of showers in the moist and weakly unstable area ahead of the front. Some activity will continue to stream in thru the morning though the back edge of the precip will drift across the area from NW to SE by midday or so. While winds may start to turn northwesterly in spots during the early part of the day, the actual cold front looks to push east of the Appalachians early this afternoon and progress south across the area by late evening. Temps today will be cooler it seems largely on account of cloud cover being slow to clear after precip tapers off this morning, and dewpoints look to remain elevated. Model LCLs are lower than usual today and lapse rates aloft remain marginally supportive of deep updrafts. PWATs will however have trended lower and the atmospheric column will be drier than what was seen Wed and Wed night. CAMs are in decent agreement on scattered showers and thunderstorms developing along the front this afternoon mainly over the NC Piedmont and eastern half of the Upstate, where we feature a PoP mention. SBCAPE of a few hundred J/kg may result, though winds aloft will be weaker and shear profiles generally not supportive of severe threat.

Temps may cool several degrees once the front passes; low clouds finally should clear the area except possibly along the NC/TN border. Breezy NW to N winds with gusts 15-25 mph possible in the late afternoon and early evening, but winds settle down more quickly behind this front than most. Expect cooler temperatures tonight 5-10 degrees below normal, the most departure from normal being in the mountains. Surface drying is more typical for May and not expected to lead to a rapid crash in dewpoints. Some high elevation spots may cool into the mid-30s, and without strong mixing or drying, conditions may prove favorable for some frost to form where the boundary layer is able to decouple. The most likely area would appear to be the northern mountain valleys.

Key message 2: Drier conditions return Friday before rain chances increase Sunday and into the start of next week.

Surface high pressure builds to start the weekend. An isolated and brief shower could occur on Saturday, but overall, conditions should remain relatively dry. By Sunday, model guidance depicts a change in the general synoptic flow and develops a series of shortwaves across the southeast. At the surface, southerly winds advect moisture ahead of a frontal boundary off to the west. Current guidance is still on track with a decent QPF response with pre-frontal showers starting as early as Saturday night, through Sunday and into Monday. This looks to be the next rainfall opportunity, with probabilities putting a 30-40% chance for rainfall amounts totaling more than 0.5". So while this looks to provide necessary rainfall, it's not a whole lot. After Monday, long range models show high pressure moving in behind the front and maintaining a calmer pattern to the end of the forecast period. Temperatures should also remain warm and closer to normal for May, with a dip behind the front and a quick return to normal.

AVIATION /12Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: Low confidence for timing, but some threat of IFR at the TAF sites thru 14-15z. Expect rapid changes between IFR and low VFR as showers come and go. Not seeing much potential for embedded thunder the remainder of the morning, so TEMPOs for IFR and SHRA without any VCTS. Winds largely look to remain light/VRB during the morning, but SW would be favored by the large scale pattern. Cold front set to move across the region over the course of the day, establishing NW flow first at KAVL, later elsewhere. Some mixing of NW winds may occur in the early part of the day and result in a period of W to NW winds at the other sites, with diurnal mixing also allowing cigs to lift toward 16-18z. Those W-NW winds appear likely to back into the SW quadrant before the actual cold front pushes in this afternoon and invokes a shift to NW that should last the remainder of the period, with a few gusts of 15-20 kt likely in the late aftn and early evening. Some risk of MVFR cigs until cold front passes. Cold front also may spawn a few lower-impact SHRA or TSRA, warranting PROB30 for KCLT, KHKY, KGSP, KGMU. That chance is too low to mention at KAVL or KAND.

Outlook: Dry and VFR conditions return Thursday night and Friday. Cannot rule out restrictions Saturday with return of some moisture. Another cold front impacts the region late this weekend into early next week bringing another round of rain and associated restrictions.

GSP WATCHES/WARNINGS/ADVISORIES

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