textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 18Z TAFs.

Increased PoPs and rainfall amounts for Thursday morning.

Freezing temperatures appear possible over portions of the NC mountains early Sunday.

KEY MESSAGES

1. A cold front brings scattered showers and thunderstorms this afternoon and evening, a few of which could become strong to severe. Confidence on the severe threat remains low. Yet another round of early morning showers are possible around daybreak Thursday. 2. Another low pressure system develops to our south late Friday into Saturday, bringing more rainfall to the western Carolinas. Freezing temperatures are possible over portions of the NC mountains early Sunday.

DISCUSSION

Key message 1: A cold front brings scattered showers and thunderstorms this afternoon and evening, a few of which could become strong to severe. Confidence on the severe threat remains low. Yet another round of early morning showers are possible around daybreak Thursday.

Strong closed upper low centered over the Upper Great Lakes will send a trailing cold front to the area later today into early Thursday. Ahead of the front, clouds have cleared and allowed for the atmosphere to recover, leading to 1500-2500 J/kg of SBCAPE this afternoon. Strong deep layer shear between 40-50 kts will be enough to support a few isolated supercells to develop as the front encroaches from the west. The better forcing for ascent will be situated east of the area, so the overall coverage will be limited with most of the guidance favoring the NC zones and the northern Upstate. Low-level shear of 20-25 kts and effective SRH values above 100 should be enough to promote the risk of an isolated tornado, mainly east of the mountains. The main threat will be a couple of damaging wind gusts and large hail (up to 1" in diameter) associated with the stronger storms. Gusty winds will also accompany the cold front, especially across the mountains through tonight and Thursday before the pressure gradient relaxes by Thursday night. Gusts are expected to remain below advisory level criteria.

Convection over the lower MS Valley and the Deep South this afternoon and evening will organize into an MCS. The flow aloft favors a southwest to northeast movement and high res guidance have come to better agreement of the remnant MCS reaching the CWFA overnight tonight into the morning hours Thursday. Environmental parameters are not favorable for a conditional severe threat, but should bring beneficial stratiform precipitation across the area, especially over northeast GA, Upstate SC, and portions of the CLT metro. Can't rule out some thunder, but any convective elements will remain elevated. Most of the rainfall should exit the area by late morning Thursday as continued dry air filters in behind the front. Skies should scatter out with breezy conditions during the day Thursday as a weak downslope component gets going with afternoon highs near normal for late April.

Key message 2: Another low pressure system develops to our south late Friday into Saturday, bringing more rainfall to the western Carolinas. Freezing temperatures are possible over portions of the NC mountains early Sunday.

Relatively weak sfc high pressure will have spread over our area by early Friday as low-level winds veer to N/NE. As the day wears on, the high will weaken, sfc winds swing around to SLY, and cloud cover steadily increases from the south and west. This will help keep temperatures below climatology on Friday, with highs in the low 70s outside the mtns.

Active weather will return Friday night into Saturday morning. By late Friday, the long-range guidance continues to expand a broad baroclinic zone across much of the deep south ahead of an upper shortwave ejecting out of west Texas. The associated sfc frontal zone appears to remain south of our fcst area thru the weekend, with the bulk of the latest guidance now depicting cyclogenesis over the Gulf Coast/Florida Panhandle early Sat. This is expected to produce a round of moist upglide across the western Carolinas which should result in more widespread showers across our area from early Sat morning into Sat evening. Overall precip amounts have trended down compared to the previous fcst, but we are still expecting at least 1" roughly along and south of I-85 and approximately 0.5 to 0.8" north of I-85. In addition, some amount of elevated instability may be present over our area Sat afternoon/evening, but any thunder/lightning would likely be isolated.

Behind this system, cool/dry high pressure will spread back over the region Sat night and Sunday. Low temperatures near freezing are expected Sunday morning over the NC mountains. Thus, Frost Advisories and/or Freeze Warnings may be needed for portions of the mountains. Otherwise, we should remain mostly dry thru early next week with temperatures steadily warming back above normal.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

At KCLT and elsewhere: MVFR to low VFR cigs remain in place this afternoon and all sites should eventually go VFR over the next few hours. A line of showers and thunderstorms may develop along and ahead of a cold front, mainly across the NC terminals this afternoon and evening, so placed a PROB30 for TSRA and associated restrictions. South to southwest winds have picked up some low-end gusts, while KAVL turns out of the north to northwest, becoming gusty at times through the period. Winds turn out of the north to northwest behind a cold front later this evening at all sites and remain that direction through the end of the period with sporadic low-end gusts. Another round of showers and thunderstorms could push through the area late tonight through around daybreak Thursday. This will come from the south, giving the Upstate terminals the best chance of precipitation. Placed a PROB30 at all sites for SHRA and associated restrictions. Wouldn't be surprised if the current trends continue, a TEMPO or prevailing line for SHRA will be placed at the Upstate sites. Clouds should scatter rather quickly during the day Thursday following the second round of showers and storms and all site should be VFR by the late morning hours.

Outlook: Mostly dry and mostly VFR Friday before another disturbance brings rain chances and associated restrictions back Friday evening into Saturday. Dry high pressure for Sunday.

GSP WATCHES/WARNINGS/ADVISORIES

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