textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes for this forecast update.

Aviation discussion was updated for the 12z TAFs.

KEY MESSAGES

1. Another frontal system arrives from the west this later today through Sunday. The front may briefly stall across our area early next week, bringing back high rain chances, below-normal temperatures, and some potential for locally heavy rainfall. 2. A return to typical mid-summer weather is expected next week.

DISCUSSION

Key message 1: Another frontal system arrives from the west this later today through Sunday. The front may briefly stall across our area early next week, bringing back high rain chances, below-normal temperatures, and some potential for locally heavy rainfall.

Stout upper ridging remains entrenched over the Desert Southwest with active northern stream flow extending from the Pacific Northwest into the Mississippi Valley. A large upper low is pushing onshore over the Pacific Northwest with a lead wave dropping out of the Midwest into the Mississippi Valley. This will be the main feature of interest through the weekend as it slowly slides across the Ohio Valley and into the Appalachians. An attendant surface reflection will accompany the wave with a frontal boundary draped from central Kentucky into Tennessee. Low-level mass response will strengthen a 20-35kt low-level jet that is forecast to translate across the region late this afternoon through Sunday. This will foster the advection of a deep plume of anomalous moisture with PWATs surging to 2-2.4", which will challenge daily records at Peachtree City, GA and Greensboro, NC upper air sites.

The 00z suite of CAMs is in generally good agreement that a band of convection on the leading edge of the moisture plume will push into the mountains and northeast Georgia late morning into early afternoon and through the Upstate and foothills/Piedmont through peak heating. Additional clusters of upstream convection are also progged to develop from Alabama into eastern Tennessee and Georgia. This activity will accompany the low-level jet and at least some extent will persist into the forecast area overnight into early Sunday morning outside of the traditional diurnal window. The main concern will be the potential for several instances of locally heavy rainfall and flash flooding from training and slow moving storms, especially along the southern escarpment. CAMs diverge, however, with how this activity evolves overnight and the extent of coverage. The HRRR solution is the most aggressive with a notable cluster of storms over the Upstate and southern escarpment and highlights the potential for isolated rainfall totals of 3-6" over a short several hour window. Should this materialize, isolated flash flooding would definitely be possible owing to very moist profiles and modest moisture transport within the cloud-bearing layer.

Heading into Sunday, the deep moisture plume will be predominately situated from along and east of I-85 as the upper trough axis and frontal boundary approaches. Extensive cloud cover and ongoing convection during the morning hours casts uncertainty as to the extent of destabilization and development of additional showers and storms by the afternoon and evening. Given the abundance of deep moisture and forcing from the wave it would appear likely that at least scattered to numerous showers and storms should redevelop in a modestly unstable airmass pending at least a few hours of daytime heating. Once again, any additional activity will pose a risk for isolated flash flooding, especially for any storms that train over areas that have already seen higher rainfall totals. A secondary threat will exist for an isolated strong to marginally severe storm or two both Saturday and Sunday. Deep-layer shear increases on Sunday with the approach of the wave, but thermodynamic parameters will be lackluster otherwise. Near moist adiabatic lapse rates and a dearth of DCAPE within a very moist profile and tall/skinny CAPE profiles will generally preclude a threat for damaging winds. Nonetheless a few isolated water loaded downburst will remain possible.

Key message 2: A return to typical mid-summer weather is expected next week.

Guidance begins to diverge regarding the evolution of the synoptic pattern as we head into early next week. Some guidance keeps some semblance of a closed upper low over the Southern Appalachians while others shift the wave northeast of the area. At the same time, a subtropical ridge will build offshore. At the very least, the region will reside within a relative weakness between two upper ridges, which will favor at least traditional diurnal convection each day. Should a weak upper low linger across the region we could see higher than climo PoPs hang on longer into the week.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: MVFR to IFR stratus has overspread much of the area early this morning, especially east of the mountains. Ceilings reductions will continue through around mid morning before low clouds finally lift and scatter with a return to VFR. This will be short lived as multiple rounds of showers and thunderstorms are expected from late this afternoon through the overnight hours and into early Sunday morning. Associated visibility and ceiling restrictions can be expected with any of this activity. Another bout of low stratus is expected to develop again tonight as a very moist airmass remains in place. Widespread IFR is expected with periods of LIFR not out of the question along with the potential for some patchy fog. Light and variable winds will remain light roughly out of the south.

Outlook: Shower and thunderstorm chances and associated restrictions expected to continue through at least the middle of next week. While convection will be most active during the afternoon/evening, some degree of overnight and early morning coverage is anticipated tonight into early Sunday. Widespread low clouds and fog are possible Sunday morning and again Monday morning. A return to mostly diurnal convection and early morning mountain valley fog/stratus is expected during the new work week.

GSP WATCHES/WARNINGS/ADVISORIES

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