textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes for this forecast cycle.

Updated the Aviation Discussion for the 06z TAFs.

KEY MESSAGES

1. Near-normal temperatures with unseasonably low humidity and no threat for afternoon/evening storms continues today. 2. Another frontal system is expected to arrive from the west this weekend, possibly stalling across our area early next week, bringing back high rain chances, below-normal temperatures, and some potential for locally heavy rainfall.

DISCUSSION

Key message 1: Near-normal temperatures with unseasonably low humidity and no threat for afternoon/evening storms continues today.

Weak shortwave ridging slides over the Appalachians today on the eve of a notable pattern change. This will support another day of continued average temperatures for late July in concert with unseasonably low humidity. Climatologically dry air with sub-1" PWATs is entrenched across the region with very dry forecast soundings. A deep well mixed boundary layer extending up to ~800mb will foster efficient mixing of surface dewpoints with values falling off below 60 this afternoon and humidity values below 40%. While a few shallow isolated cumulus clouds will be possible during peak heating, the environment will be rather hostile to convective development with no threat for traditional summer afternoon/evening thunderstorms.

Key message 2: Another frontal system is expected to arrive from the west this weekend, possibly stalling across our area early next week, bringing back high rain chances, below-normal temperatures, and some potential for locally heavy rainfall.

A notable pattern change commences on Saturday with a return to wet and unsettled weather through the weekend and into next week. By Saturday morning, a stout 600dam H5 ridge will be well established over the Desert Southwest with active northern stream flow draped from the Pacific Northwest through the Northern Rockies. A lead shortwave trough will be dropping down the eastern flank of the ridge across the Central Plains and into the Mississippi Valley. The synoptic pattern also begins to amplify with a tall Bermuda high building across the Atlantic, thus slowing the eastward progression of the upper wave through the weekend. Height falls will gradually overspread the Appalachians with an attendant surface low sliding across the Ohio Valley and a surface frontal boundary pushing across central Kentucky and Tennessee. Low-level mass response will strengthen a 30-40kt low-level jet progged to translate across the western Carolinas and northeast Georgia Saturday into early Sunday morning. This will draw an anomalous plume of deep moisture into the region with PWATs surging to 2-2.4", well north of the 90th percentile for early August. Increasing moisture will also support modest instability with 2000-3000 J/kg of surface-based CAPE. The net result will be a rapid increase in rain chances through the day Saturday and into the overnight hours. Guidance is in relatively good agreement that a broad zone of convection will develop within the deep moisture axis and shift east across the area. The presence of the upper wave and low-level jet will likely support convective elements through the overnight into early Sunday past the traditional diurnal convective window. The main hazard of concern will be for locally heavy rainfall and isolated to widely scattered flash flooding from any slow moving/training storms within the high PWAT environment and increased moisture transport within the cloud- bearing layer. A secondary threat for isolated strong to severe storms cannot be completely discounted, but the near-storm environment is not overly impressive. DCAPE is forecast to be underwhelming within such a moist environment, which is further supported by near moist adiabatic mid-level lapse rates and tall/skinny CAPE profiles. Nonetheless, a few isolated water loaded downbursts could be realized.

The upper trough axis and surface frontal boundary approach the area on Sunday and could kickoff another round of numerous to widespread showers and storms, although the intensity of such is somewhat uncertain owing to what will likely be widespread cloud cover. Given the slow progression of the upper wave, attention will once again need to be paid to any slow moving and training storms as the axis of deep moisture will still be well established at least east of the mountains.

Heading into next week, the 00z suite of guidance and their respective ensemble members begin to diverge with respect to the evolution of the synoptic pattern. There are indications that the upper trough may evolve into a closed low and linger across the Southern Appalachians as it becomes cutoff from the main northern stream flow and sandwiched between the Desert Southwest ridge and a building Bermuda high to the east. Regardless, at the very least a general weakness between the two upper ridges will reside across the region with a typical warm and humid late summer pattern featuring daily diurnal convective chances. The weekend low-level jet and plume of deep moisture will have shifted east at this point, thus lowering the potential for a more focused flooding threat. However, locally heavy rainfall and an isolated flooding threat can never be ruled out with any summer storm, especially following several prior days of expected rainfall.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR conditions will prevail at all terminals through the TAF period. A few high clouds will drift across the region through tonight and Friday with a shallow afternoon cumulus field between 5-6kft during the afternoon. High clouds will increase in coverage overnight into Saturday morning as moisture streams back into the region. Winds will be light and variable overnight and generally out of the south/southwest during the day, but still light.

Outlook: Shower and thunderstorm chances and associated restrictions return Saturday and continue into early next week. Low stratus and/or fog will be possible each night starting Friday night, mainly in the mountain valleys. More widespread low clouds and fog expected Saturday night through Sunday morning.

GSP WATCHES/WARNINGS/ADVISORIES

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