textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion.

KEY MESSAGES

1. Isolated severe storms are possible through this evening, with an escalating risk of heavy rainfall and potential flash flooding tonight through at least Friday. A Flood Watch is in effect tonight through Thursday for much of western NC. 2. A return to typical late July weather is expected next week.

DISCUSSION

Key message 1: Isolated severe storms are possible through this evening, with an escalating risk of heavy rainfall and potential flash flooding tonight through at least Friday. A Flood Watch is in effect tonight through Thursday for much of western NC.

The axis of an upper trough will remain over the East through the end of the week, bringing an active period of convective weather. As of this afternoon, the leading edge of a slow-moving frontal zone extends from Middle Tennessee ENE to roughly along the WV/VA border. This boundary has been moving even slower than earlier model guidance had suggested, and this slow-moving trend is likely to continue until an area of more substantial height falls passes over the Mid-Atlantic on Thu. With mean cloud-bearing winds oriented almost perfectly with the boundary orientation, showers and scattered thunderstorms continue to train ahead of the boundary across east Tennessee and NW NC/southern VA, and this training activity will continue to build gradually southeast into our CWA through the afternoon. As a SW low level jet increases to 25-30 kts tonight, convergence along the boundary will increase and will interact with tropical PWATs of over 2," warm cloud depths of over 12 kft, and modest instability pooling along the boundary to result in training, highly efficient rainfall-producing convection that will gradually sag south through the overnight, with widespread 1-2" rainfall amounts and localized amounts as high as 5" expected across much of western NC by sunrise Thursday. A Flood Watch for flash flooding has been issued for almost all of our NC counties from this evening through 00Z Friday.

For locations farther south, training showers may not even reach the Upstate and northeast GA until after sunrise, and with lingering concerns about this timing and how convective trends will evolve on Thursday, these areas will be left out of the Watch for the time being. However, it is likely that some areal extension to the south will occur at some point tonight, as the main heavy rainfall threat sags south with the boundary.

With the aforementioned height falls passing north of the area tomorrow...followed by a 1020+ mb area of surface high pressure that will build into the Northeast by the end of the day...developing inverted ridge east of the Appalachians should push the boundary clear of the CWA Thu night. This will spell a diminishing flash flood threat late tomorrow into tomorrow night/Friday, but some marginal threat is expected to continue through the end of the week, as a series of weak upper waves interact with an environment that is generally stable, yet still moist (albeit not as tropical.)

Other than near-normal temps across the southern tier Thu afternoon, temperatures (especially max temps) are forecast to be at least 5-10 degrees below normal through Saturday under the influence of ENE low level, anomalously low heights, and plenty of afternoon storms and/or showers.

Key message 2: A return to typical late July weather is expected next week.

A consensus of global models indicates the axis of a long wave trough will shift to...or just off the Atlantic Coast by Sunday, and generally remain there through at least mid-week, with the flow aloft slowly becoming more anticyclonic as a ridge builds strongly across the Rockies/Great Plains. The result will be a return of seasonably hot conditions, with temps forecast to be within a couple of degrees either side of climo from Sunday onward. This time frame will be much less convectively active with showers/storms more strongly tied to the diurnal heating cycle as well, with PoPs mostly in the 10-30% range warranted each day.

AVIATION /00Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR conditions to start but restrictions come in overnight as SHRA and TSRA arrive at all terminals. PROB30 for TSRA at almost all sites overnight for TSRA and MVFR vsby/cigs. SHRA continues Thursday morning with a few TSRA possible at the NC terminals. Winds shift from W/SW to NE between 18z-22z Thursday. MVFR/IFR cigs and SHRA/-TSRA remain toward the end of the forecast at most sites.

Outlook: Restrictions continue through Friday as showers and thunderstorms remain. Low stratus and/or fog are possible each morning in the mountain valleys and locations which receive heavy rainfall.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Flood Watch through Thursday evening for NCZ033-035>037- 048>053-056>059-062>065-068>072-501>510. SC...None.