textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The severe storm threat continues today but chance is lower. Forecast heavy rainfall the threat for isolated flash flooding continues, but the best locations and timing remain uncertain.

KEY MESSAGES

1. Very active convective weather is expected through the end of the week, as a frontal boundary moves into the area and stalls. Isolated severe storms are possible this afternoon and evening, with an escalating risk of heavy rainfall and potentially flash flooding from tonight through at least Friday. 2. A return to typical late July weather is expected next week.

DISCUSSION

Key message 1: Very active convective weather is expected through the end of the week, as a frontal boundary moves into the area and stalls. Isolated severe storms are possible this afternoon and evening, with an escalating risk of heavy rainfall and potentially flash flooding from tonight through at least Friday.

A slow moving frontal zone will settle over the area today and stalling near the area on Thursday as a couple of short waves rotate through an upper trough developing over the East Coast. This will lead to increased forcing and an increase in bulk shear. Convection will be slower to start than usual, mainly late afternoon into the evening and storms will be slow to move south across the area. The air mass remains moist and unstable, but mid-level lapse rates will be meager. Still, the low-level lapse rates may increase enough with daytime heating for an isolated severe storm with damaging winds.

The bigger concern will end up being the potential for heavy rainfall as periods of thunderstorms cross the area along and near the stalled frontal boundary through Friday. PW values are forecast to surge to 2" (or more) over the area within deep SW flow, and forecast soundings depict very deep warm cloud depths of over 12 kft, along with notable instability lingering well past the typical diurnal cycle. With mean cloud-bearing winds of 20 kts or more expected, cell movement will be relatively fast. However, the mean flow will be oriented almost perfectly parallel to the boundary, creating the threat for training, highly efficient rainfall- producing cells. That said, uncertainty remains regarding the best location for training cells as the guidance continues to favor different locations. The resulting QPF also varies in amounts and locations. Given this uncertainty, will hold off on issuing a Flood Watch at this time. Prefer to wait for some consolidation in the guidance to target the main threat areas instead of issuing a forecast area-wide watch that remains in effect for days.

The boundary is forecast to remain over, or near our area through at least Friday, but high pressure building into the northeast Conus will try to nudge the boundary south of the CWA. Thus, more stable air will gradually filter into the area with time. The potential for heavy rainfall and isolated flash flooding will nevertheless continue into Thursday, as the potential for training, highly efficient rain showers and storms will continue. This threat should gradually relent toward the weekend as the boundary settles farther south. After a day of near-normal heat today, max temperatures are forecast to cool to well-below normal levels Thu through Sat under the influence of cooling easterly flow, anomalously low heights, and widespread cloud cover.

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

The global models largely agree that deep-layer drying will occur across the region by Sunday, thus bringing an end to the period of relatively cool and very active convective weather that is anticipated late this week. Forecast temps return to near- normal Sunday, then above normal early next week. Convection is still in the forecast each day Sunday-Tuesday, but PoPs are mainly in the 20-30% range and timing of said PoPs is largely tied to the diurnal heating cycle.

AVIATION /07Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Generally VFR early this morning but some brief MVFR cigs possible at KAVL. S to SW wind early this morning with an occasional low end gust. Latest guidance shows a few SHRA possible over the mountains or I-40 corridor early this morning. However, they should stay away from KAVL and KHKY.

The guidance shows better chances of convection sliding south to the TAF sites during the afternoon. Have timed PROB30s based on the timing of the convection moving southward. S to SW wind picks up speed through the morning becoming gusty. The potential for convection lingers overnight as a cold front drops into the area. Have gone with -SHRA and PROB30s for the continuing convective chances. Gusts end and wind speeds diminish during evening outside of TSRA.

Outlook: Active forecast to continue with potential for convection through the weekend, as a cold front stalls in or near the terminal forecast area. Low stratus and/or fog will be possible each morning, especially in the mountain valleys and locations which receive especially heavy rainfall.

GSP WATCHES/WARNINGS/ADVISORIES

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