textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Forecast rainfall totals and the threat for isolated flash flooding are increasing for the period from late Wed through Thu.

KEY MESSAGES

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

DISCUSSION

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

In the absence of any good upper forcing, convection has remained isolated and unimpressive this evening. The remnants of organized convection may move into the forecast area from the NW at some point during the overnight/early Wed morning, but the severe storm potential appears minimal in the absence of organizing shear values, weak instability, and with this convection becoming increasingly displaced from large scale forcing. The leading edge of a frontal zone is forecast to move into the forecast area during Wed afternoon, and scattered-to-numerous convective coverage is forecast within a moderately unstable air mass along/ahead of the front Wed, with 50-70 PoPs warranted across the NW half of the area, with mainly 20-40% across the southeast half. Accelerating mid-level flow associated with multiple rounds of height falls will result in increased deep layer shear of 25-30 kts, so another threat for isolated severe storms will exist Wed afternoon/evening.

The increasingly slower-moving frontal zone is forecast to settle over the forecast area overnight Wed/early Thu, possibly even stalling somewhere over the area. Meanwhile, precipitable water 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 swift (for mid-summer, anyway.) However, the mean flow will be oriented almost perfectly parallel to the boundary...creating the threat for training, highly efficient rainfall-producing cells. This threat is confirmed by some extreme localized rainfall totals in convection-allowing models. It is very possible that a Flood Watch for flash flooding will eventually be issued for Wed night/early Thu, but there is too much uncertainty about the eventual position of the boundary to entertain any headlines at the current time.

The boundary is forecast to remain over, or near our area through at least Thursday, but a slowly-developing inverted ridge associated with 1020+ surface 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 week's end as the boundary settles farther south. After a day of near-normal heat on Wed, 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 slightly above climo early in the new work 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 /00Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Still expecting VFR for the bulk of the 00Z TAF period. As expected, isolated convection developed over the northern Blue Ridge this afternoon, as well as some unanticipated showers over the I-77 corridor, and most recently, the Savannah River Valley...but no impacts to the TAF sites are expected, with all activity so far remaining widely scattered at best and struggling to maintain its integrity. A dry overnight period is expected, with some mid- and high-level cloud cover. It's not out of the question that high-end MVFR ceilings may briefly develop at KAVL after midnight, but guidance favors low-end VFR ceilings.

Hi-res models have slowed considerably with respect to a line of showers and thunderstorms arriving in the area Wednesday. While a few forecast ensembles still depict showers developing over the I-40 corridor as early as 15Z, the model consensus now keeps upstream convection at bay until at least 18Z. At least somewhat organized convection should then cross the terminal forecast area from northwest to southeast through late Wednesday evening. Showers and thunderstorms may persist past the end of the 00Z TAF window...even at CLT, where precipitation could remain after 06Z Thursday. Some flight restrictions are of course likely to develop wherever convection does; both TSRA and associated restrictions are now handled with VCSH and PROB30s where appropriate.

Outlook: Active forecast to continue with potential for convection through the end of the week, as a cold front stalls in or near the terminal forecast area. Convection may persist well into the overnight again on Thursday night. 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.