textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated details about convective potential this afternoon and tonight. Temperatures have trended lower for Monday and Tuesday and precip chances have increased both of those days.

KEY MESSAGES

1. Very active convective weather is expected to continue through early next week. Isolated severe storms and localized flash flooding will be possible each day through Monday. Otherwise, a cooling trend is expected, with well-below normal temperatures forecast by Monday. 2. A return to seasonably hot conditions and typical coverage of diurnal showers and storms is expected during the latter half of next week.

DISCUSSION

Key message 1: Very active convective weather is expected to continue through early next week. Isolated severe storms and localized flash flooding will be possible each day through Monday. Otherwise, a cooling trend is expected, with well-below normal temperatures forecast by Monday.

We remain under a westerly flow regime aloft, between a digging trough over the NE quadrant of the CONUS and the broad subtropical ridge. A slow-moving cold front is present south of the trough. Deep upper anticyclone over the Desert Southwest will build north over the Rockies and Plains through early Monday. Downstream of that ridge and along/south of the sfc front, seasonably high CAPE in the mid-MS Valley has allowed MCSs to develop each of the last couple of evenings, and with the pattern allowing 0-3km shear of 15-25 kt in an area of weak warm advection, those MCSs have been able to survive to the NC mountains. Outflow from this morning's round of upstream convection helped drive strong to severe storm clusters over GA/SC earlier today, but that activity has shifted south. Convective debris clouds have now cleared over parts of western NC and it remains possible some redevelopment will occur there. Certainly cannot rule out redevelopment in GA/SC, though the newer cold pool and track of extant activity to our west suggests that portion of the area is more likely to remain settled this afternoon and early evening. Wherever storms do fire along outflows, given the shear, the cells are likely to organize into clusters and pose a risk of damaging winds. SPC upgraded much of the area to a Slight Risk on this morning's Day 1 update, but the associated threat likely has peaked for most of the Slight Risk area, perhaps excluding our far south.

Tonight, a well developed 500mb circulation embedded in a deep shortwave now over MO/IL will drift southeast into the lower OH and TN valleys, while the front oozes southward also. The general pattern supporting nocturnal MCSs thus continues, but they look to initiate a little further south than the previous couple of days, still making a run to the southeast but possibly even missing our CWA. The cold front will however arrive from the north during the day Sunday, inducing cooler temperatures especially in the northern CWA, but encountering a still very humid airmass. The shortwave also appears to provide some DPVA during the afternoon such that numerous to widespread showers/storms are expected to develop again...assuming outflows don't spoil the show. 0-3km and 0-6km shear is also improved as a result further increasing the potential for organized storms. The CWA is in SPC Marginal Risk for Day 2, but a slight risk is now included for the southeast zones where the best overlap of forcing/shear are seen.

Models seem to be trending more in favor of the cold front slowing down Sunday night into Monday due to the influence of the shortwave, which by that time likely will have a closed circulation as low as 850mb and a weak sfc reflection as well. A period of enhanced easterly 925-850mb winds continues to be expected along the front in that time period, so some training of elevated convection remains possible along it, but southerly low-level flow is an additional concern for anchored cells near the Blue Ridge Escarpment. Clouds and precip will keep temps below normal Monday, though the moisture plume along the front appears likely to still be over the area and so the threat of at least heavy rainfall will continue. WPC Day 2 Slight for Sunday and Sunday night, and Marginal for Day 3 (Mon-Mon night), but if soils become saturated and models remain consistent in this slower solution, could see Monday's outlook being upgraded at some point. The slower trend is already reflected in PoPs now having been raised to likely over most of the SW half of the CWA on Tuesday, with the moist easterly flow regime likely still in place there, with temps also still cooler and peaking 7-10 below normal.

Key message 2: A return to seasonably hot conditions and typical coverage of diurnal showers and storms is expected during the latter half of next week.

By late Tuesday a Rex block appears to set up over the CONUS as the anomalously deep upper ridge expands across the northern tier and the remnant upper low retrogrades beneath it. So while it is not exactly looking like a typical July pattern at that time, drying does appear to occur such that temperatures trend back near normal by Wednesday with seasonably weak flow aloft. Another trough may dig east of the ridge later in the week, setting up a trend toward NW flow again. Confidence is lower than normal but it would appear our sensible weather returns to something closer to usual midsummer, as temps warm back to near or above normal and PoPs are highest over the mountains.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Convective outflows have pushed past all the terminals already today and it appears slightly more likely than not that will preclude development of any more SHRA/TSRA through sunset. However, there is time for at least modest destabilization to occur as convective remnant clouds clear this afternoon, and there remain weak disturbances aloft that could help some clustering of storms. KAVL only has VCSH, KCLT/KHKY have PROB30, and the SC sites have a TEMPO, with KGSP/KGMU potentially still having VCTS as storms drift further away from the area. The outflows turned winds NW'ly at KCLT and the SC sites, but as the outflow mixes out winds should again favor WSW with time this afternoon. If storms redevelop there, they most likely would approach from the SW so the afternoon PROB30 includes G30KT from that direction.

Winds will be lighter tonight compared to Fri night, and crossover temperatures are expected to be higher, so there is somewhat higher potential for daybreak fog/stratus. KCLT and KAVL both retain a mention for FEW clouds at IFR level, but KAVL now has MVFR vsby. Expect this to be reevaluated on the 00 and 06z TAF sets. Similar to Thu and Fri nights, conditions tonight will be favorable for storm complexes to develop upstream and approach the Carolinas. Recent model trends suggest this activity is not likely to reach the TAF sites with direct TS impacts, though the potential remains for cold pools or cloud cover to limit TS development during the day Sunday. A better defined cold front will however arrive from the north late in the day and that, and/or differential heating along outflows, could still lead to clusters of storms developing. Confidence remains low, but all in all felt a PROB30 at KCLT was warranted for Sun aftn.

Outlook: Greater than normal coverage of SHRA/TSRA will continue across our area Sunday into Monday as multiple rounds of storms are expected across the region. Drier conditions should develop Tuesday into Wednesday. Low stratus and/or fog will be possible each morning in the mtn valleys and in locations that see appreciable rainfall the previous afternoon/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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