textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Rain chances have trended down about 10-15 percent from Thursday through Saturday.

Updated the aviation discussion for the 18Z TAFs.

KEY MESSAGES

1. Very active convective weather is expected to continue through at least Monday. Isolated severe storms and localized flash flooding will be possible today through Monday. Otherwise, a cooling trend is expected, with well-below normal high temperatures forecast Monday and Tuesday. 2. A return to seasonably hot and drier conditions is expected during the latter half of next week.

DISCUSSION

Key message 1: Very active convective weather is expected to continue through at least Monday. Isolated severe storms and localized flash flooding will be possible today through Monday. Otherwise, a cooling trend is expected, with well-below normal high temperatures forecast Monday and Tuesday.

A 500 mb vort max can be seen drifting over the OH/TN valleys toward the Southern Appalachians on WV imagery. This feature will help support a couple more rounds of convection across the area. The first will be with daytime heating, as the moist air mass in place has destabilized to around 2000-2500 J/kg. With the approaching wave, bulk shear will increase slightly, approaching 25-30 kt, which is decent for mid-July. The CAMs show hints of multicell clusters and even some rotating updrafts, especially across the Upstate and NE GA Piedmont where mid clouds have cleared out the most. The Day 1 Convective Outlook has been adjusted with slight risk trimmed to the southern third or so of the CWFA. Damaging wind gusts will be the primary threat, but with a little more shear, large hail up to quarter-sized will be possible.

Tonight, a weak backdoor cold front will slide south thru the area, helping trigger additional convection thru this evening. Then, low-level flow turns out of the E/SE on the north side of the front, with upslope forcing enough to trigger another round of showers late overnight thru early Monday morning. The 12z CAMs are still not in great agreement on coverage and placement, but seem to favor the northern NC escarpment and adjacent foothills. The signal is still too subtle to warrant a Flash Flood Watch. Nevertheless, a slight risk of excessive rainfall continues tonight, with PWATs around 2" and warm cloud depths over 10 kft.

There should be a relative lull in shower coverage mid to late Monday morning, as an area of low clouds spreads over the forecast area and persists until around midday. A 1021-1023 mb sfc high over the Mid-Atlantic may be strong enough combined with low clouds and spotty precip to produce an in-situ CAD. Temps will be about 10-15 deg below normal across most of the area as a result. The clouds and cool temps will limit instability. But guidance shows pretty solid coverage of diurnal PoPs again Monday aftn-eve. A decent amount of elevated CAPE should support at least scattered tstms embedded within numerous to widespread showers. PWATs will remain around 2"+, so excessive rain will still be a concern, although muted slightly by less available instability.

Relatively cool/mostly stable and showery conditions continue into Tuesday...although guidance signals advection of drier air into the heart of the forecast area from the N and NE. Still can't rule out some degree of a heavy rainfall threat around the southern and western periphery of the CWA Tuesday, but much of the area may well be free of convection Tue afternoon. Forecast maxes remain around 10 degrees below normal Tue.

Key message 2: A return to seasonably hot and drier conditions is expected during the latter half of next week.

An upper ridge will persist across the central and northern Great Plains, while a weak upper low will retrograde across the Southeast toward TX. Upper ridging will gradually nose east/southeast into our region thru the end of the week, resulting in seasonably hot conditions to return to the CWFA Thursday thru next weekend. The 12z guidance has trended a little drier for Thursday and Friday, then some increase in diurnal convection over the weekend. Generally isolated to widely scattered convection in the mountains, with very isolated activity in the Piedmont until slightly higher PoPs Saturday and Sunday.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: Scattered convection has started to fire as of the 18z TAF issuance across the Upstate and coverage should continue to increase thru the rest of the aftn hours. Hence, TEMPO for TS still looks good. The NC sites have more mid and high clouds lingering from this morning, and guidance delays redevelopment of convection slightly; therefore, TEMPOs were pushed back an hour or two with the 18z TAFs. Otherwise, expect lingering showers and isolated thunderstorms thru the evening. A backdoor front will slip in from the NE and toggle winds out of the NE or ENE, but should remain light. Moist easterly flow will support expansion of MVFR to IFR cigs forming across the area overnight thru Monday morning. The low clouds may persist into early aftn, before beginning to lift to MVFR. Confidence is low to moderate on how much TS will be able to form due to the wedge-like air mass. But scattered to numerous SHRA are expected with enough potential for TSRA to warrant a PROB30 at KCLT late Monday aftn.

Outlook: Greater than normal coverage of SHRA/TSRA will continue across our fcst area 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.