textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Confidence is increasing again that cooler air will infiltrate the area early this week, and max temps have trended down (10+ degrees below normal) for Mon/Tue.

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 each day through Monday. Otherwise, a cooling trend is expected, with well-below normal high temperatures forecast Monday and Tuesday. 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 at least Monday. Isolated severe storms and localized flash flooding will be possible each day through Monday. Otherwise, a cooling trend is expected, with well-below normal high temperatures forecast Monday and Tuesday.

Isolated to scattered convection continues across the region this morning, with the best coverage noted across the NC mountains. However, even this activity has been weakening substantially as it crosses the crest of the Appalachians from east TN such that morning PoPs will mainly be 10-30% for locations south and east from the counties bordering TN. Cannot rule out an isolated flash flood event this morning across the lower reaches of the French Broad basin, as areas of Madison County and vicinity have seen solid 2-4 inch rainfall totals in the past 24 hours.

A backdoor front will gradually sag southwest across the forecast area throughout the day...although the main push of lower theta-e air into our area is forecast to hold off until tonight. With the bulk of the forecast area expected to remain free of convection and thick cloud cover this morning, much of the area (especially east of the mountains) should have no issue destabilizing to the tune of 2000 J/kg (give or take) of sbCAPE. Meanwhile, an area of height falls will drift south across the Ohio and TN Valleys this morning...with a weak upper low expected to take shape across the TN Valley by late today. Accelerating mid-level flow is forecast to improve shear parameters (around 25 kts through a deep layer) across our area today. In other words...shear will be improved over the past couple of days...when convection has been able to make the absolute most of relatively modest buoyancy to produce areas of severe weather. The SPC slight risk for severe storms looks prudent and well placed across the Piedmont this afternoon, especially in light of the fact that the synoptic and mesoscale pattern should favor good coverage (PoPs of 70-80% in most locations) of diurnal showers and storms.

Of equal concern will be the increased potential for locally excessive rainfall, especially later in the day through tonight as the flow turns easterly upslope...which will interact with very moist conditions (PWATs around 2") and warm cloud depths over 10 kft to up the heavy rainfall ante. Cell training could also become a concern along the surface boundary. Combined with locally poor antecedent conditions resulting from heavy rainfall over the past three days warrants a slight risk for excessive rainfall across much of the CWA through tonight.

Precip chances remain high Monday into Monday night, as moisture content remains high and low level upslope flow persists. Having said that, widespread thicker cloud cover and advection of lower theta-e air into the CWA is expected to result in weak-at-most instability Monday afternoon, so the heavy rainfall threats begins to get a little murkier. As such, the bigger story Monday may be the much cooler weather expected as a consequence of the easterly flow/cloudy/showery regime. Forecast maxes are more than 10 degrees below climo.

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 conditions and typical coverage of diurnal showers and storms is expected during the latter half of next week.

By mid-week, a Rex block if forecast to become established across the central Conus, with a broadening anticyclone/attendant ridge forecast to be centered across the central/northern Great Plains. As weakening upper low retrogrades across the Deep South, heights will rise across our region, bringing a return of seasonably hot conditions. Low level moisture and instability are forecast to be adequate to support typical coverage of diurnal convection from Wed onward...with mainly widely scattered activity expected across the mountains, and isolated coverage elsewhere.

AVIATION /07Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Convection is moving across the southern Appalachians early this morning, and could impact KAVL between now and 09Z. The bulk of the activity is expected to pass north of there, so will ride with a VCSH for the time being. Little-to-none of this convection is expected to survive the trek over the high terrain, so will omit any convective mention at the other TAF sites for this morning. Ample low level moisture creates some cause for concern of fog and/or low stratus developing over the area this morning. We're already seeing it in the usual mountain valleys west of KAVL, but localized areas of low stratus are also beginning to show themselves across the Piedmont. Confidence in cig restrictions at the TAF sites is low, but the potential is sufficient to include tempos for FEW/SCT LIFR/IFR cloud layers at all sites by daybreak. MVFR visby is also included at the climatologically favored sites at KAVL and KHKY.

The afternoon/evening is expected to be more active convection-wise than yesterday, such that tempos for TSRA are warranted at all sites... beginning as early as 17Z at KAVL, and as late as 20Z at KCLT. The convective potential will linger into the evening, and Prob30s replace tempos after the allowed 4-hour tempo window. In fact, conditions will be favorable for at least isolated convection to continue well into the overnight. Generally light WSW winds this morning are expected to turn to the NE this afternoon.

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.