textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Little in the way of forecast changes for this cycle, although confidence in PoPs and afternoon Heat Index forecast through mid- week is modest at best, mainly owing to the potential for multiple remnant convective complexes to pass near the forecast area.

KEY MESSAGES

1. Mainly isolated to scattered diurnal convection will continue each day through the week, with some potential for isolated pulse severe storms that produce damaging wind gusts and localized excessive rainfall. Somewhat organized convection may impact the area at times through at least mid-week, possibly enhancing the damaging wind gust threat. 2. Hot and humid for the next seven days at least. Some Piedmont/Upstate locations are forecast to daily afternoon heat index between 100-105 each afternoon...especially from tomorrow through the end of the week.

DISCUSSION

Key message 1: Mainly isolated to scattered diurnal convection will continue each day through the week, with some potential for isolated pulse severe storms that produce damaging wind gusts and localized excessive rainfall. Somewhat organized convection may impact the area at times through at least mid-week, possibly enhancing the damaging wind gust threat.

Convection has flared up to our south along an outflow boundary that sagged through the forecast area this morning. North of the boundary, conditions are still quite moist and unstable across our area, but westerly low level flow, some degree of drying aloft and subtle overturning north of the boundary is resulting in a very flat and sparse cumulus field. We still expect isolated-to-widely scattered activity to eventually develop, but this afternoon certainly looks to be less active compared to previous days. Having said that, still can't rule out some localized areas of training cells leading to excessive rainfall and perhaps a few storms producing strong-to-severe microbursts.

With a fairly strong upper ridge forecast to gradually move east across the central Conus, and the axis of a trough expected to remain just off the East Coast, NW flow aloft will become well- established across our region by tomorrow. Meanwhile, frequent short wave troughs are forecast to move along a baroclinic zone from the Midwest through the Mid-Atlantic through at least Wed, interacting with a very unstable upstream air mass along the northern periphery of the ridge to produce a veritable wave train of convective complexes. The remnants of a couple of these could at least brush our CWA through mid-week. Damaging wind gust probabilities could be enhanced with any of these remnants if they happen to align with diurnal instability. However, this is all very uncertain at this point...confidence will be quite low beyond the 6-12 hour period as long as this pattern persists.

By Friday...the global models suggest the anticyclone...which is forecast to be centered somewhere over the TN Valley or Deep South will begin to intensify again...possibly pushing the westerlies/NW flow aloft and therefore the MCS track into the Ohio Valley and Mid-Atlantic. Nevertheless, conditions are expected to remain sufficiently hot and muggy to support diurnal convection into early next week.

Key message 2: Hot and humid for the next seven days at least. Some Piedmont/Upstate locations are forecast to daily afternoon heat index between 100-105 each afternoon...especially from tomorrow through the end of the week.

Heights will gradually increase through the next couple of days, while persistent, mostly weak westerly low level flow is forecast. These factors are expected to produce hot conditions, with temperatures forecast to be around 5 degrees above climo through at least the end of this week. While conditions will certainly be muggy, drier profiles aloft/lower precipitable water values and the westerly downslope flow should promote better afternoon mixing...with surface dewpoints likely dropping into the 60s most days. Additionally, there is the complicating factor of the potential for cloud cover and/or outflow air associated with remnant convective complexes shaving a couple of degrees off heating potential. That's all to say that confidence that Heat Advisory criteria will be reached anywhere in the CWA over the next couple of days is low. Nevertheless, there should be plenty of 100+ H.I. through mid-week...especially across the lower elevations of the Piedmont. By the weekend, a consensus of the global models indicates some strengthening of an anticyclone over the Deep South/TN Valley. While confidence in surface temps/dewpoints is inherently low at that time frame, the forecast pattern will certainly be more favorable for persistent excessive heat.

AVIATION /18Z MONDAY THROUGH SATURDAY/

At KCLT and elsewhere: Coverage of convection is expected to be much less this afternoon in comparison with recent days...supported by the very unimpressive state of the cumulus field as of 17Z. The convective mention is limited to a Prob30 for TSRA at KAVL from 19-22Z. With much less convective coverage today, slightly drier profiles, and general westerly low level flow, the potential for fog/low stratus is expected to be largely confined to the mountain valleys (and perhaps KHKY) late tonight/early Tue. MVFR visby w/ SCT LIFR clouds are advertised at KAVL by daybreak (at least brief periods of IFR/LIFR restrictions are certainly possible), and tempo MVFR conditions are forecast at KHKY. Otherwise, VFR is forecast through the period. General WSW surface winds at 5-10 kts are expected this afternoon...becoming light/vrbl this evening, then perhaps WNW at around 5 kts Tue morning.

Outlook: Isolated-to-scattered diurnal showers and thunderstorms with associated flight restrictions are expected through the week. Additionally, with flow aloft turning NW, the remnants of organized complexes of convection originating from the Midwest could push into the area at just about any time from Tue afternoon through at least Thu. Patchy fog and low stratus is possible each morning, especially in the mtn valleys and anywhere that received significant rainfall the previous afternoon/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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