textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated aviation discussion for the 12Z TAFs.

KEY MESSAGES

1. Another hot day today, before a cooling trend sets in for the weekend, with below normal temperatures forecast early next week. If you have outdoor activity planned in the afternoons through Saturday, make sure to stay hydrated and take frequent breaks to cool off and avoid heat-related stress. 2. An active period of convective weather is anticipated from this afternoon through early next week. While mostly diurnal activity is expected, convection may impact the area outside of the traditional afternoon/evening time frame, especially Sunday night. Isolated strong-to-severe thunderstorms and isolated flash flooding are possible each day through Monday.

DISCUSSION

Key message 1: Another hot day today, before a cooling trend sets in for the weekend, with below normal temperatures forecast early next week. If you have outdoor activity planned in the afternoons through Saturday, make sure to stay hydrated and take frequent breaks to cool off and avoid heat-related stress.

Persistent westerly deep layer flow will favor continued hot conditions across our region today, with max temps expected to again be around 5 degrees above normal. The westerly flow will also support relatively deep mixing, with gusty Piedmont winds expected along with dewpoints mixing out to the upper 60s across much of the area. Some locations across the far southern part of the CWA could see Heat Index above 100 for a couple of hours this afternoon, but Heat Advisory conditions are not anticipated.

The pattern is expected to become more active in terms of convection over the weekend into early next week, with some potential for upstream activity to move into the CWA at just about any time of day. Increased cloud cover is expected to initiate a slight cooling trend over the weekend, such that highs Sunday are forecast to be slightly below normal...the first such day in about two weeks. By Monday, broad 1020+ mb high pressure building into the northeast quadrant of the country is expected to push a backdoor cold front into our forecast area, with extensive cloud cover/easterly flow and plenty of showers and storms bringing much cooler conditions...highs Monday around 10 degrees below climo.

Relatively cool conditions are forecast to persist into Tue before normal temps return during mid-week. Hotter-than-normal weather may return late next week, as upper ridging associated with a large anticyclone across the northern Great Plains spills into the Southeast.

Key message 2: An active period of convective weather is anticipated from this afternoon through early next week. While mostly diurnal activity is expected, convection may impact the area outside of the traditional afternoon/evening time frame, especially Sunday night. Isolated strong-to-severe thunderstorms and isolated flash flooding are possible each day through Monday.

A weakly-organized area of convection is ongoing early this morning from eastern Kentucky into middle Tennessee, with some recent dissipation of activity noted across the southern part of this area. Short wave trough/MCV associated with this activity and usual terrain effects are expected to be the primary impetus for enhancing convective coverage across our area later today, with mostly 50-70 PoPs across western NC...and more like 20-40% across the Upstate and northeast GA...as steering currents will remain such that the bulk of any convection originating from the high terrain should generally be carried due east. Surface-based CAPE of 2000-2500 J/kg and low level shear that should be adequate for some degree of cell clustering along outflows will support the potential for a few severe weather events, mainly in the form of downburst winds.

The upper flow pattern will amplify over the weekend, with height rising strongly across the western Conus. This will result in developing NW flow aloft across our region, with embedded short wave troughs and/or MCVs associated with upstream convective complexes periodically passing over our CWA through early next week. The result could be areas of convection wandering into the area at just about any time...including outside the usual afternoon/evening diurnal window. The NC mountains will be especially susceptible to multiple rounds of convection, and a somewhat organized heavy-to-excessive rainfall threat could develop there as early as Saturday. Having said that, locally heavy rainfall/isolated flash flooding will be possible just about anywhere through the weekend. Shear parameters will generally remain too weak for noteworthy convective organization, but the usual microburst threat from pulse/multi-cell clusters can be expected each afternoon/evening through Sunday.

The strongest signal for an excessive rainfall threat appears to be Sunday night into early Monday, when developing ESE upslope flow associated with a slow-moving backdoor cold front interacting with deep moisture and deep warm cloud depths of well over 10 kft may yield slow-moving, efficient rainfall-producing cells. The time of day/lack of significant instability is an obvious limiting factor, but this is a decent excessive rainfall set up for our forecast area. Stable conditions north of the boundary will result in diminishing coverage and intensity of showers later Monday into Tuesday. The air mass quickly modifies under July sun during mid-week, bringing a return of typical heat and humidity, although diurnal convection is expected to remain largely confined to the mountains during the latter half of the week.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Prevailing VFR conditions are forecast to continue at the TAF sites through this period. Fog/low stratus is again confined to the mountains valleys W through N of KAVL this morning, and is not expected to expand to KAVL. Otherwise, diurnal convective chances are higher across the Terminal Forecast Area today in comparison with the past couple of days...and Prob30s for TSRA are warranted at all sites from early/mid afternoon into early evening. At least some of these will need to be evaluated for potential upgrades to tempos once 12Z guidance sources come available. As we move into the weekend, the pattern is expected to become more conducive to convection moving into the area beyond the traditional afternoon/early evening time frame, and this can't be ruled out late this evening or overnight/early Saturday. For now, the greater chances will be highlighted with this afternoon/evening's Prob30s, but a VCSH has been introduced at most sites for the overnight period. Winds will be SW through the period, generally 4-8 kts, expect most sites (KAVL and KHKY being the exceptions) should experience a stretch of gusty conditions during the afternoon.

Outlook: Better coverage of SHRA/TSRA continues for all terminals Saturday into Monday as multiple rounds of storms move towards and across the area. Drier conditions should develop Tuesday into Wednesday. Low stratus and/or fog will be possible each morning in the mountain valleys and in locations that see appreciable rainfall the previous afternoon/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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