textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No substantive changes.

KEY MESSAGES

1. A very hot Independence Day weekend will steadily give way to more typical early summer heat during the new week. Diurnal convection is expected to remain largely confined to the mountains today, but a transition to a more active diurnal convective cycle is expected beginning Sunday. Those with outdoor plans for the holiday weekend should prepare for elevated heat risk. Stay hydrated, take frequent breaks in air-conditioned or shaded areas, and never leave children or pets unattended in vehicles.

DISCUSSION

Key message 1: A very hot Independence Day weekend will steadily give way to more typical early summer heat during the new week. Diurnal convection is expected to remain largely confined to the mountains today, but a transition to a more active diurnal convective cycle is expected beginning Sunday. Those with outdoor plans for the holiday weekend should prepare for elevated heat risk. Stay hydrated, take frequent breaks in air-conditioned or shaded areas, and never leave children or pets unattended in vehicles.

The latest surface analysis indicates surface dewpoints have mixed out to the upper 60s across much of the forecast area, with some lower 70s lingering across the southwest zones, and a few mid-60s noted across the mtn valleys and the northeast corner of the CWA. Heat Indices are generally struggling to reach Heat Advisory criteria of 105, but we still expect at least local instances of 105+ while there are multiple large outdoor festivities underway...and the duration of this heat event is increasingly becoming a factor in potential heat stress, as we are sitting on about Day 5 of very hot conditions. Thus, the Heat Advisory will remain in effect until 8 PM.

The air mass has become quite unstable (2500-3500 J/kg of sbCAPE) this afternoon, and isolated deep convection is ongoing...mainly across the mountains. The flow aloft has basically vanished, as the anticyclone has broken down/evolved into more of a nebulous region of ridging across the Southeast. As such, cells are moving very slowly...and locally heavy rainfall and an isolated flash flood threat may prove to be the biggest concern going forward. Having said that, the robust instability...along with potential cell mergers should result in a few strong-to-severe updraft pulses resulting in microbursts.

A transition to more of a cyclonic upper flow pattern is expected across the southern/central Appalachians and vicinity on Sunday, with deep, albeit weak SW flow resulting in an increase in deep layer moisture. Temperatures will remain above normal...and a few spots across the lowest elevations could again reach the century mark...but temps overall should be a little cooler than today. With the increasing moisture, surface dewpoints will have less of a tendency to mix out during the afternoon. Instability should again be quite robust, although moist adiabatic mid-level lapse rates should yield sbCAPE more typical of early summer. Overall, conditions will be much more favorable for deep convection Sunday afternoon/evening, with likely PoPs warranted across the mountains, and generally 30-50% chances elsewhere. Typical isolated strong-to-severe pulse storms along with areas of locally heavy/possibly excessive rainfall can be expected. We still can't rule out portions of the eastern and southern fringe of the CWA at least flirting with Heat Advisory criteria before the convection develops in earnest, but confidence in this is modest at best, and after collaboration with neighboring NWS offices, it was decided to withhold heat products for Sunday.

The westerlies are forecast to remain quite active to our north through the new week, with multiple rounds of height falls forecast. This will steadily lower heights across our region as the week progresses. A weak frontal system may cross the region Tuesday in association with one of these height fall surges...possibly enhancing diurnal convective chances and a slight uptick in the severe storm potential. Either way, thermal profiles and the synoptic pattern should support solid scattered-to-numerous coverage of diurnal showers and storms Mon and Tue. Somewhat drier air may advect into the region by Wed in the wake of aforementioned front, but this is unlikely to be enough to preclude diurnal convection during the latter half of the week. Nevertheless, PoPs for Wed-Fri do decrease a bit from their Mon/Tue peak. Temperatures otherwise steadily decrease through the first half of the week...to around 5 degrees above climo for Monday...to just slightly above normal for Wed/Thu.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: VFR conditions are forecast to prevail at the TAF sites through this forecast period. Isolated-to-widely scattered convection will continue across mainly the mountains and foothills through the afternoon. In terms of the TAF sites, the most likely location to see convection is KHKY, and a Prob30 for TSRA is included there between 20-23Z. Certainly can't rule out a cell passing near the other sites... particularly KAVL, but chances are deemed too small to include a mention at this time. Low stratus and/or fog will redevelop in the mountain valleys overnight into early Sunday, but restrictions are again very likely to remain confined to the valleys W through N of KAVL. Moisture ramps up during the daylight hours Sunday, and this will lead to much improved chances for diurnal convection...with Prob30s entering the picture at KAVL by 16Z, and at KCLT by 20Z. Convective chances are otherwise expected to hold off until 18Z at the other sites. Winds will be mostly light SSW through the period.

Outlook: Mainly afternoon/evening shower and thunderstorm chances will continue through the week. Low stratus and/or fog will be possible each morning, especially in the mountain valleys, but also in locations that see appreciable rainfall the previous afternoon/evening.

CLIMATE

RECORDS FOR 07-04

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 97 1948 68 1976 70 2016 51 1986 KCLT 99 1993 70 1968 76 1993 55 1933 1955 KGSP 100 1993 70 1988 75 2018 58 2021 1996 1933

RECORDS FOR 07-05

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 96 1948 71 1976 70 2024 46 1967 2018 1933 KCLT 101 2024 66 1892 78 2024 57 1967 KGSP 101 2024 70 1976 78 2016 58 1967 1933 1892

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory until 8 PM EDT this evening for GAZ018-026-028- 029. NC...Heat Advisory until 8 PM EDT this evening for NCZ035>037-056- 057-068>072-082-502-504-506-508-510. SC...Heat Advisory until 8 PM EDT this evening for SCZ008>014-019- 104>109.