textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion.

KEY MESSAGES

1. Showers and thunderstorms are forecast through the weekend, mainly in the afternoons and evenings, with isolated severe storms possible. Storms Saturday and Sunday may pose an additional risk of heavy rainfall. 2. Hot and humid conditions continue through the weekend, followed by drier and slightly below-normal temperatures Monday and early next week.

DISCUSSION

Key message 1: Showers and thunderstorms are forecast through the weekend, mainly in the afternoons and evenings, with isolated severe storms possible. Storms Saturday and Sunday may pose an additional risk of heavy rainfall.

A narrow upper ridge extends up the Eastern Seaboard as of midday Friday. An upper low south of Hudson Bay will advance slowly eastward through the weekend; shortwaves rotating through the surrounding trough across the Great Lakes region will gradually suppress that ridge and promote gradual height falls through the remainder of the weekend and into the beginning of next week. A weak cold front trails the first of those shortwaves, and is expected to cross the southern Appalachians and stall across the GSP CWA early Saturday morning. That front should reactivate later Saturday or Saturday night, in advance of a stronger shortwave and cold front moving thru the Ohio Valley Sunday.

As of midday Friday a boundary, the leading edge of a remnant cold pool from overnight MCS, extends from near Knoxville to Huntsville and advancing slowly east and southeast. 12z CAMs did not initialize this well, but there appears potential for the boundary to initiate storms as it crosses the Appalachians into our western CWA. LCLs look to be on the high side this afternoon given the near-record temperatures, despite the muggy dewpoints under the ridge which likely will keep the activity somewhat isolated, except perhaps along the cold pool boundary. 2000-2500 J/kg SBCAPE appears a safe bet and sfc-6km shear will be no more than 20 kt, so pulse mode would be favored. Sfc-midlevel delta-theta-e is 30-35 K on the NAM due to the profiles being dry aloft, which is higher than we see on most days this time of year; that probably weighed into SPC's decision to include most of our CWA in a risk area on the Day 1 outlook for storms to produce damaging microbursts. The sparse CAM response could be an indicator that the dry air is too much of a limiting factor for deep convection without strong forcing, but as noted above, poor performance of CAMs with this morning's upstream convection and on recent afternoons leads us to place less weight to their depictions than on a typical afternoon. Assuming the cold pool mixes out with time, there remains the potential for the shortwave itself to initiate storms this evening with LCLs lowering and potential for outflow boundaries to focus new development.

Moisture pooling near the stalled front Saturday could have the effect of enhancing development near the boundary but reducing delta-theta-e and DCAPE, with severe potential lower as a result. However that could lead the heavy rain threat to tick upward a bit, due to the extra moisture and due to steering flow being slightly weaker and also parallel to the front. The HRRR trends more active diurnally on Saturday, though the other CAMs depict even less coverage; confidence remains low as do PoPs, remaining no better than ~30%. Some models depict nondiurnal convective coverage Saturday night as the front reactivates. Sunday, PWATs and 0-6km shear increase with trough developing ahead of the OH Valley front and shortwave. Convergence within the trough is expected to produce better convective coverage than the previous two days, with both heavy/excessive rain and clusters of storms posing a damaging wind/hail threat appearing possible.

The cold front should settle southward across the CWA Monday, though PWATs remain at or above climo in the southeast half of the area through early Tuesday per LREF, the Euro and Canadian ensembles continuing to be quicker to dry us out than the GEFS. 20-50% PoPs linger through early Tuesday before decreasing that afternoon into Wednesday. Chances increase again in advance of the next front Thursday.

Key message 2: Hot and humid conditions continue through the weekend, followed by drier and slightly below-normal temperatures Monday and early next week.

Though the upper ridge will slowly be suppressed through the weekend, no significant airmass change is expected through Sunday. Temps will top out in the lower to mid 90s this (Friday) afternoon east of the mountains though it looks questionable whether records will be reached. Soupy dewpoints in the upper 60s to lower 70s will make for very high heat indices. A Heat Advisory remains in effect for Mecklenburg-Union-Cabarrus counties in NC where heat indices at calibrated stations are expected to push past 105. The weak front reaching the area tonight should bring slightly lower dewpoints to the northern zones Saturday, and temperatures back off just enough across the area to preclude any heat headlines at this time. Dewpoints and temperatures may creep upward again Sunday but confidence is low owing to the position of the front and also the potential for convective outflows to keep temps in check that day.

A more robust cold front arriving by early Monday is likely to induce a more substantial drop in dewpoints over much of the area Monday afternoon, and also bring temps back to around if not below normal Monday. That airmass should stick around for Tuesday, modifying slightly Wednesday and further on Thursday as SW flow redevelops ahead of the next system.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Isolated showers and TSRA should be trending downward through midnight, with confidence only for VCSH at this time. KAVL could see a brief period of MVFR vsby before sunrise from BR. This is mainly due to rain during the day. Winds become more N/NW overnight and very light, with period of calm possible. Winds toggle SW by Saturday afternoon. Lower chances for any SHRA or TSRA outside the mountains, but keep a PROB30 at KAVL during the afternoon. Skies remain VFR and SCT through the entire TAF period.

Outlook: Diurnal convection and associated restrictions are expected into early next week. Valley fog and low stratus is possible each morning, primarily in the mountain valleys.

GSP WATCHES/WARNINGS/ADVISORIES

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