textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Timing of the cold front mid-week has sped up a bit and is now expected Wednesday night into early Thursday morning.

Trending slightly cooler for highs on both Thursday and Friday as cold air damming develops behind the front.

KEY MESSAGES

1. A summer-like pattern will continue through mid-week, with above normal temperatures and spotty diurnal convection expected, mainly today and on Wednesday. An isolated severe thunderstorm will be possible this afternoon and evening. 2. A cold front brings better rain chances mid to late week but rainfall amounts are expected to remain light so any drought relief will be limited. Above normal highs linger through Thursday before cold air damming develops behind the front, allowing for the return of near to below normal highs Friday and Saturday.

DISCUSSION

Key message 1: A summer-like pattern will continue through mid-week, with above normal temperatures and spotty diurnal convection expected, mainly today and on Wednesday. An isolated severe thunderstorm will be possible this afternoon and evening.

The latest mesoanalysis depicts a plume of 8+ C/km mid-level lapse rates associated with remnant elevated mixed layer covering much of the southern Appalachians and vicinity this afternoon. As a result, and despite surface dewpoints that have only managed to creep up into the upper 50s/lower 60s, sbCAPE of 2000+ J/kg is analyzed in much of the area along/southeast of the I-85 corridor. Associated cumulus field is steadily getting healthy across north/central GA and extreme southwest NC. Soundings from high resolution models depict an air mass that is weakly capped and/or features high levels of free convection across our forecast area, but conditions become more hospitable for convective initiation to our west.

A general consensus of high resolution guidance is that showers and storms will initiate upstream by late afternoon/early evening with some potential for activity to wander into the upper Savannah River Valley this evening. Given the absence of any organized areas of upward vertical velocity, coverage should be widely scattered at most, but we still feel that 20-30 PoPs are warranted across ~the western quarter of our area. With an upper ridge quickly building along the East Coast...shunting the stronger westerlies north and west of our area, shear parameters are underwhelming to say the least, but we still can't rule out a pulse severe storm or two.

In response to the building ridge, low level Bermuda high is forecast to intensify/expand over the next couple of days...allowing the surface ridge to build into the Southeast...turning trajectories off the western Atlantic rather than the Gulf. This will begin to circulate subsidence-induced drier air into the CWA tonight, with afternoon dewpoints only expected in the 50s Monday and Tuesday. Meanwhile, temps will be steadily warming aloft. These factors are expected to result in much weaker PM instability...and little in the way of diurnal convection is expected Mon and Tue.

By Wednesday, the upper ridge/Bermuda high is forecast to begin weakening as a strong short wave trough sweeps across eastern Canada/the northeast Conus. Moisture and instability profiles are therefore expected to improve across our areas, especially as the leading edge of a frontal zone approaches the southern Appalachians from the west. PoPs for diurnal convection increase to likely across the mountains by Wed evening. Shear parameters will improve somewhat during this time, but signals for any organized severe storm threat area weak at best. Otherwise, temperatures will remain 6-9 degrees above normal through the first half of the week.

Key message 2: A cold front brings better rain chances mid to late week but rainfall amounts are expected to remain light so any drought relief will be limited. Above normal highs linger through Thursday before cold air damming develops behind the front, allowing for the return of near to below normal highs Friday and Saturday.

The main change of note is that the timing of the cold front has sped up a bit compared to this time yesterday and is now expected to track across the GSP forecast area Wednesday night into early Thursday morning. As a result of this earlier FROPA timing, highs have trended down ever so slightly on Thursday compared to the previous forecast. However, highs are still expected to remain warm and above normal on Thursday. The front stalls across the Carolinas Friday into Saturday as weak cold air damming develops behind the front (per 06Z the GFS), bringing near to below normal highs back to the area. The front should then reactivate and lift back north as a warm front over the Carolinas on Sunday. The 06Z GFS depicts the wedge eroding by Sunday, allowing just above normal highs to return. This front will bring better rain chances to the area starting Wednesday evening, with rain chances sticking around through at least the weekend. Although we are trending more towards a wetter pattern with higher chances for rain mid to late week, rainfall amounts appear to remain light at this time (especially considering how long rain could stick around). Most locations could see rainfall amounts ~1-1.5" Wednesday evening through Sunday, with some isolated locations along the Blue Ridge Escarpment possibly seeing 2"+. Will continue to monitor QPF trends closely in the coming days but for now it still looks like there would be limited relief from the drought with these rainfall totals. With the timing of the front coming through prior to peak heating on Thursday, and with CAD expected behind front Friday into Saturday, any potential for severe weather should remain low for most of the period. However, with the warm sector returning Sunday behind the warm front, better instability may develop during peak heating, but confidence is low with this being day 7.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: A cumulus field continues to steadily expand across the far western part of the Terminal Forecast Area this afternoon within a moderately unstable air mass. Isolated-to-widely scattered deep convection may develop west of the area by late afternoon, and wander into our western areas by this evening. However, even at KAND and KAVL, the chance of a shower or storm is only 20%, and a mention will be omitted from those TAFs for the time being. For the other TAF sites, the chance is 10% at most. VFR is therefore forecast through the period, with FEW/SCT clouds in the 045-060 range forecast at most sites this afternoon/evening. Having said that, there are hints in the guidance that continued moist southerly flow into the area could result in low cloud development by daybreak Monday (similar to Sunday morning.) However, signals in statistical and raw model guidance remain very mixed at this juncture, and have opted to handle the potential with FEW/SCT low MVFR clouds at most sites, except for SCT IFR clouds at KAND. Otherwise, the pattern of SW winds at 5-10 kts from late morning through the afternoon becoming light/variable during the evening will continue.

Outlook: VFR to persist through mid-week, except perhaps in mountain valleys, where patchy morning fog/low stratus may develop each morning. An active cold front may bring restrictions associated with convective precip by late week.

GSP WATCHES/WARNINGS/ADVISORIES

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