textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Temperatures and Heat Index have trended down slightly for next week. Updated the aviation discussion for the 06Z TAFs.

KEY MESSAGES

1. Mainly diurnal thunderstorm activity is expected today and again Sunday, with a few storms posing a threat of isolated flash flooding and perhaps one or two severe storms producing damaging wind gusts. 2. Temperatures will climb back above normal on Monday, after which it will be hot and humid each day through mid-week. Some Piedmont/Upstate locations could have afternoon heat index between 99-103 beginning Monday.

DISCUSSION

Key message 1: Mainly diurnal thunderstorm activity is expected today and again Sunday, with a few storms posing a threat of isolated flash flooding and perhaps one or two severe storms producing damaging wind gusts.

Not much change from previous thinking after a look at the new model guidance. The weekend should be a transition between the relatively active pattern of late with a weakness aloft to our west, and a warmer regime as the Atlantic Subtropical Ridge builds in from the east by Monday. Overall, the pattern highly favors diurnal convection today and Sunday with the best coverage over the mtns and foothills. As noted by the day shift, the SW flow will tend to keep most of the convection near the higher terrain, so those areas will get a likely precip prob both days. Meanwhile, guidance suggests areas south/east of I-85 will have more isolated convection today, and maybe better coverage Sunday afternoon. Either way, outflow boundaries could allow for storms to propagate anywhere. Buoyancy should top out in the 2500-3500 J/kg range with just enough mid-level dry air to provide enough potential for strong downdrafts that could produce a few severe wind gusts. Shear of 15-20 kt might also allow for some weak organization. The heavy rain threat is probably the bigger concern, though a modest one at that. Precipitable water will remain high and warm cloud depth will also remain high, so the Marginal Risk of excessive rain looks reasonable. An isolated flash flood or two can't be ruled out. Temperatures should be close to normal both days.

Key message 2: Temperatures will climb back above normal on Monday, after which it will be hot and humid each day through mid-week. Some Piedmont/Upstate locations could have afternoon heat index between 99-103 beginning Monday.

The latest guidance continues to show the Atlantic Subtropical Ridge building back in from the east on Monday, though perhaps suppressed more to the south than previously shown. As a result, temperatures have trended down ever so slightly, which helps to keep the heat index merely in the hot and uncomfortable range instead of up into the Advisory range. Thus, confidence in the need for any advisories has gone down. The slightly more westerly flow favors mountain convection each afternoon, with some moving out over the western Piedmont late in the day. What is more interesting is that we might get more into an MCS track by Wednesday afternoon as we come more under the influence of the trof passing to our north mid-week. This is not really reflected in the precip probs yet, though a more widespread chance is shown for Thursday. If the trend continues, we could be closer to a boundary sagging southward by the end of the week, which would help keep temps further in check. Confidence in any of this is relatively low.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Satellite imagery shows low stratus and fog developing in the mtn valleys at this hour, indicating that KAVL should have a good shot at ceiling/vis restrictions developing in the pre-dawn hours. For now, we limit the fcst to IFR until there is a better indication of something worse, though we note that guidance suggests LIFR by 10Z. Elsewhere, VFR. Wind should become light SW after sunrise. Won't rule out a very brief broken MVFR ceiling anywhere as convective clouds form in mid/late morning, but for now the chance is too low to mention in the TAF. Thunderstorms are expected again this afternoon, with the best chances over the mtns and foothills of NC, thus KAVL gets the TEMPO and KHKY gets the PROB30 for associated restrictions. The other terminals have too low of a chance to include the storms at this point. The convection should die off in the middle part of the evening, after which only debris clouds remain.

Outlook: Scattered diurnal showers and thunderstorms, with associated flight restrictions are expected each day through early next week. Patchy fog and low stratus will be possible each morning, especially in the mtn valleys and anywhere that received significant rainfall the previous day/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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