Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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007 FXUS62 KGSP 291428 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1028 AM EDT Sat Jun 29 2024 .SYNOPSIS... Seasonable temperatures are expected today as shower and thunderstorm chances increase ahead of a robust cold front arriving Sunday. Drier and cooler conditions will filter in on Monday and Tuesday behind the front, before summer heat returns on Wednesday and beyond. && .NEAR TERM /THROUGH TONIGHT/... As of 1000 AM Saturday...Very moist air mass in place today, resulting in plenty of cloud cover that is slowly breaking up. Expect moderate instability this aftn and scattered to numerous showers forming across the mountains with help from differential heating. Overall steering flow is more westerly today, which will help carry some of that activity into the Piedmont. The 12z GSO and FFC soundings still show an inversion around 600 mb. So thunder chances remain low (but slightly better than yesterday). Thus, have cut back the PotThunder grids to mainly slgt chc to low-end chc, while keeping solid chc to likely PoPs. The rest of the fcst is on track. Otherwise...We remain under a subtle weakness in the broad upper ridge stretched out across the srn Plains, across the Southeast, and offshore. At low levels, a light southerly flow of moisture is expected to continue through late tonight. In this environment, a shower could pop up at any time at any location, though the lack of mid/upper forcing means that coverage will be isolated to scattered at best. Eventually, as we warm up and destabilize in the late morning and into the afternoon, we should see an increase in convective coverage concentrated over the mtns where differential heating and weak mechanical lift will help develop the precip. Guidance remains in line with this scenario, so precip probs are allowed to climb into the likely range this afternoon over the mtns, but kept at chance east of the mtns. The convection should persist well into the evening as the overall flow changes little. Eventually, the chances will move to the TN side of the mtns as a cold front over the OH Valley region approaches. Within this regime, the risk of severe thunderstorms looks low because of poor lapse rates, a lack of mid-level drying, a moist boundary layer, and no shear. High temps today should be similar to Friday, and lows will be mild once again tonight. && .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/... As of 212 AM EDT Saturday: Sunday`ll be a dynamically-active day as a trough axis sharpens up over the Mid-Atlantic on the leading periphery of a deeper upper wave dipping down out of the Great Lakes. Operational guidance still depicts the little "kink" in low-level flow early Sunday in response to that initial shortwave, which should lead to an initial suppressive effect on convective initiation while helping temperatures get a jump start earlier in the day. A cold front will be making tracks across the central Tennessee Valley at this point, but it`ll take until late morning for the bulk of the forecast area to get into the prefrontal trough...so initially the atmosphere won`t be super conducive to convection. Indeed, even at 18z many forecast profiles depict shallow lapse rates or even modest low-level subsidence at sites east of the NC mountains. At best, though, this can be expected to merely delay CI. By Sunday afternoon, as the frontal circulation draws near and low-level dynamics become enhanced, lapse rates will somewhat steepen and any low-level inversion should mix out entirely...giving way to impressive sbCAPE values on the order of 2000 J/kg across much of the forecast area. Especially toward late afternoon and evening, the upper height gradient will tighten up a bit, enhancing deep layer shear across these areas. And with some lingering dry air aloft, the most recent HREF cycle is hinting at DCAPE values of 800-1000 J/kg...enough to support an outside risk of severe weather (the primary threat being damaging wind, and large hail a close secondary). So, the SPC`s recent inclusion of most of the area in a Marginal Risk of Severe Weather for Sunday looks reasonable enough. Separately, Sunday may present a heat risk...with dewpoints forecast to climb into the mid 70s alongside temps in the mid 90s. Heat index values could exceed 100 degrees across much of the NC Piedmont and SC Upstate, and may well climb to values near 105. There`s high potential for the forecast to fail, though...it`s going to be a delicate balance between increasing heat / moisture versus convection. Cloud cover due to developing convection, as well as the rainfall itself, could easily skewer a higher-end temperature forecast. And indeed, the latest HREF cycle is hinting at lower probabilities of hitting advisory-criteria heat index values than before. This`ll need to be monitored closely over the next 36 hours. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 241 AM EDT Saturday: High pressure will build into the area Monday and beyond as an active frontal boundary pushes east of the forecast area. Upper heights will rise sharply through mid-week as an upper anticyclone deepens over the Southeast...while at the surface, high pressure will migrate across the Ohio Valley and eventually, off the Mid-Atlantic coast. This`ll actually set up a brief period of cold-air damming on Tuesday, such that high temperatures, which will already have fallen significantly on Monday following the FROPA, will take another hit. Expect highs to stay in the low- to mid-80s, with relatively dry dewpoints, both afternoons. Thereafter, temperatures will rise steadily as the heat dome intensifies and increasingly S flow develops over the forecast area. Forecast profiles each afternoon look increasingly conducive to diurnal convection, though sans any discernible synoptic forcing, we`ll be relying on parcels to auto-convection, which could put coverage into question. More than likely, any meaningful shower activity will be focused across the mountains, where the terrain will serve as a trigger. By Friday, northern stream troughing looks like it may put a dent in the upper pattern and carry a cold front into the Ohio Valley...although this forecaster finds such a scenario a little dubious. Whatever the case may be...temperatures by this point in time will be back in the 90s, and we may well be once again looking at heat index problems on and after Independence Day. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: A light moist southerly flow continues across the region at daybreak. Altho most of the shower activity has weakened or dissipated, new showers could develop at any time and place this morning, but the chances are too low to include. Otherwise, the guidance still suggests that low clouds will be an ongoing concern through mid/late morning, with MVFR ceiling at times at most terminals, maybe starting out in the IFR category, especially near the Blue Ridge Escarpment. Wind will stay SSE to SSW and light. Any ceiling will eventually lift to VFR and scatter out by afternoon, thereafter we will look for better coverage of showers and thunderstorms in the mid/late afternoon. Confidence is high enough only at KAVL at this time, so this will be handled with a PROB30 outside the mtns until confidence in timing and coverage improves. Expect a similar evolution late Saturday night, with more low clouds possible. Outlook: A cold front approaches the area on Sunday, resulting in higher aftn/eve convective coverage. The front pushes trough and stalls out just south of the area, resulting in quieter weather for Monday and Tuesday. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...MPR NEAR TERM...ARK/PM SHORT TERM...MPR LONG TERM...MPR AVIATION...PM