Area Forecast Discussion
Issued by NWS Greer, SC

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588 FXUS62 KGSP 281401 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1001 AM EDT Fri Jun 28 2024 .SYNOPSIS... Seasonable temperatures are expected today and tomorrow as shower and thunderstorm activity increases ahead of an approaching cold front. Hot and humid conditions on Sunday will precede the front, which will usher in drier and cooler conditions Monday and Tuesday. The summer heat will return Wednesday and beyond. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1000 AM...Plenty of cloud cover atop the forecast area this morning, but not as much as this time yesterday. Overall, expect a little more sunshine, allowing for scattered showers with a few mainly garden-variety thunderstorms this afternoon. Warming aloft will make for weak mid-level lapse rates and thus severe storm potential will remain low. Only minor tweaks to the grids for this update, but did lower the PoPs and thunder chances slightly given the model trends. Over the next 24 hours, a subtle weakness aloft will linger between the old upper anticyclone over the srn Plains and the reflection of the Atlantic Subtropical High off the Southeast Coast...or maybe what we see in the model guidance is just the result of the moist southeasterly low level flow toward the srn Appalachians. Either way, coverage of showers and storms is expected to be a bit greater this afternoon than the past few days. That being said, expectations are tempered somewhat by our recent lack of widespread rain, and wondering if the models are...a little over-enthusiastic. The precip probs were kept fairly close to the old forecast, but if anything might be overdone. Severe weather chances appear to be low. High temps should stay close to normal. For tonight, the persistence of the weak low level moist flow from the south should support a continued rain chance with a focus on the Blue Ridge Escarpment. Isolated and locally heavy rain would not be a surprise, but confidence is low. Min temps look a bit warmer than normal.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 249 AM EDT Friday: A moist subtropical airmass will continue to advect into the forecast are on Saturday as ill-defined upper flow over the immediate forecast area does little to interact with deep SSE flow at the low levels. Forecast profiles on Saturday appear moist through the depth of the troposphere, especially during the afternoon hours. Operational guidance depicts some 1000-1500 J/kg sbCAPE developing during the afternoon and evening across most of the forecast area...but with anemic lapse rates that should hinder more robust updrafts from developing. And, without much synoptic forcing, it`ll be tough to trigger initiation en masse, which should once again limit convective coverage. So, despite juiced-up PWs exceeding 2" in some areas, the threat of either severe weather or heavy rainfall appears limited. Temperatures Saturday afternoon should climb into the upper 80s or lower 90s along the I-77 corridor. With profiles as moist as they are, any amount of diurnal mixing will do little to lower dewpoints, so afternoon heat index values may surge into the upper 90s...possibly hitting 100 in isolated locations along the southeastern fringe of the forecast area. On Sunday, z500 troughing over the Great Lakes will sharpen, and a shortwave will dig across the central Midwest and Ohio Valley, pushing a surface cold front into the Cumberland Plateau and upper / central Tennessee Valley through afternoon. Oddly, guidance depicts a W wind developing at the low levels Sunday morning in response to a subtle shortwave crossing the region...which will both boost temperatures and lower dewpoints, as well as have an initial suppressive impact on convection east of the mountains where normally the dynamics might support better initiation and stronger updrafts. Instead, the mountains still look like a focus for afternoon showers and thunder...although the Piedmont and Upstate zones should eventually catch up and destabilize despite the transient downsloping effect. Forecast sbCAPE values range from <1000 J/kg in ensemble members that depict a stronger downsloping effect, to over 2000 J/kg in the more aggressive guidance. A dry slot aloft may support some limited downburst threat...but no widespread severe risk is expected. Like Saturday, there will be limited dry air aloft to mix down on Sunday afternoon, so dewpoints will struggle to lower during peak heating. With temperatures expected to soar into low- to mid-90s on Sunday afternoon, and dewpoints ever soupier...wide swaths of the Upstate and Piedmont can expect to see heat index values in excess of 100 degrees, with some potential for values of 104-106 degrees, mostly south of I-85. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 311 AM EDT Friday: By Monday, the cold front should be pulling east of the forecast area and ushering in somewhat cooler and drier air in for the start of the new week. Temperatures on Monday and Tuesday will only climb into the mid/upper 80s...particularly on Tuesday, which will boast an almost CAD-like pattern with high pressure drifting off the New England coast and driving NE flow across the Carolinas. The latest cycle of ensemble guidance has trended away from much active weather on Monday afternoon, depicting a stronger subsidence inversion across most of the area and very limited moisture. So, maintained token PoPs across the southeastern tier and across the mountains, where some widely scattered showers will be possible. Further quiet conditions are expected on Tuesday, with an even more robust subsidence inversion in place. By midweek, temperatures will be on the rise again as high pressure slides off the Atlantic coast and winds turn back around out of the south. Deep ridging will once again develop over the Southeastern CONUS, maintaining suppressed profiles across the western Carolinas...so although some diurnally-driven afternoon showers can`t be ruled out, the case for robust afternoon convection appears weak through the end of the seven day forecast. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/... At KCLT and elsewhere: A light southeasterly low level upslope flow has been established across the region, and in response we are seeing some sct/bkn low clouds forming in patches, mainly closer to the Blue Ridge Escarpment. This could result in a temporary MVFR ceiling restriction in some locations, while KAVL deals with low stratus in the Fr Broad Valley that dropped the ceiling to LIFR. The low clouds in the valleys should mix out by 14Z at the latest. Expect the wind direction to come up from the SE once we begin to mix out the morning inversion. Can`t rule out another brief MVFR condition as cumulus form early in the day. The big question will be the extent of deep convection this afternoon. Most of the CAMs have fairly sparse coverage that suggests we stick to PROB30s for this issuance. Hopefully the next few runs of the HRRR lend confidence to go with a TEMPO at some terminals. The threat for thunderstorms should taper off in mid-evening. Thereafter, the guidance is hitting the low cloud development before daybreak Saturday, so all terminals will get at least an MVFR. There are some indications that a more widespread IFR could materialize, but this was left out for now. Outlook: Another cold front approaches the area Sunday night, keeping unsettled weather around through early next week. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...MPR NEAR TERM...ARK/PM SHORT TERM...MPR LONG TERM...MPR AVIATION...PM