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Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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267 FXUS62 KGSP 131800 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 200 PM EDT Tue Aug 13 2024 .SYNOPSIS... Dry high pressure gradually builds in from the north through Thursday, pushing a stalled front further south of the area. The high will drift east off the East Coast Friday, as a cold front approaches from the west over the weekend. The front will bring a return of better chances of shower and thunderstorms through Sunday, then lower chances early next week. && .NEAR TERM /THROUGH TONIGHT/... ...Key Messages... 1) Isolated Strong to Severe Storms Possible this Afternoon and Early Evening, mainly across the Upstate and northeast Georgia 2) Heavy rain and localized flooding possible in a few locations over the Upstate and northeast Georgia 3) Cooler Temperatures over the foothills and western Piedmont of NC As of 130 PM EDT Tuesday: A few more changes for the afternoon, as radar and satellite imagery suggest an almost-weak-wedgelike scenario for the rest of the daylight hours. One boundary lies across the lower Piedmont/Lakelands, where mesoanalysis indicates pooled moisture has allowed for sfc-based CAPE on the order of 1500-2500 J/kg. The dCAPE is also modest in that same area in northeast Georgia, with values almost high enough to support strong downdrafts and outflows. Thus, a non-zero severe thunderstorm threat, with damaging wind gusts the main concern. Another boundary was noted in the satellite imagery, slowly sagging south across the northern part of the Upstate. Along and south of this boundary, we might have an outside chance at some heavy rain and localized flooding, as we retain a very moist air mass that has precipitable water approaching two inches. The orientation of the boundary looks favorable for storms to move/train W-to-E, thus the small threat. We`ll keep our eyes on that. As for temps, a large area of rain-cooled air covered by low stratus/stratocu has temps trapped well down into the 70s over NC east of the mtns. So, it looks like it is time to cut back on the high temp forecast as the trends suggest the clouds will not thin quick enough to save the high temp forecast. Otherwise, upper troughing will remain over the Carolinas through the near term while periodic shortwaves track over the region from the northwest. Meanwhile at the sfc, the southern periphery of surface high pressure centered over the Great Lakes region will extend into the Carolinas while a front stalls over the Southeast. This will lead to a more convectively active day compared to yesterday. Although deep layer shear will range from 35-45 kts, lapse rates look relatively poor and the sfc-based CAPE might make it to around 1500 J/kg during peak heating. Damaging wind gusts will be the main concern with any severe storms that develop. The excessive rainfall threat will remain isolated with higher shear values in place allowing for faster storm motion. However, flash flood guidance continues to be on the low side (mainly across the eastern two-thirds of the CWA) due to the rainfall from Debby last week and the recent rainfall over the weekend. Convection should gradually end from west to east this evening, with drier conditions moving in tonight. Lows tonight will end up near climo to a few degrees above climo. Patchy fog may develop across the NC mountains and Foothills overnight into daybreak Wednesday. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 145 AM EDT Tuesday: The short term looks fairly quiet, as the axis of a deep upper trough will swing east off the East Coast, and upper ridging builds in from the Lower Mississippi Valley Wednesday thru Thursday. At the sfc, dry air will filter in from the north, limiting instability. A light easterly or southeasterly low-level flow may be enough to force a few isolated showers and perhaps a garden-variety tstm or two Wednesday and Thursday aftns in the mountains, but otherwise, it should be partly cloudy with temps slightly below normal. Highs in the mountain valleys and Piedmont will be mainly in the low to mid 80s and lows in the mid to upper 60s. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 230 AM Tuesday...The medium range guidance is in good agreement on a vigorous shortwave diving thru the western Great Lakes Friday, carving out a fairly deep upper trough across the Ohio Valley Saturday, then persisting as a long wave trough across the eastern CONUS into early next week. An associated low pressure system will bring a cold front into the Ohio Valley and possibly crossing the forecast area over the weekend. The front is expected to stall out roughly across the Midlands or Coastal Plain Sunday or Monday. Overall, the front will have limited moisture to work with, but should support scattered to numerous showers and thunderstorms in the NC mountains with help of NWLY upslope flow Friday and Saturday. Downsloping east of the mountains may limit coverage, but support a return of near-normal to slightly above-normal temps. Drier air filters in behind the front Sunday into Monday, lowering PoPs and bringing temps down a couple degrees. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... At KCLT and elsewhere: An almost-wedgelike configuration is observed in the satellite imagery, with a large area of low cloudiness covering most of the foothills and northwest Piedmont. Thus, KCLT and KHKY will be in-and-out of a low cloud MVFR ceiling through the afternoon (mostly IN), with a light northeast wind. The Upstate terminals will also have some low clouds developing through the afternoon, but will remain mostly OUT of the restriction. Precip probs are looking less and less based on the recent output of the CAMs, so we will amend by watching radar trends. Thus, KCLT and the Upstate terminals get a TEMPO for shower activity and associated restrictions, but the others don`t. Most of the lowest cloud deck will finally mix out late in the day, leading to a period of VFR in the evening and early overnight. Wind should go light/variable. Tricky forecast overnight. Previous guidance suggested prevailing VFR, but the last model run before issuance made a major change to widespread IFR/LIFR restrictions at most terminals in the pre-dawn hours. Rather than jump on the big change, preferred to wait to see some consistency over the next few runs before adding it in. Be on the lookout for changes, though. Wind should come back up from the ENE in the morning owing to high pressure to our N. We could easily pop up a low cloud ceiling restriction with minimal heating, but this was left out for the time being. Outlook: Expect mostly diurnal isolated to scattered diurnal convection through Thursday. Coverage of convection increases Friday into Saturday as a cold front approaches out of the west. Patchy fog and low stratus will be possible each night in the mountain valleys and in any locations that received heavy rainfall the day prior. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...AR/PM SHORT TERM...ARK LONG TERM...ARK AVIATION...AR/PM