Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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614 FXUS62 KGSP 160744 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 344 AM EDT Mon Sep 16 2024 .SYNOPSIS... A low developing off the South Carolina coast is expected to track northwestward into the Carolinas later today, possibly become a Tropical Storm, bringing heavy rain and gusty winds. The low will slowly drift inland tonight through Tuesday, and weaken. By Wednesday, the remnant low will stall out over the Mid-Atlantic, while high pressure begins to build into the region from the north and west late in the week. && .NEAR TERM /THROUGH TONIGHT/... As of 330 AM EDT Monday: Broad deck of lower clouds associated with Potential Tropical Cyclone 8 is covering at least the eastern two-thirds of the CFWA. Still hard to pinpoint a defined circulation, but indications on satellite show the storm intensifying. Once a circulation center becomes established, model guidance should have a better handle on the overall track (more on this below). The area of precip remains to the southeast and will have to overcome a dry boundary layer for rain droplets to reach the surface, which won`t be an issue after a while as moist 40-50 kt easterly LLJ begins to filter in after daybreak Monday. Can`t rule out a few spotty showers in the vicinity of the eastern Blue Ridge Escarpment overnight, but not much fanfare outside of that for the rest of the overnight period. Weak blocking pattern will remain in place across the eastern CONUS through a good portion of the forecast period, as an anticyclone weakens over the Great Lakes and a east-to-west-oriented trough lingers atop the Southeast. As mentioned above, Potential Tropical Cyclone 8 is currently situated about 115 miles east-southeast of Charleston, SC and will begin its gradual track inland, eventually making landfall late Monday. Model guidance still differ a good bit on overall timing and location of the tropical disturbance as the operational GFS still fast tracks the system into the heart of SC by Monday evening, while the ECMWF/Canadian/NAM are just making landfall at this point. Even ensemble members on the track guidance diverge after the 12-24 hour period, but the general consensus is for the storm to move onshore late Monday evening, while quickly enduring an extratropical transition as the main axis of precip remains on the northern and western peripheral of the disturbance as the deformation zone shifts across the area in response. With the center of the storm shifting slightly further south across the Midlands with each run, the heaviest rainfall in the current forecast shows the main QPF axis stretching from the CLT metro area, along the NC/SC state line, and eventually the favorable upslope locations along the eastern Blue Ridge Escarpment as a stout easterly component will keep rainfall ongoing in this area. CAMs, hurricane guidance, and operational models show the system somewhat stalling in the area, which will carry mentionable PoPs longer, especially in the favorable upslope zones along the Blue Ridge Escarpment as the easterly component remains established in this area. Hydro issues may become a concern for the upslope regions along the Escarpment and the CLT metro, but dry antecedent conditions and rainfall amounts being more spread out over time are the reasons for not issuing a Flood Watch at this time. Current QPF response includes 2-4+" in the NC Piedmont and 3-5+" along the eastern Blue Ridge Escarpment and 1-2" of rainfall in the northern Upstate. A sharp gradient will cutoff any potential of significant rainfall in northeast Georgia and the western Upstate as the system stalls for a short time period and very slowly tracks northward during the short term forecast period. Other hazards in association with Potential Tropical Cyclone 8 are wind gusts as aforementioned model guidance indicate a 40-50+ kt LLJ punching in on the north and northwest peripheral of the system. Gusts of 30 kts, with an occasional gust of 35+ kts can`t be ruled out, mainly in the CLT metro and higher elevations that will tap into the higher gusts. The NAMNest and HRRR display 30-40+ kts of surface wind gusts in these same areas. The current forecast still doesn`t support enough speed for a Wind Advisory and with dry antecedent conditions, didn`t think an impact based Wind Advisory was necessary. One thing to note, with the southern trend that the center of the tropical cyclone continues on, this may place the northwest Piedmont and portions of the I-40 corridor into the front right quadrant of the storm, which could lead to an environment favorable for an isolated tornado or two in this region late Monday night into Tuesday morning. Confidence is low on this development, but the chance is nonzero and is supported by the 00Z HREF, which places a few updraft helicity streaks in this area. Otherwise, temperatures will induce a tight gradient for afternoon highs as the southwest zones will end up close to normal, while the rest of the CFWA will be 5-10 degrees below normal due to extensive cloud cover and much better precip chances. Overnight lows will run at or slightly above normal Monday night as extensive cloud cover and rainfall continue and remain over at least the northeastern half of the CFWA. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... As of 315 AM Monday: Overall, the models are trending a little slower and south with the track of Potential Tropical Cyclone #8 as it tracks into SC, which may be named Hermine at some point before making landfall along the Carolina coast. This results in lingering heavy rain potential across the NC Foothills and Piedmont Tuesday, as the Escarpment keeps a solid upslope ELY flow thru the day. The low will be weakening and right over the forecast area, so winds should become fairly light, although may remain breezy in the far northern zones. PoPs and sky cover were bumped up, and max temps lowered based on these trends. Additional QPF after 12z Tuesday may be 1-3" along the Escarpment per the latest HREF PMM. The low is expected to continue to fill and drift north of the forecast area Tuesday night into Wednesday. The 850 mb flow will turn out of the W/SW during the day, allowing for some clearing of clouds east of the mountains. Sfc dewpts still in the mid 60s combined with a rebound in temps into the 80s may result in a few hundred J/kg of sbCAPE by mid-aftn. So isolated to scattered showers with a few tstms possible mainly in the mountains is expected Wednesday, and should largely dissipate in the evening. Highs will be near normal, while lows remain slightly above normal. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 245 AM Monday: An upper low associated with the remnants of PTC #8 is forecast by the medium range guidance to drift east and open up along the East Coast late in the week. To our west. a long ridge axis extending from the Southern Plains to the Great Lakes will begin to build into the region. At the sfc, a weak low may try to organize off the mid-Atlantic coast, while an inverted ridge noses south along the Appalachians. Fairly weak flow across the region will likely keep some lingering moisture around, and with more sunshine Thursday, could see some instability for diurnal convection. Similar setup continues for Friday, with slight chc to low-end chc diurnal PoPs for mainly shower, but isolated tstms possible. Temps will be slightly above normal. By Saturday, the sfc high over New England strengthens significantly, as a Rex Block pattern sets up across the eastern Conus. The operational models disagree on how organized a sfc low will get off the coast, but agree that some moisture may be advanced in ELY/NELY flow into the area Saturday thru Sunday. This may result in a CAD-like setup over the forecast area, with increasing clouds and slightly below-normal highs and above-normal lows. PoPs still look low given the shallow moisture. && .AVIATION /08Z MONDAY THROUGH FRIDAY/... At KCLT and elsewhere: Moist easterly flow just above the surface layer in response to Potential Tropical Cyclone Eight, located offshore the Grand Strand will support BKN/OVC MVFR low clouds across most of the area through daybreak Monday. Conditions may improve at KAVL and the Upstate terminals during the afternoon, but should continue to slowly deteriorate at KCLT and KHKY as the tropical disturbance inches closer to the coast and eventually making landfall late Monday. As a result, rain chances will increase by the afternoon and remain in place through the evening, which is reflected in the prevailing lines at these TAF sites. Low-level wind shear may become an issue at KCLT and potentially KHKY, but confidence is higher at KCLT for a TAF mention. Confidence lowers at the upstate sites and KAVL as model guidance differ on the timing and extent of the rainfall, but rain and associated restrictions should eventually reach KGSP/KGMU/KAVL by the end of the forecast period. Showers are possible at KAND, but confidence is too low through the 06Z forecast period to include nothing more than VCSH in the prevailing line due to the sharp cutoff of precip, but still expect for MVFR cigs to filter in before the end of the TAF period. Eventually, MVFR/IFR should overspread all terminals by Monday night, with vsby restrictions being included, especially with any ongoing rainfall. Winds will take on a northeasterly component, but may back more northerly by late Monday evening, depending on the location of the center of Potential Tropical Cyclone Eight. Gusts in the 25-30 kt range are possible, mainly at KCLT and KHKY, with some gusts potentially exceeding 30 kts. 20-25 kt gusts are included at the upstate terminals and KAVL as the storm system pushes towards the region late in the period. Outlook: Gusty winds and some instances of low-level wind shear will impact KCLT and KHKY through Monday night and even into parts of Tuesday. The best chance for rainfall and associated restrictions will reside at KCLT and KHKY, based on the current forecast of Potential Tropical Cyclone Eight. MVFR/IFR restrictions are likely at all TAF sites by late Monday evening through at least daybreak Tuesday. Still some uncertainty on the extent of rainfall coverage and associated restrictions beyond daybreak Tuesday, but winds and precip will begin to taper off at some point during the day Tuesday. Spotty showers and occasional restrictions will be possible through the remainder of the week. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...ARK NEAR TERM...CAC SHORT TERM...ARK LONG TERM...ARK AVIATION...CAC