Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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996 FXUS62 KGSP 161757 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 157 PM EDT Mon Sep 16 2024 .SYNOPSIS... A semi tropical low will move inland over the Carolinas tonight and Tuesday bringing heavy rain and gusty winds. The low will weaken through mid week with less wind and rain. 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 1200 PM EDT Monday: Main change this afternoon was the issuance of a Flood Watch for Flash Flooding for the Charlotte Metro area as rainfall amounts from 2-4 inches are expected, with locally higher amounts from 4.5-5.0 inches possible. Otherwise, the broad deck of lower clouds associated with Potential Tropical Cyclone 8 continues over eastern two-thirds of the CWA. Still hard to pinpoint a defined circulation with the system and it has not become any better organized since this morning. The area of light rain continues pushing west across the eastern zones of the CWA so increased PoPs slightly to account for this. 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. Potential Tropical Cyclone 8 is currently situated about 100 miles east of Charleston, SC and will begin its gradual track inland before making landfall sometime this evening. Model guidance still differs on the overall timing and location of the potential tropical cyclone as the operational GFS still fast tracks the system into the heart of SC by this evening, while the ECMWF/Canadian/NAM are slower and do not show the potential tropical cyclone making landfall until this evening. 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 this 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. The other main hazard in association with Potential Tropical Cyclone 8 will be wind gusts as model guidance sources indicate a 40-50+ kt LLJ punching in on the north and northwest periphery of the system. Did issue a mostly impact based Wind Advisory for most locations along I-77 as the last few runs of the HRRR continue to show 30-40 kt gusts in these locations. These winds may allow for some scattered trees to come down and may lead to isolated power outages. Also, 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 very 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 CWA 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 as extensive cloud cover and rainfall continue and remain over at least the northeastern half of the CWA. && .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 MONDAY/... 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 /18Z MONDAY THROUGH SATURDAY/... At KCLT and elsewhere: Messy 18Z TAF forecast period thanks to Potential Tropical Cyclone (PTC) 8. PTC 8 will gradually push farther inland across the Carolinas through tonight before stalling over the terminals on Tuesday. Light rain from PTC 8 is currently impacting KCLT and KHKY this afternoon with the rest of the terminals remaining dry as of 18Z. This rain shield will continue expanding westward today into tonight, while precip intensity gradually ramps up. Handled the onset of rain with TEMPOs (outside of KCLT and KHKY where it`s currently raining). Have prevailing RA/+RA for all terminals after precip onset due to the expectation that the low will stall. The precip shield should allow vsbys to range mostly from IFR to MVFR levels, although the heaviest bands of rain may allow cigs to drop to LIFR levels at times. BKN to OVC skies are in place at KHKY and KCLT this afternoon, and should expand westward through this evening. Cigs will gradually lower through the overnight hours, with IFR to LIFR restrictions expected for most terminals. However, KAND may only see cigs drop to MVFR levels. Breezy winds remain in place across most of the terminals this afternoon, and should linger through late tonight before tapering off around daybreak Tuesday. Low-level wind shear may also be an issue, mainly this evening into the overnight hours so have this accounted for in the TAFs. There will be very little improvement (regarding precip, cigs, and vsbys) throughout the day Tuesday, so restrictions should linger through the end of the 18Z TAF period. Wind direction will generally be N/NE east of the mountains through early Tuesday morning. Winds at KAVL are currently VRB but should gradually toggle more N/WNW this evening into Tuesday. Winds at KCLT and KHKY should gradually turn more E/ESE Tuesday morning into Tuesday afternoon while the SC Upstate sites will see winds with a N`ly component. Outlook: An upper low will meander over the Carolinas Tuesday evening before gradually weakening and lifting northeast Wednesday into Thursday. This will act to keep precip chances and possible restrictions around through Thursday. Friday into the weekend high pressure builds into the region from the north allowing drier weather and VFR conditions to return. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...Wind Advisory until 2 AM EDT Tuesday for NCZ057-070>072-082. Flood Watch from 8 PM EDT this evening through Tuesday morning for NCZ070>072-082. SC...Wind Advisory until 2 AM EDT Tuesday for SCZ009-014. Flood Watch from 8 PM EDT this evening through Tuesday morning for SCZ009. && $$ SYNOPSIS...DEO NEAR TERM...AR/CAC SHORT TERM...ARK LONG TERM...ARK AVIATION...AR