Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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711 FXUS62 KGSP 260025 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 825 PM EDT Wed Sep 25 2024 .SYNOPSIS... Tropical Cyclone Helene is forecast to make landfall over the Florida Panhandle late Thursday, with the remnant circulation moving quickly north bringing gusty winds and heavy rainfall to our area Thursday night into Friday. Expect a calmer weekend with temperatures near seasonal normals. && .NEAR TERM /THROUGH THURSDAY/... As of 755 pm...A wide band of rain continues to stream northward in a plume of deep tropical moisture to the east of a stalled frontal boundary over east TN. Thus, our Predecessor Rain Event (PRE) was well underway. The eastern edge of the rain band has moved very little, and it is along this eastern edge where there has been enough overlap of shear, buoyancy, and forcing to keep deep convection going. In addition, the shear was supporting occasional rotating storms, and that may continue for the next several hours according to the 22Z run of the HRRR. There will also be a severe storm threat from metro CLT up across the NW Piedmont, which retains 1500-2000 J/kg of sfc-based CAPE and deep layer shear of 40-45 kt. A brief tornado would not be out of the question, after what we have already seen today. That being said, the latest guidance keeps the primary rain band generally to the west of a line from Abbeville to Spartanburg to Forest City and Lenoir through the evening. The decision was made on the 5 pm Advisory for Helene to upgrade to a Tropical Storm Warning based on coordination with neighboring offices and a compelling trend in the consensus of the guidance regarding the wind gust potential, which mostly ramps up Thursday evening. Confidence was high enough to make the upgrade at 5 pm to give the most lead time, rather than wait until the late evening or early morning cycles when prep time would have been lost. Otherwise, all signs point to a significant event across the western Carolinas and northeast Georgia. Although a subtle westward shift in the track is noted, Helene will be a large and fast-moving storm, so the shift doesn`t really matter. We still expect a very rare and significant amount of rainfall and some areas across the mtns and adjacent zones will see catastrophic and damaging flooding by the later near term. Once the rainfall gets above ten inches of rain or more, we often see numerous reports of landslides across the high terrain. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 130 PM EDT Wednesday: ***MAJOR RAINFALL EVENT FOR THE SOUTHERN APPALACHIANS AND VICINITY WITH POTENTIAL FOR WIDESPREAD FLASH FLOODING*** The PRE will be coming to an end by the beginning portions of the forecast period as the initial tropical rain bands from Hurricane Helene begins to move in from the south as Helene will be in the midst of making landfall as a major hurricane near the Big Bend of Florida Thursday evening. Large amounts of rainfall will have already fallen across the Blue Ridge Escarpment, southeast facing slopes, western Upstate South Carolina, and northeast Georgia. Sandwiched between a closed upper low over the mid-Mississippi Valley and an upper anticyclone over the western Atlantic will inadvertently sling shot Helene into the Southeast Thursday night before the storm takes on a more western track as the closed upper low overtakes the track by Friday afternoon. The heaviest rainfall rates will be within the tropical rainbands and with already rising streams, creeks, and rivers, expect for significant flash flooding to take place in locations that will get hammered by the PRE in the near-term forecast. The areas of most concern will be the southern Appalachians and adjacent foothills as flash flooding becomes widespread, with locally severe and life- threatening flooding likely. The rainfall is expected to taper off by Friday afternoon as Helene becomes absorbed by the closed upper low and drier air filters in. Before everything is all said and done, 5-10" of rainfall will be possible in locations west of HWY 321 and 10-15+" on the southeastern facing slope along the Blue Ridge. Still expect moderate to major flooding on some main stem rivers including: French Broad River Basin, Pigeon River and Tuckasegee River Basin, and Saluda and Broad River Basins. These areas are forecasted to see river floods that rival 1995 (TC Jerry) and 2004. With the amount of rain and heavy rainfall rates, it`s likely that numerous landslides will occur over the mountains, especially the Blue Ridge Escarpments and result in a couple of large, damaging debris flows/slope failures as well. With the fast track north to the area that Helene will encompass, the frictional force that usually derails any tropical cyclone`s wind fields won`t have enough time to totally wipe out the strong wind field associated with Helene. In this case, wind gusts of 40-50, some 60-70 mph will be possible in the western portions of the CFWA Thursday night into Friday morning. In this case, we have issued a Tropical Storm Watch for the North Carolina mountains/foothills, Upstate South Carolina, and northeast Georgia. The impacts will be significant from winds as soils will already be super saturated and any strong wind gusts will easily knock down numerous trees and lead to numerous power outages across the locations in the Tropical Storm Watch. The tornado threat will be the lowest concern of the big three that come with any tropical system, but definitely nonzero, especially with the initial rainbands that work into the region Thursday evening into the overnight period. The biggest threat will be in locations along and south of the I-85 corridor as better instability will become established in this region and will be in the favorable front-right quadrant of Helene. Most instability will diminish overnight as more stable northeasterly flow filters in thanks to Helene. Most model guidance lifts Helene out of the region by Friday afternoon with conditions quickly improving. Flooding will be ongoing Friday afternoon and evening, while mainstem river flooding may persist into the weekend. Saturday will be much improved and provides relief from the impacts that will occur Thursday and Friday, with a lot of locations cleaning up after the storm. The closed upper low is expected to engulf Helene, but should gradually push east. This could lead to a few showers in the mountains Saturday afternoon, but expect for rainfall to remain light in comparison to the rainfall from Thursday and Friday. Temperatures will be near-normal Saturday as well in part to the lower heights and any lingering cloud cover. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 245 PM EDT Wednesday: Closed upper low is expected to engulf the remnants of Helene, while gradually pushing across the Ohio and Tennessee Valleys over the weekend before setting shop over the Mid-Atlantic and Carolinas by the start of the new workweek. In this case, showers will return back into the forecast for Sunday and Monday as the upper low crosses the area and a moisture starved front will swing across the region by midweek. Signs of true Fall weather from the model guidance is evident beyond D6. Temperatures will remain near-normal through much of the period as elevated cloud cover and PoPs remain in the forecast. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... At KCLT and elsewhere: A main band of rain and thunderstorms has made very little eastward progress this afternoon and evening, as predicted, thus most terminals remain VFR at issuance time, with the exception of KAVL. Unfortunately, it`s all downhill from here. An area of TSRA will pass just east of KCLT over the next hour, otherwise, we should make it thru the evening VFR there until lower clouds develop after midnight/04Z. Elsewhere, a gradual eastward creep in the rain band will bring the other terminals into the restrictions associated with storms by 03Z at the latest, and probably even before that. Ceilings drop overnight as more upslope forcing arrives and deep tropical moisture streams up from the south, down through MVFR to IFR through the pre-dawn hours. Once there, we should stay down for the count in the LIFR/IFR range, with frequent heavier showers moving through and winds becoming gusty from the SE after daybreak. Lots of details are being left out for brevity for Thursday. Outlook: Helene will bring widespread low CIGs, heavy rain, strong winds, and low level wind shear Thurs nite into Fri before moving out of the area late Fri into Sat. Isolated to sct mainly diurnal convection will be possible thru the weekend. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Flood Watch through Friday afternoon for GAZ010-017-018-026- 028-029. Tropical Storm Warning for GAZ010-017-018-026-028-029. NC...Flood Watch through Friday afternoon for NCZ033-035-048>053- 056-058-059-062>065-068-069-501>510. Tropical Storm Warning for NCZ033-048>053-056-058-059-062>065- 068>071-082-501>510. SC...Flood Watch through Friday afternoon for SCZ008-010>013-019- 101>109. Tropical Storm Warning for SCZ008>014-019-101>109. && $$ SYNOPSIS...DEO NEAR TERM...PM SHORT TERM...CAC LONG TERM...CAC AVIATION...PM