Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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357 FXUS62 KGSP 251736 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 136 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 TONIGHT/... 1000 AM Update...An upper trof axis has setup west of the FA and is instigating an active PRE along a sfc frontal zone. Showers and thunderstorms have developed and will continue to train along the front into the sw/rn NC mtns an nrn NE GA. So far, new rainfall amts have approached an inch across wrn Graham Co with little impact as far as flooding is concerned. Good confidence is had in this pattern holding rather steady state into the afternoon and evening, which will increase the threat of flash flooding over the aforementioned areas. The WPC has a Day 2 Moderate Risk area that includes our western zones and we have a Flood Watch in effect for that same area. In addition, our area will be under the gun again this aftn/evening for strong to severe convection with some organization possible due to the continued high shear environment. These storms will be capable of producing high rainfall rates and pose a risk of localized flooding outside/east of the PRE area mentioned above. All this precip is expected to occur before the rainfall associated with TC Helene begins moving into our area on Thurs. Thus, more widespread flooding is likely with this tropical system. Please remain weather aware over the next few days, the potential for major to catastrophic flooding over parts of our fcst area is looking more likely. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 130 PM EDT Wednesday: ***MAJOR RAINFALL EVENT FOR THE SOUTHERN APPALACHIANS AND VICINITY WITH POTENTIAL FOR WIDESPREAD FLASH FLOODING*** The predecessor rainfall event 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 THROUGH TUESDAY/... As of 325 am EDT Wednesday: Upper low is forecast to be centered somewhere over the Ohio Valley at the start of the extended, and should begin wobbling east over the weekend, as a series of strong short wave brush the northern part of the Conus. We should have an opportunity to dry out over the weekend, as precip chances are expected to be limited to spotty, mainly diurnal showers. Precip chances ramp up a bit (although remaining of the token chance variety) early in the work week as the upper low opens up and ejects over the region. Any additional rainfall amounts should (hopefully) be light during this time frame. Temperatures are forecast to be close to normal through the period. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... At KCLT and elsewhere: The TAF set will start out mostly VFR with MVFR lingering at KHKY for a few more hrs this afternoon. Precip and thunder chances outside the the mtns will be tricky, but enuf destabilization will likely develop sct showers with isol thunder at least, so went with a TEMPO thunder all sites later this afternoon into the evening. Flight conds will begin to deteriorate overnight with IFR conds, mainly due to CIGs, likely at all sites before daybreak. High moisture levels persist Thu and will expect little improvement in CIGs thru the day. Winds remain se/ly outside the mtns and more varying at KAVL with low-end gusts all sites by the late period. Outlook: A tropical cyclone will bring low CIGs, heavy rain, and strong winds Thurs 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 Watch 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 Watch for NCZ033-048>053-058-059-062>065-068- 501>510. SC...Flood Watch through Friday afternoon for SCZ008-010>013-019- 101>109. Tropical Storm Watch for SCZ008-010>013-019-101>109. && $$ SYNOPSIS...JPT NEAR TERM...JPT/SBK SHORT TERM...JDL LONG TERM...JDL AVIATION...SBK