Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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729 FXUS62 KGSP 251407 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1007 AM EDT Wed Sep 25 2024 .SYNOPSIS... A moist and cooler airmass will remain in place across the forecast area through mid-week. 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. && .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 245 am Wednesday: ***MAJOR RAINFALL EVENT FOR THE SOUTHERN APPALACHIANS AND VICINITY WITH POTENTIAL FOR WIDESPREAD FLASH FLOODING*** Predecessor rainfall event will be ongoing at the start of the short term, and isolated flash flooding will likely already be developing in areas along and near the Blue Ridge escarpment. With inverted surface trough interacting with fire hose of moisture extending north from T.C. Helene being the primary impetus for the PRE event, rainfall rates could diminish somewhat during the afternoon as the surface trough begins shifting west. However, any relief will be brief as SE upslope flow into the escarpment will be relentless, and the outer rain bands from Helene will will likely begin invading the CWA by evening as the cyclone begins its fairly quick trek through Georgia. The heaviest rainfall rates of the entire event will occur within tropical bands Thu night into Fri morning on top of already saturated soils across the southern Appalachians and adjacent foothills. Flash flooding is expected to become widespread in these areas during this time frame, with locally severe and life- threatening flooding likely. By the time the rainfall finally tapers off Friday morning, 48-hour storm totals are expected to range from 5-9 inches across much of the area west of ~U.S. Highway 321, with as much as 15 inches expected on southeastern-facing slopes in the vicinity of the Blue Ridge. At least moderate flooding is also likely to develop on some main stem rivers. (Amounts of 3-5 inches are expected for areas to the east). It`s also looking more likely that we will see numerous landslides in the mountainous counties encompassing the Blue Ridge escarpment...with a couple of large, damaging debris flows/slope failures possible. Helene will likely be of tropical storm strength as far north as central Georgia, so wind gusts of 50 to 65 mph remain a distinct possibility Thu night/early Fri across far western portions of the CWA (i.e., near the GA/SC border & southwest NC)...as well as in the high elevations. Most of the remainder of the area should see gusts in the 35 to 45 mph range. No decisions will be made on this shift regarding wind headlines. It should be stated that even if winds don`t quite reach those magnitudes, numerous to widespread downed trees will still be a strong possibility across the western half of the area due to the saturated soil conditions. In terms of the tornado threat...can`t rule out an isolated spin-up across southern portions of the area on Thursday...especially with any initial rainbands punching into the area late in the day. However, any sbCAPE is expected to diminish quickly as the sun sets and E/NE flow developing north of Helene helps to stabilize the atmosphere. Quickly improving conditions are expected throughout Fri morning into the afternoon, although ongoing flooding will likely continue well into the afternoon, while main stem river flooding may continue for several days. && .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 /14Z WEDNESDAY THROUGH SUNDAY/... At KCLT and elsewhere: Expect MVFR to IFR cigs to linger over the area thru the morning with cigs improving to MVFR by noon or so, and then low-end VFR by mid-afternoon. Another round of convection is expected again this afternoon/evening which I account for with PROB30s at all taf sites. Cigs are expected to lower to IFR/LIFR again later tonight and stay that way through the end of the taf period. Winds will pick up from the S to SE during the late mor- ning and will eventually back to a more ELY direction by evening and remain ELY thru early Thursday. Outlook: Fog/low stratus will likely develop again the next few mornings, especially in the mtn valleys. A tropical cyclone will bring heavy rain and strong winds Thurs into Fri before moving out of the area late Fri into Sat. Isolated to sct mainly diurnal con- vection will be possible thru the weekend. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Flood Watch through Friday afternoon for GAZ010-017-018-026- 028-029. NC...Flood Watch through Friday afternoon for NCZ033-048>053-058- 059-062>065-501>510. SC...Flood Watch through Friday afternoon for SCZ010-101>108. && $$ SYNOPSIS...JPT NEAR TERM...JPT/SBK SHORT TERM...JDL LONG TERM...JDL AVIATION...JPT