Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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831 FXUS62 KCHS 261254 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 854 AM EDT Thu Sep 26 2024 ...TORNADO WATCH IN EFFECT FOR ALL LAND AREAS AND THE ADJACENT COASTAL WATERS THROUGH 9 PM... .SYNOPSIS... Hurricane Helene is forecast to move inland across portions of Florida and Georgia later today into Friday. Multiple impacts are expected across our area today and tonight. Drier weather then arrives later Friday and persists into next week. && .NEAR TERM /THROUGH TONIGHT/... Today and Tonight: Aloft, a ridge will be centered far off the Southeast Coast while a large cutoff low remains over the middle Mississippi River Valley. At the sfc, high pressure extending south along the East Coast and into the Southeast will retreat offshore in advance of Hurricane Helene, which is forecast to quickly track north across the eastern Gulf of Mexico during the day, then make landfall as a Major Hurricane across the Florida Panhandle around nightfall. Given the speed of Helene, strong winds are expected further inland than typical. Additionally, wind fields will expand as Helene gains latitude, likely producing impacts well east of its center, which is forecast to track north across Central Georgia. For this reason, a Tropical Storm Warning remains in effect for the entire forecast area today and through the night, including across nearby coastal waters. The latest forecast will keep Southeast Georgia and Southeast South Carolina along Helene`s eastern side throughout the day and night, bringing multiple hazards to the immediate area. For full details, please refer to weather.gov/chs and the Hurricane Local Statement (HLS). Tornadoes: Perhaps the greatest concern in regards to impacts associated with Helene will be the risk for tropical tornadoes across the entire area, especially given the local area remains situated in the favorable northeast quadrant of Helene for quite some time today and tonight. Guidance continues to support conditions favorable for tornadoes, with long and looping hodographs, ample buoyancy along a northward shifting CAPE axis (SBCAPE 1500-2000 J/kg) extending from the nearby Atlantic and onshore, sfc temps in the upper 70s to lower 80s, and 0-3 km SRH values near 300-400 m2/s2. Additionally, the latest HREF run gives a fairly strong indication of updraft helicity tracks supportive of tornadic waterspouts that shift onshore, coinciding with rainbands associated with Helene this afternoon and the night. The latest SPC Day 1 Convective Outlook does include an Enhanced Risk for severe weather across the majority of the local forecast area, with 10% tornado probabilities driving the bulk of the risk. Have a tornado plan in place in case a warning is issued, but keep in mind that tornadoes of this type can form with little advance warning and move very quickly (potentially in excess of 45 to 55 mph). Winds: A Tropical Storm Warning is in effect for the entire forecast area. Based on the 5 AM NHC advisory, the Earliest Reasonable Arrival Time of Tropical Storm Forecast Winds (39+ mph) in our area is mid to late afternoon across Georgia, and early evening across South Carolina. The Most Likely Arrival of these winds is early evening across Georgia and during Thursday night across South Carolina. Gusty winds as high as 40 to 60 mph could produce downed and/or uprooted trees, isolated to scattered power outages locally, with greatest impacts anticipated during the night. Flooding Rainfall: Multiple rounds of showers/rainbands in association with Helene will begin to impact the area this morning, initially across Georgia counties, then spread north across South Carolina counties during the afternoon. These showers along with a few thunderstorms will then persist through the night, before considerably drier air wraps around the southern edge of departing Helene and overtakes the entire forecast area by around daybreak Friday. Rainfall totals are mainly in the 1.5 to 3 inch range, but with locally higher amounts possible in heavy rainbands. The risk currently does not support the issuance of a Flood Watch, especially with dry soils in many places per NASA Sport guidance. That said, area rivers and creeks will rise in response to the rainfall, with some flooding possible, even after the rains come to an end. Storm Surge: The latest forecast has storm surge values in the 1 to 3 foot range. As such, coastal flood products might be required at a later time. Additional information can be found in the Tides/Coastal Flooding section below. && .SHORT TERM /FRIDAY THROUGH SATURDAY/... Strong upper low lingers near the Tennessee Valley through the end of the week. Attendant surface low/remnants of Helene remain quasi- stacked despite trending more baroclinic, but steadily fills. A few lingering showers Friday morning will quickly diminish by midday Friday, with strong subsidence and mainly dry conditions persisting across SE SC/GA thereafter. Breezy S to SW winds Friday morning trend moderate by Friday afternoon, with humid conditions persisting into the weekend and temps remaining very close to normal. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... A mid and upper low initially over the Ohio Valley will eventually open into a wave as it reaches the Mid-Atlantic coast Tuesday. Forcing for ascent remains north of the area, with subsidence aloft persisting locally, keeping a mainly dry forecast in place. At the surface, weak ridging persists early in the week, with a cold front (which appears mainly dry) crossing mid to late week. Temps generally near to a few degrees below normal through the period. && .AVIATION /13Z THURSDAY THROUGH MONDAY/... VFR conditions will persist through about 14-15Z at CHS and JZI terminals. Otherwise, showers and potentially a few thunderstorms will continue to spread north across the region today, resulting in at least prevailing MVFR conditions at CHS/JZI/SAV terminals through 12Z Friday. There is some potential for IFR conditions to develop at all terminals after 00Z Friday, but currently low-end MVFR conditions are forecast. In addition to moderate and possibly heavy showers, wind speeds will noticeably increase this afternoon and especially late afternoon into the evening as Helene gains latitude and passes inland. By 02-04Z Friday, southeast winds gusting up to 35-40 kt should prevail at all terminals, beginning at the SAV terminal first. Gusts could top out near 45-50 kt at SAV tonight by the end of the TAF period. LLWS will likely be needed at all terminals at some point starting around 00Z Friday. Future TAF issuances will likely address this issue. Extended Aviation Outlook: Extended Aviation Outlook: Prevailing VFR, though with persistent elevated low level moisture and a strong subsidence inversion, will need to monitor for morning radiation fog and afternoon MVFR strato- cu potential through the long term. && .MARINE... Today and Tonight: Dangerous marine conditions will occur as Helene strengthens across the eastern Gulf of Mexico during the day, then moves onshore across the Florida Panhandle around nightfall and continues north across Central Georgia during the night. Helene is a large tropical cyclone with wind fields that are expected to expand wide as the cyclone gains latitude, favoring impacts well to the east. Tropical Storm Warnings will remain in effect for all local waters as a result. Winds as high as 45-50 kt will start impacting the Georgia waters this evening, then spread into the South Carolina waters overnight. Seas will build as high as 10-15 feet within 20 nm from shore, and up to 17 feet across the outer Georgia waters tonight. Mariners are also alerted to the risk for waterspouts this afternoon and through much of the night. Some of these will be tornadic waterspouts, moving much quicker and producing much stronger winds than most waterspouts in our area. Very large medium period swell associated with the large wind field of Helene peaks Friday morning, gradually subsiding into the weekend. Near Tropical Storm Force winds early Friday morning will subside to more moderate winds by Friday afternoon as well. Light to moderate winds persist Friday night into early next week as high pressure moves overhead and more seasonable 2-3 ft seas prevail. Rip Currents and High Surf: Conditions at the beach will be dangerous today through Friday, as winds and seas build in response to Hurricane Helene tracking across the northeast Gulf of Mexico and into the interior of the Southeast. A High Risk of Rip Currents will be in effect for all beaches both days, and breakers will reach at least 5 feet in height starting this afternoon and persisting through at least Friday afternoon or evening. Areas of significant beach erosion are also likely during this time. && .TIDES/COASTAL FLOODING... Strengthening onshore winds will lead to a continuation of the elevated tides today, and minor coastal flooding is likely along the Charleston and Colleton County coasts with the afternoon high tide. The threat for coastal flooding is then expected for the entire coast through tonight and early Friday due to impacts from Hurricane Helene. Storm surge inundation of 1 to 3 ft is possible along the coast of southeast Georgia and Southern South Carolina. Keep in mind that if the higher surge and/or heavy rains occur near or during the time of the high tides, the coastal flooding could be worse. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Tropical Storm Warning for GAZ087-088-099>101-114>119-137>141. High Risk for Rip Currents until 8 PM EDT this evening for GAZ117- 119-139-141. High Surf Advisory from 2 PM this afternoon to 5 PM EDT Friday for GAZ117-119-139-141. SC...Tropical Storm Warning for SCZ040-042>045-047>052. High Risk for Rip Currents until 8 PM EDT this evening for SCZ048>051. High Surf Advisory from 5 PM this afternoon to 8 PM EDT Friday for SCZ048>051. MARINE...Tropical Storm Warning for AMZ330-350-352-354-374. && $$ NEAR TERM... SHORT TERM...CEB LONG TERM...CEB AVIATION...CEB/DPB MARINE...