Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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969 FXUS62 KCHS 251212 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 812 AM EDT Wed Sep 25 2024 .SYNOPSIS... High pressure will linger across the region today. Tropical Cyclone Helene is forecasted to track northward across the Gulf of Mexico, then move inland across portions of Florida and Georgia Thursday into Friday. Multiple impacts are expected across our area Thursday and Thursday night. Drier weather then arrives later Friday and persists into the weekend. && .NEAR TERM /THROUGH TONIGHT/... Today: Aloft, a ridge centered off the Southeast Coast will become further displaced offshore while a large trough begins to cutoff near the middle Mississippi River Valley late day. At the sfc, high pressure placed along the East Coast will extend across Southeast South Carolina and Southeast Georgia for much of the day well ahead of a cold front progressing east across the Deep South. Some hires guidance does suggest showers currently occurring offshore to brush the coast this morning, and perhaps make some progress further inland across Southeast Georgia. Given latest radar trends, have added slight chance pops to the latest forecast to account for this activity. Otherwise, most areas should remain dry into early-mid afternoon hours. By late afternoon, deeper moisture will begin to advect north across inland Georgia, which could lead to a few showers and/or isolated thunderstorm in a marginally unstable environment displaying SBCAPE between 500-1000 J/kg. Increasing clouds throughout the day should limit high temps a few degrees cooler than the previous day. In general, temps should peak in the mid-upper 80s, warmest across the Georgia interior. Tonight: The mid-upper low near the middle Mississippi River Valley becomes cutoff, slowly nudging south with time while the first signs of vort energy traverse the region. At the sfc, high pressure will begin showing signs of being forced offshore late night as deep tropical moisture advances north across the Southeast United States well in advance of Tropical Cyclone Helene making northward progress into the Gulf of Mexico. Given the setup, expect few/scattered showers along with isolated thunderstorms to shift into Southeast Georgia early evening, then become numerous to potentially widespread showers with a few embedded thunderstorms a few hours prior to daybreak as precip activity continues to advance into Southeast South Carolina. Lows should remain mild overnight, generally ranging in the low to mid 70s. Locations near the coast could stay in the upper 70s. && .SHORT TERM /THURSDAY THROUGH FRIDAY/... Thursday: The NHC is forecasting Tropical Cyclone Helene to move northward across the eastern Gulf of Mexico Thursday, reaching the Big Bend coast of Florida late Thursday. The center is then forecasted to pass to our west Thursday night into daybreak Friday. Helene will bring multiple impacts to our area Thursday and Thursday night. For full details, please refer the the HLS. Winds: A Tropical Storm Watch is in effect for our entire area. Tornadoes: As Helene passes west of our region, hodographs rapidly elongate with an impressive amount of instability forecast for a tropical cyclone. Both the GFS and NAM have MLCAPE values at 500 J/kg or greater with Helicity values around 300 m2/s2. Critical angles are also near 90 degrees, which would make most of the vorticity streamwise, or suitable for the development of tropical tornadoes. As such, the SPC has raised our GA counties to a Enhanced Risk for Thursday, with our SC counties remaining a Slight Risk. If models stay consistent, we expect this Enhanced Risk to be expanded further north. Flooding Rainfall: Multiple rounds of showers/rainbands in association with Helene will begin to impact the area Thursday morning (for our GA counties). By Thursday afternoon, showers will have overspread our entire areas. Showers and a isolated rumble of thunder will then persist through Thursday night into early Friday morning. Showers will come to an end across GA by Friday afternoon and SC. Rainfall totals of 2 - 4" are possible for GA and 1.5 - 3" across SC. Storm Surge: Currently surge values are in the 1 - 3 ft range MHHW. As such, coastal flood products might be required. More on this can be found in the Tides/Coastal Flooding section. Friday: Helene will be across northern GA and SC Friday morning with dry air wrapping around the southern flank of the storm. This is due to a cutoff low located near MS/AL which will cause Helene to accelerate north. As this occurs, the main barotropic low will absorb Helene with the system rapidly becoming baroclinic. The absorption of Helene will feature a potent dry conveyor belt on the southern flank of the storm with a weak front expected to pass through Friday morning. The frontal boundary will be more of moisture boundary as PWATs fall from 2.5"+ (in association with Helene) to less than 1". It should also be noted that GFS forecast soundings have a deep mixed layer Friday afternoon with gusts 20 to 30 mph possible at times in the afternoon. A Lake Wind Advisory might be required for Lake Moultrie. Expect high temperatures in the mid 80s. Lows Friday night will mostly be in the upper 60s. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... The closed mid-level low will shift east over the Ohio River Valley on Sunday. Then opening into a wave across the east coast early next week. Isolated to scattered showers and thunderstorms will remain across the forecast area through the period, especially during the afternoon and evening. High temperatures are forecast to range between 80 to 85 degrees with lows between 65 to 70. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... A few showers could drift onshore and impact the SAV and/or JZI terminal this morning, producing brief MVFR conditions. Otherwise, expect VFR conditions to prevail at CHS/JZI/SAV terminals through at least 00Z Thursday with occasionally gusty winds up to 15-20 kt at CHS and SAV terminals between 18Z this afternoon to 00Z Thursday. Thereafter, some showers are expected to arrive from the southwest late tonight, impacting the SAV first by around 08Z and perhaps the CHS/JZI terminals near 12Z Thursday. Prevailing showers and MVFR cigs have been introduced at the SAV terminal at 08Z Thursday. Have opted to keep VFR at CHS/JZI for the 12Z Wednesday TAF issuance. Extended Aviation Outlook: Wednesday night: All terminals VFR with winds out of the southeast. Thursday: Tropical Cyclone Helene will be across the northeast Gulf of Mexico Thursday morning heading to the north/ northeast. Initially, all terminals will be VFR in the early morning hours with high clouds thickening and bases slowly lowering. By mid- morning hours, MVFR conditions are likely at KSAV and KCHS. Transient IFR cigs will then be possible by the afternoon hours as precipitation from Helene spreads over the entire region. Visibilities will also be reduced to IFR or lower at times due to the heavy rainfall. Restrictions in cigs, vsbys, heavy rain, and gusty winds will then persist Thursday evening into early Friday morning. Friday: Tropical Cyclone Helene will quickly start to pull away from the region with a return to VFR conditions. Southwest winds will be gusty at times. Saturday and Sunday: VFR conditions. No precipitation is expected on Saturday with a slight chance of showers and thunderstorms on Sunday. && .MARINE... Today and Tonight: High pressure will prevail across local waters for much of the day, leading to quiet marine conditions. In general, southeast winds will range between 10-15 kt with seas of 1-3 ft across nearshore waters and 3-4 ft across offshore Georgia waters that gradually build into the evening. Overnight, the pressure gradient will become noticeably stronger between high pressure becoming more displaced offshore well in advance of Tropical Cyclone Helene making northern progress into the Gulf of Mexico. This will lead to gradually deteriorating marine conditions through the night, with southeast winds gusting up to 15-20 kt across South Carolina nearshore waters and 20-25 kt across Georgia waters, highest after midnight. Seas will also build, ranging between 3-5 ft across nearshore waters and 5-7 ft across offshore Georgia waters. Thursday through Friday: Expect dangerous marine conditions as a northward moving tropical system in the Gulf of Mexico strengthens. Fine details are likely to change depending on the exact track and strength of the system. At this juncture, guidance indicates ESE gusts could range between 25-30 kt by Thursday morning. The tropical cyclone is expected to track inland and make it closest approach late Thursday night/early Friday, which is when tropical storm force gusts or greater are likely across the local waters. A Tropical Storm Watch has been issued for the SC and GA waters. Tornadic waterspouts will also be possible during this time frame, in addition to bands of heavy rain. Seas are expected to peak between 8- 13 ft within 20 nm and 14-17 ft over the outer GA waters from 20-60 nm late Thursday night. Conditions will begin to improve Friday night. Saturday through Sunday: As the tropical system rapidly weakens to our north, seas will drop to 2-3 ft with westerly winds shifting out of the north around 10-15 kt. High Surf and Rip Currents: Today: Onshore winds and swells with 10 to 11 seconds will develop along the Southeast Georgia and Southeast South Carolina coasts. Local calculation and rip current MOS support a Moderate Risk for rip currents along the entire coast. Breaking waves of 5 feet or greater are likely to occur at the beaches Thursday into Thursday night. A High Surf Advisory will likely be needed for all area beaches. Significant beach erosion is likely to occur during this time. A high risk for rip currents has been posted for the SC/GA coast on Thursday as well. Additionally, an elevated risk for rip currents is possible Friday with lingering gusty winds, large breakers, and long period swells. && .TIDES/COASTAL FLOODING... On Thursday, strengthening onshore winds could lead to an increasing tidal departure, with some coastal flooding possible along the Charleston and Colleton County coasts Thursday afternoon. The threat for coastal flooding is then expected for the entire coast through the end of the week due to impacts from a tropical cyclone. Storm surge inundation of 1 to 3 ft is possible along the coast of southeast Georgia and the South Carolina Lowcountry. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Tropical Storm Watch for GAZ087-088-099>101-114>119-137>141. SC...Tropical Storm Watch for SCZ040-042>045-047>052. MARINE...Tropical Storm Watch for AMZ330-350-352-354-374. && $$ NEAR TERM...DPB SHORT TERM...Haines LONG TERM...Haines AVIATION...DPB/Haines MARINE...DPB/Haines