Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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041 FXUS62 KCHS 240603 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 203 AM EDT Tue Sep 24 2024 .SYNOPSIS... High pressure will linger across the region through Wednesday. A tropical cyclone is forecast to track northward across the Gulf of Mexico, then inland across portions of Florida, Georgia, and possibly South Carolina. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Tonight: Although an isolated shower can not be ruled out across far inland locations overnight, expect the vast majority of the local area to remain dry with a mid-lvl ridge placed directly over the region and broad sfc high pressure extending over land from the western Atlantic. Some guidance suggests that low stratus could spread into portions of the Charleston Tri-County near sunrise. Additionally, some patchy fog could develop across some areas in southeast South Carolina and southeast Georgia near sunrise and persist for an hour or so after daybreak. No significant visibility reductions are expected though. Low temps should range in the low to mid 70s, warmest along the coast. && .SHORT TERM /6 AM THIS MORNING THROUGH THURSDAY/... On Tuesday, H5 ridge axis is expected to slide east across the forecast area during the day. Forecast soundings indicate weak lapse rates under the ridge, limiting the convective potential. The forecast will generally remain dry, with SCHC PoPs across portions of the CHS Tri-county. High temperatures should remain around 90 degrees. Wednesday, the forecast area will remain under the influence of the H5 ridge centered over the western Atlantic. At the sfc, high pressure centered over the Mid-Atlantic States is expected to ridge south across the region. It is possible that a sea breeze could trigger isolated showers during the afternoon. However, at this time, the forecast will remain dry. High temperatures should remain in the upper 80s. Thursday: Details of the forecast will largely depend on the intensity, placement, and timing of Potential Tropical Cyclone (PTC) Nine sourced from the Gulf of Mexico. See latest NHC Tropical Weather Outlook for latest information on PTC 9. Based on the collaborated low position, the center of the system is expected to reach N. FL by Thursday afternoon. The circulation associated with the system will remain quite large. In fact, Tropical Storm force winds could reach the GA waters by Thursday afternoon. GFS and ECMWF indicates that the leading edge of a rain band will lift north, pushing across SE GA in the morning, north of the Savannah River in the afternoon. In addition, onshore winds could lead to an increasing tidal departure, with some coastal flooding possible with the high tide Thursday afternoon. It is possible that some excessive rainfall issues may develop with any heavy rainfall that coincides with high tide. High temperatures will be limited to 80 to 85 degrees. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Thursday night, PTC 9 will pass between a H5 ridge centered over the western Atlantic and a closed H5 low over the MississippI River Valley. PTC 9 should accelerate across FL and GA Thursday night. The circulation associated with the system will cover the entire forecast area Thursday night. Given the expected wind fields and rising values of tropical storm force wind probs, it is expected that tropical headlines will be needed for portions of the area. Tornadoes: GFS shows the circulation from the system drawing instability sourced from the Gulf Stream across SE GA/SC Thursday night. Forecast soundings show the CAPE across the coastal counties will surge to 500-1000 J/kg with LI as low as -5. Interestingly, EHI values along the coast peak between 1-3 units by 6Z Friday. Hodographs indicate a strongly sheared environment, with 0-1 km SRH in excess of 500 m2/s2. This environment appears very suitable to fast-tracked tornadoes, both over land and water. The system is forecast to begin to pull away from the region on Friday. Storm total rainfall amounts from the system is expected to generally range from 3-4 inches, with locally higher amounts. Ongoing gusty winds combined with the wet ground may result in tree and power line damage. The forecast is even more uncertain for Friday night into the weekend, it is possible that dry air could wrap around the central CONUS closed low and bring drier conditions to the region. Or, some guidance indicates that a strong CAD may develop over the region, with rain, thick cloud cover and cool NE winds. The forecast will keep scattered convection with day to day highs in the 80s. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... VFR conditions are expected to prevail at CHS/JZI/SAV terminals through 06Z Wednesday. However, some shallow ground fog could reduce vsbys to 6 SM late tonight into early morning. Extended Aviation Outlook: VFR conditions expected to prevail through Wednesday. Gusty conditions are possible at the terminals during the end of the week. Potential for periodic flight restrictions are increasing. && .MARINE... Tonight: Broad high pressure will prevail, yielding tranquil marine conditions. Expect south winds 5-10 kt across most local waters. Seas will generally be 1-3 ft, largest across offshore Georgia waters. Tuesday and Wednesday: Broad high pressure offshore will result in southeast winds around 10 kt or less on Tuesday, increasing around 15 kts on Wednesday. Seas will average 1-3 ft for the nearshore waters out 20 NM and 3-4 ft for the outer GA waters from 20-60 NM. Wednesday night through Saturday, there will be an increase in winds due to a potentially developing tropical system in the Gulf of Mexico. Thursday morning, guidance indicates that gusts may range between 25-30 kts with seas build to 5-6 ft within 20 NM and 8-9 ft across the outer GA waters. Tropical Storm conditions expected late Thursday afternoon until Friday morning. Seas are expect to peak between 10 to 13 ft within 20 NM and 14-15 ft over the outer GA waters Thursday night. Conditions will gradually improve late Friday through Saturday. High Surf and Rip Currents: Five foot breakers are possible at the beaches Thursday night. Additionally, an elevated risk for rip currents is likely Thursday and Friday with gusty winds, large breakers, and long period (10 second) swells. && .TIDES/COASTAL FLOODING... Although the astronomical high tides are lowering, minor coastal flooding is expected with the daytime high tide cycle along the Charleston and Colleton County coasts Tuesday, and potentially Wednesday. Strengthening onshore winds could lead to an increasing tidal departure, with some coastal flooding possible with the high tide Thursday afternoon. This threat is expected to continue through the end of the week due to impacts from a tropical cyclone. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...DPB SHORT TERM...NED LONG TERM...NED AVIATION...DPB/NED MARINE...DPB/NED