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Area Forecast Discussion
576 FXUS62 KCHS 310023 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 823 PM EDT TUE AUG 30 2016 .SYNOPSIS... Tropical Depression Nine is expected to intensify into a tropical storm and track from the eastern Gulf of Mexico northeast across northern Florida and off the Southeast U.S. coast through Friday. Cooler and drier high pressure will then return from the north this weekend and persist into early next week before another area of low pressure possibly develops offshore. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... The latest radar indicates that most of the convective activity has ended over the land and only isolated showers and thunderstorms currently going over the Atlantic waters. Models continue to show precip chances increasing overnight over the coastal waters within a coastal trough of low pressure. These showers may spread into the coastal counties later tonight and into the pre-dawn hours. Have maintained the highest pops over the waters with chance pops over the coastal counties, mainly after midnight. Lows in the lower 70s most areas. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/... A weak low pressure system off the Southeast U.S. coast will deteriorate as it moves off toward the northeast through the day on Wednesday. The strongest destabilization and forcing in the area of the low will mainly remain offshore, though its proximity to the coast will keep the potential for showers and thunderstorms in the forecast for mainly Charleston County and the immediate coast of Southeast South Carolina and Northeast Georgia. The highest moisture values will remain off the coast with the low Wednesday, so the flooding threat appears low other than low-lying areas at high tide. Moving into Thursday and Friday, the primary challenge is nailing down Tropical Depression 9`s impacts on the area. 12Z GFS and EURO deterministic runs, as well as the majority of the GFS ensemble runs, have trended the storm further north/closer to the coast as it moves back into Atlantic waters Friday. Have increased QPF across the area to account for this, with storm total rainfall amounts near the coast now in the 4-6 inch range. While wind impacts from this storm remain uncertain due to significant timing and strength differences between models, there is growing confidence in flooding being a threat with this storm (WPC has the immediate coast under a marginal risk for excessive rainfall Thursday), especially if heavy rainfall occurs around high tide in Downtown Charleston. Please continue to monitor the latest forecasts as the situation will become more clear in the coming days. Northerly low and mid level flow will fill in as early as Friday afternoon as the storm moves toward Cape Hatteras, bringing cooler and drier conditions to the area. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... The latest forecast from the NHC indicates a tropical storm will be moving off the northeast FL coast overnight Thursday and then staying offshore as it moves away from the GA/SC coasts through late week. This will be followed by high pressure building from the north this weekend into early next week. There is still quite a bit of uncertainty regarding the track/strength of the tropical cyclone so it is best to stay tuned to the latest forecast. Cooler and drier conditions should then prevail for the weekend into Monday, although a few showers can`t be ruled out from time to time mainly near the coast. By Tuesday another area of low pressure could develop offshore and head toward the coast. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Tonight: Expect VFR to prevail much of the night. Increasing chances for showers and isolated thunderstorms after 06z...but have kept VFR through the night for now. Mesoscale models indicate better chances for showers and some thunderstorms closer to KSAV tonight. Extended Aviation Outlook: Ceiling and visibility restrictions are possible Wednesday and Wednesday night at both KCHS and KSAV as showers and thunderstorms mainly offshore may approach the terminals. Additional impacts from showers/thunderstorms and winds are likely Thursday night into Friday, depending on the track of Tropical Depression 9 which is forecast by the NHC to be a tropical storm as it passes northeast off the GA/SC coasts. && .MARINE... Tonight: Have made only cosmetic changes to the forecast. Still expect Small Craft Advisory conditions over the offshore GA waters to continue, mainly for seas of 5 to 7 feet. Wind speeds expected to increase later tonight, especially south of the Savannah River, where the pressure gradient looks a little stronger on the west/northwest side of the coastal trough. Wednesday through Sunday: Weak low pressure off the Southeast coast will bring easterly winds in the 10 to 15 kt range, with mainly 3 to 5 foot seas Wednesday and Wednesday evening. Conditions will deteriorate rapidly early Thursday mornings as a tropical storm is forecast by the NHC to move off the GA/SC coasts. Tropical Storm force winds and seas as high as 5 to 10 feet are possible across the local waters beginning Thursday night and lasting into Friday. There is still some uncertainty in the strength/track of the tropical cyclone so mariners should stay tuned to the latest forecasts. Rip currents: Numerous rip currents were reported at area beaches today. High risk of rip currents continues for Wednesday due to solid groundswell from Gaston, persistent onshore winds, and high tidal ranges. Long period swells and onshore flow are expected to persist, thus an enhanced risk will continue through at least Friday. && .TIDES/COASTAL FLOODING... The latest tide observations for Charleston Harbor indicate that the water levels will stay just below Coastal Flood Advisory levels this evening. Have decided to go ahead and cancel the Coastal Flood Advisory for this evening. Still expect tides to remain elevated due to ongoing east/northeast winds, long period swell and influences from the approaching new moon through much of the work week. The extratropical surge guidance suggests evening high tides through Thursday will be very close to levels which would produce shallow coastal flooding, especially along the lower South Carolina coast. More Coastal Flood Advisories may eventually be needed. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through Wednesday afternoon for GAZ117-119-139-141. SC...High Risk for Rip Currents through Wednesday afternoon for SCZ048>051. MARINE...Small Craft Advisory until noon EDT Wednesday for AMZ374. && $$ SYNOPSIS... NEAR TERM...RFM SHORT TERM... LONG TERM...RJB AVIATION... MARINE... TIDES/COASTAL FLOODING...