141 FXUS62 KCHS 181956 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 356 PM EDT Fri Jun 18 2021 .SYNOPSIS... High pressure will prevail through Saturday. Unsettled weather is expected Saturday night through the middle of next week as a Gulf low moves through, followed by a cold front. Broad low pressure will linger across the region into late next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... The forecast area will remain on the western periphery of high pressure tonight. Meanwhile, a potential tropical cyclone is progged to move onshore in Louisiana. While moisture levels will continue to increase, there is not much in the way of forcing to work with. So aside from any convection near the Altamaha this evening, no rainfall is expected through 12z. Expect mid and high level clouds to increase and thicken. Temperatures will be near normal with lows mainly in the lower 70s, although a bit more mild at the beaches. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/... Saturday: NHC forecast indicates that center of the tropical cyclone will track across southern MS during the daylight hours. H5 shortwave ridging will remain across the forecast area during the morning, with heights falling slightly during the afternoon and evening. Deeper moisture will arrive during the late afternoon and evening, with PW values peaking around 2 inches. Isolated to scattered showers and thunderstorms are forecast to develop by late afternoon, showers and isolated thunderstorms will gradually increase Saturday night. High temperatures are forecast to range from the mid to upper 80s across SE GA to the low 90s over the SC Lowcountry. Sunday: Model agreement with the placement and timing of the low center is quite poor. NHC forecast indicates that the center of the low will track NE across AL and GA. Much of the Deep South and Southeast U.S. will remain under a broad H5 low. The feature of interest is a sfc trough that extends east-southeast from the center of the low to the coast of SE GA and SC. This feature should provide enough llvl moisture convergence to support a band of moderate showers and isolated thunderstorms by Sunday afternoon and evening. The environment will feature PW values between 1.8 to 2 inches, supporting periods of heavy downpours. WPC high highlighted a portion of the CWA with a slight risk of excessive rainfall. Model differences are the greatest with the details surrounding severe weather. GFS indicates that the best overlap of shear and instability will occur across the CWA Sunday afternoon, the NAM is as late as the Midnight to 3 AM time frame. The potential exist for short-lived tornadoes given the shear and instability, SPC has highlighted the region with a marginal risk. Monday: The shield of rain associated with the tropical cyclone will depart to the north. In the wake of the system, winds across the CWA will remain from the SW, pinning the sea breeze to the coast. However, a weak shortwave will pass over the forecast area, triggering showers and thunderstorms during the afternoon and evening. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... The axis of a long wave mid-level trough will ripple over the region on Tuesday. The broad H5 trough is forecast to stall over the East Coast of the U.S. as deep ridging builds over the Great Plains and western Atlantic and persists through late week. A surface cold front is expected to sweep west to east across the CWA Tuesday into Tuesday night. The combination of synoptic scale forcing, PW in excess of 2 inches, and K-index near 40 should support widespread showers and thunderstorms will develop across the SC Lowcountry and SE GA Tuesday afternoon. Scattered to numerous showers and thunderstorms are expected to develop each afternoon and evening under the lingering H5 trough and llvl moisture. Temperatures through the long term should remain a few degrees below normal. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... VFR is expected through 18z Saturday. Any mention of rain would arrive after the valid TAF cycle. Extended Aviation Outlook: Restrictive ceilings may develop across the terminals Saturday night into Sunday morning. Flight restrictions are possible in showers/thunderstorms through the middle of next week. && .MARINE... Tonight: The coastal waters will remain on the western periphery of high pressure. Southerly flow this evening will turn more southwesterly late tonight towards daybreak. Speeds will average 15 knots or less with seas 1-3 feet. Saturday through Wednesday: The pressure gradient will tighten Saturday through Sunday night as low pressure tracks inland over the Deep South and Southeast U.S. The sfc pattern will yield strengthening S to SW winds. Small Craft Advisories could be issued for Sunday night for most marine areas due to a combination of 6 ft seas and 25 kt wind gusts. Conditions should slightly improve by Tuesday or Tuesday night though SCA conditions may persist in some areas until mid-week. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...ETM SHORT TERM...NED LONG TERM...NED AVIATION...ETM/NED MARINE...ETM/NED