871 FXUS62 KCHS 180910 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 510 AM EDT Fri May 18 2018 .SYNOPSIS... Low pressure will linger just west of the area through Saturday. Atlantic high pressure will then prevail into the middle of next week before low pressure likely returns later in the week. && .NEAR TERM /THROUGH TONIGHT/... As of 5 AM: Over the past hour, IR satellite image indicated cooling cloud top temperatures associated with the convection off the GA coast. Regional radar composite detected a converging area of showers across the marine zones, tracking north at 25 kts. Based on radar/satellite trends and latest run of the HRRR, I will expand likely PoPs across the SC Lowcountry by 11Z. In addition, I will increase QPF to indicate around a half inch across the coastal SC counties. Thunderstorm mention will be limited to isolated at this time. As of 4 AM: The GOES-16 mid level water vapor indicated a closed low across the central CONUS with a large plume of tropical sourced moisture across the eastern U.S. and western Atlantic. The water vapor product indicated a vort max over northern Florida, tracking to the north. The environment across much of the SE U.S. will feature widespread weak to moderate instability, low LFC, and deep moisture through the near term. Given the environment and the track of the vort max, I would expect convection to develop well off the GA coast this morning, streaming north over the SC Lowcountry this morning and afternoon. I will configure PoPs to range from likely across the CHS Tri-county to solid chance across inland GA through today. Based on pre-dawn satellite and radar composite, I may increase PoPs and QPF across the SC Lowcountry if convection develops as expected. High temperatures are forecast to range from around 80 across the SC Lowcountry to the low 80s across SE GA. Tonight, guidance suggests that drier air sourced from the lower Mississippi River Valley may drift over the western forecast area overnight. Showers and thunderstorms may remain isolated to scattered overnight. However, the greatest coverage is expected to drift east over the Atlantic and the NC coast. I will indicate a PoP gradient of SCHC across inland GA to likely across the Charleston Co. waters. Low temperatures are forecast to range from the upper 60s inland to around 70 along the coast. && .SHORT TERM /SATURDAY THROUGH MONDAY/... The area will remain on the western fringes of Atlantic high pressure centered to the east. An upper trough will linger to the west Saturday before weakening Sunday and Monday although an upper low looks to develop near northern Florida/southern GA Monday. This pattern will mean the deep feed of tropical moisture will linger over or near the area most of the time and with some upper shortwave energy around this should translate to higher than normal rain chances. However, chances should generally diminish each day. Overall no significant severe storms and/or flooding are anticipated. High temperatures should be near to even a bit above normal while low temperatures remain above normal. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... Upper low pressure will linger mainly just west of the area through the period. By the end of the period surface low pressure could develop near the western Caribbean Sea and track northeastward toward Florida. Way too early to tell if this low will become a tropical cyclone but we'll definitely keep an eye on it for a potential track near the Southeast U.S. Coast next weekend since the GFS and ECMWF have been pretty consistent for a while now suggesting such potential. In general it looks like temperatures and rain chances will stay fairly close to or even slightly above normal. && .AVIATION /09Z FRIDAY THROUGH TUESDAY/... MVFR ceilings are expected to continue across KCHS through most of the pre-dawn hours. KSAV ceiling forecast is more challenging for this morning. KSAV observations leading up to the 6Z TAF has remained VFR. However, the majority of guidance indicates that conditions were forecast to be at least MVFR. However, I will highlight a TEMPO period from 10Z-14Z with MVFR ceilings. Cyclonic mid level flow, widespread instability, and low LFC should yield scattered thunderstorms during this afternoon and evening. I will include a PROB30 at both terminals for thunderstorms. Otherwise, TAFs will feature south winds between 10-15kts and LVFR cloud bases. Extended Aviation Outlook: High chance of restrictions at times from low clouds and rain, mainly each afternoon. && .MARINE... Today through tonight, steady south winds will be supported by high pressure centered near Bermuda and low pressure over western TN. Using a blend of guidance, speeds are expected to range from 10 to 15 kts. Gusts may approach 20 kts across portions of AMZ350 and AMZ374. Wave heights are not expected to vary much given the steady south winds. Heights across AMZ350 and AMZ374 are forecast to range from 4 to 6 ft, Small Craft Advisories will remain in effect. Conditions are expected to improve slightly, likely allowing the Small Craft Advisory to expire at 8 AM. Otherwise, an approaching mid level disturbance will likely result in increase coverage of showers and thunderstorms across the marine zones this morning and afternoon, then shifting east tonight. It is possible that one or two storms may produce gusts around 34 kts. Saturday through Wednesday: The area will generally remain between low pressure to the west and high pressure to the east through the period. Although there will be some wind surges, mainly each afternoon near the coast due to the sea breeze, no significant winds are expected. However, seas should meet Advisory levels near the Gulf Stream through Saturday as the pressure gradient remains elevated enough. Rip Currents: Lingering swell combined with a modest onshore flow will support a moderate risk for rip currents at all beaches Friday. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory until 8 AM EDT this morning for AMZ352. Small Craft Advisory until 8 AM EDT Sunday for AMZ350-374. && $$ NEAR TERM...NED SHORT TERM...RJB LONG TERM...RJB AVIATION...NED/RJB MARINE...NED/RJB