789 FXUS62 KCHS 162324 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 724 PM EDT Sat Jul 16 2022 .SYNOPSIS... A surface trough will linger near the area for much of this week. High pressure will build in next weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Lingering outflow boundaries from convection, plus the sea breeze will keep a couple of stray showers and maybe one or two t-storms going through 9 pm and mainly far inland, before completely dissipating. Meanwhile, additional convection will develop across the Atlantic, mainly out near the Gulf Stream where the instability and moisture is better than elsewhere. The HRRR continues to depict a scenario similar to what the previous forecaster mentioned, sending an outflow into the local waters from the south. However, recently that model has slowed this down a bit, which is more in line with the HREF. Given low level moisture convergence, weak isentropic ascent and a shear axis aloft, we still anticipate an enhancement of convection from the ocean, and moving onshore at the least the coastal counties after midnight. We maintain slight chance/chance PoPs, but depending upon later radar trends, adjustments could be required. Temps will be similar to or a tad warmer than last night. Previous discussion... Latest SPC mesoanalysis indicated that MLCAPE across the forecast area ranged between 1500-2000 J/kg with little to no CIN. KCLX detected a weak fine line with isolated showers just inland of the coast, drifting inland. This feature is likely associated with a sea breeze. Given the instability ahead of the sea breeze, convection should gradually increase to scattered coverage, aided by development near outflow boundaries. The convection over SE GA and the SC Lowcountry is expected to dissipate within and hour or two following sunset. GOES-East middle level water vapor showed a well developed vort max over the Gulf Coast of the FL Panhandle this afternoon. Near term guidance indicates that this feature will amplify later tonight, drifting west. HREF indicates that west of the mid-level vort max, PW values will build across the GA/SC coast, reaching 2 inches by daybreak Sunday. A wide field of 1000-1500 J/kg will linger over the Atlantic through the overnight hours. The combination of the building moisture, persistent instability, and weak synoptic forcing may favor convection developing over the marine zones, pushing onshore around dawn Sunday. Interestingly, the latest HRRR indicates that a large outflow boundary sourced from FL convection will push northward tonight, resulting an arching band of showers and thunderstorms reaching the GA coast after 6Z tonight. The HRRR solution appears too early, but the forecast will show PoPs building from the Atlantic during the pre-dawn hours, especially over the GA coast. Low temperatures are forecast to range from the low 70s inland to the mid to upper 70s along the coast. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... A weak upper trough will remain to our west through Tuesday while the Bermuda high remains well offshore. Ample moisture will continue to stream in from the Atlantic with PWs remaining at or above 2". A surface trough could develop over the area on Monday, then an upper shortwave will drop in from the northwest Tuesday afternoon. Between these features and a moderate sea breeze each day, there should be ample forcing for scattered to numerous showers and tstms. The best coverage should be in the afternoon and evening. Temps will gradually climb back to near normal on Tuesday. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As the upper ridge builds across the central US, the upper trough will shift east over the area for mid/late week. At least scattered mainly diurnal showers/tstms expected each day. The deep layered ridge could build into the area Saturday at the earliest. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... Conditions will stay VFR through at least much of the night, with some risk for convection and flight restrictions to arrive closer to daybreak. This would be in association with activity moving onshore from the ocean. Since our confidence on this happening is rather low, we opted not to show anything with the 00Z TAFs. There does look to be a better chance for convection on Sunday, especially during the late morning through mid afternoon. This occurs with the interactions of mesoscale boundaries, increasing PWat, short waves aloft and better instability due to diurnal heating. We have VCSH and VCTS at the terminals to account for this, and later TAFs can refine the forecasts if necessary. Extended Aviation Outlook: Periodic flight restrictions are possible at KCHS/KJZI/KSAV during showers/thunderstorms each afternoon/evening. && .MARINE... The marine zones will remain under a weak pressure gradient on the west side of a Bermuda high. This pattern should support light south to southwest winds through the night. Seas are expected to range between 2-4 ft. Thunderstorms will increase across the coastal waters after midnight, and some of these will be strong, producing 30 or 40 kt wind gusts, heavy rains and frequent lightning strikes. Marine Weather Statements and/or Special Marine Warning will likely be required. Sunday through Thursday: Bermuda High pressure will persist across the Atlantic, maintaining persistent southerly flow, becoming predominately southwest by Monday. As broad low pressure shifts across the Midlands Sunday into Monday we expect winds to become steady around 10-15 knots with seas 2-4 feet through the period. Gusts up to 20 knots are expected across the local waters, yet conditions should remain below Small Craft Advisory criteria. Waterspouts: Light winds could favor a waterspout risk Sunday morning. && .TIDES/COASTAL FLOODING... Tidal departures are running about 0.3-0.4 ft MLLW higher than at a similar time last night. However, astronomical tide levels are close to 0.2 ft MLLW higher than 24 hours prior. We need a departure of 0.77 ft MLLW to achieve minor coastal flooding in downtown Charleston with the 1130 PM high tide. This seems somewhat unlikely, but we thought so last night, and we reached 7.03 ft MLLW. We will continue to closely monitor. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM...JRL LONG TERM...JRL AVIATION... MARINE... TIDES/COASTAL FLOODING...