692 FXUS62 KCHS 301210 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 710 AM EST Fri Dec 30 2022 .SYNOPSIS... High pressure will prevail today. A cold front will move through Saturday night, followed by high pressure through early next week. Unsettled weather could return for the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... Today: The mid and upper levels remain highly amplified, showing ridging off the eastern seaboard, while a trough moves through the Southern Plains and northern Mexico. Embedded short wave energy within the trough will cause weak low pressure to form across southern Louisiana, while strong Atlantic high pressure holds firm to the north of Bermuda. While the local area is generally within the anticyclonic flow around the high, it is interrupted by an inverted trough across the nearby coastal waters. This trough will attempt to push toward the coast through the day, but looks to really struggle to cross the cooler shelf waters. Isentropic ascent, low level moisture convergence due to the proximity to the trough and a mid level perturbation that moves through will be enough to generate at least isolated to maybe scattered showers. Amounts will be rather paltry given large condensation pressure deficits and considerable dry air above 700 or 750 mb. Recent satellite images, simulated satellite products and cross-sections indicate that stratocumulus will continue to prevail from off the ocean within a moist southeast to south low level flow. The extent of the clouds (which as of this time looks to average about 60-80 percent sky cover) will have a bearing on temperatures. Given warm advection and at least partial sunshine developing, and utilizing a mix of the various MOS guidance (except for the cooler NAM) along with the partial thickness forecast, we arrive at max temperatures mostly 70-75F degrees. Nearby ocean temperatures that are down near 50F degrees will hold the barrier islands down much cooler with a developing northeast-east sea breeze. Tonight: High pressure will retreat further east in the Atlantic, as a warm front develops across northern Florida, stemming from the surface low moving into Mississippi. As the warm front lifts north overnight, a southerly flow will allow deep subtropical moisture to spread into the area. This is evidenced by PWat climbing to near or in excess of 1.5 inches, or just shy of how high it has gotten on record for this time of year. Despite this, the better forcing stays off to the west and northwest of the local area. This keeps the higher probabilities of showers to our west through 6 AM Saturday. We show some showers moving in from the west after midnight, with coverage peaking at 20-40 percent by morning. An upstream band of convection will move quickly east and move into western Georgia by midnight. However, the band will weaken as it continues to draw closer, moving into a more stable environment. The SREF and to some extent the HREF do show fog overnight. But given considerable cloud cover, 925 mb winds climbing to 15 or 20 kt and unfavorable condensation pressure deficits, we opted not to carry any fog in the forecast. Min temperatures will be as much as near 15F degrees above climo. && .SHORT TERM /SATURDAY THROUGH MONDAY/... Saturday: A mid-level trough will progress eastward through the Deep South, likely passing to the north of the forecast area in the evening hours. At the surface a cold front will sweep through the southeastern states, pushing offshore in the overnight hours. Rainfall is expected ahead of the cold front, with showers and possibly a thunderstorm likely by the afternoon. With shear values on the order of 40 to 50 knots and ML CAPE forecast to reach to around 300 J/kg thunderstorms are certainly possible. High temperatures are forecast to reach into the low to mid 70s. Even with the cold front passing through overnight temperatures are not expected to be chilly. The current forecast features overnight lows in the mid to upper 50s. Sunday and Monday: Zonal flow to slight ridging will develop in the upper levels while high pressure builds in at the surface. Subsidence from the building high pressure will result in a dry forecast through the period. Downsloping flow in the upper levels will likely result in high temperatures several degrees above normal. Both days feature high temperatures in the low to mid 70s. Overnight lows Sunday should only dip down into the upper 40s inland, with low 50s elsewhere. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... Broad ridging will prevail aloft through midweek while a trough over the Midwest progresses northeastward. A low pressure system is forecast to form associated with this feature passing over the Great Lakes late week. A cold front is expected to push through the forecast area late Tuesday or Wednesday bringing the region it's next shot of rainfall. Above normal temperatures will likely return to near normal after the front moves through mid-week. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... A nearby trough in the Atlantic and a moist onshore flow will produce VFR or possibly brief MVFR ceilings at the terminals with the 12Z TAFs, along with a small potential for showers into late morning and early afternoon. There might be some fog concerns late tonight, either from stratus build down and/or warm and humid air moving in from the south late. Extended Aviation Outlook: Flight restrictions are possible Saturday with showers/thunderstorms. Otherwise, prevailing VFR. && .MARINE... Today: A large and robust area of high pressure centered north of Bermuda will move little, while an inverted trough likely gets hung up near the cooler shelf waters of the local marine area. With sea breeze influences and the clockwise flow around the high, winds will be mostly NE or E at around 10 kt. Seas will average 2-4 feet. Tonight: The coastal trough looks to become a part of a warm front that lifts into the area from the southwest late at night. This happens in response to low pressure lifting from Louisiana into southern Mississippi and Alabama, while the Atlantic high pulls further east. Winds look to be easterly around 10 kt or so, with seas of 3-4 feet. There are indications of some fog trying to form after midnight, maybe advecting in from south of the area. This depends on the northward extent of the warm front, so for now we did not include in the forecast. Saturday through Wednesday: Southerly winds across the marine waters Saturday morning will shift to the SW as a cold front pushes through the region. A surge in wind speeds is expected, with 15-18 knots and gusts to around 20 knots. Seas are forecast to build, with 6 ft waves possibly impacting the offshore Charleston waters and the 20 - 60 nm offshore GA waters. A short-lived Small Craft Advisory may be needed. Winds are expected to ease Sunday as high pressure builds into the region, with winds generally less than 10 knots and seas averaging 3 to 4 feet. Another surge in winds and seas is expected midweek as another cold front passes through. Wind gusts could approach 25 knots and seas could build to 6 ft. Small Craft Advisories may be needed for all marine zones. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...