183 FXUS62 KCHS 071456 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 956 AM EST Mon Jan 7 2019 .SYNOPSIS... High pressure will extend across the region through tonight as a weak coastal trough dissipates offshore. A cold front will move through the area Tuesday night. Dry and colder high pressure will build over the area mid to late week. Periods of unsettled weather are expected to develop over the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Mid and upper ridging will prevail aloft, while a strong and elongated ridge of surface and low level high pressure covers the entire eastern seaboard, from southeast Canada to the Gulf of Mexico. Meanwhile, a weak coastal trough not too far off the coast will meander over the Atlantic, stuck across the cooler shelf waters. But this feature will be close enough, that along with increasing isentropic ascent along mainly the 290K and 295K surfaces will generate an increase in stratocumulus. Along with scattered mid and high level clouds moving through aloft, we have increased cloud cover a bit further the previous forecast. Still, nothing worse than partly or mostly sunny skies. Any late day showers due to the proximity of the trough will stay over the ocean. Although we have maintained the previous max temp forecast in the lower 70s inland, we are concerned that with an onshore fetch this might be a little too warm. We did trim back temps a couple of degrees along the coast however. && .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/... Tonight: The H8-H5 ridge will move quickly east tonight as a shortwave pivots across the Tennessee and Ohio Valleys. Weak high pressure hold its influence on the region through daybreak as the primary high shifts well out into the Atlantic and a secondary anticyclonic center reforms along the northeast gulf coast. The subtle coastal trough offshore will dissipate after midnight as the synoptic flow becomes more southerly with time. Any shower activity associated with the coastal trough should remain offshore as guidance is similar in showing any weak moisture transport and low-level convergence being directed to the upper South Carolina coast and Cape Fear region. A thick band of cirrus will move into the area overnight as a weakening branch of the subtropical jet propagates across the lower Mississippi Valley and into the Deep South. Although the boundary layer will likely decouple early resulting in calm/light winds within a baggy pressure field, thicker cloud canopies will result in warmer overnight temperatures with lows only dropping into the lower-mid 50s. Expect some patchy fog to develop after midnight as 1000 hPa condensation pressure deficits lower, but thicker clouds aloft should prevent the fog from becoming too significant. Will highlight patchy fog along/west of the I-95 corridor coincident with the axis of lowest 1000 hPa condensation pressure deficits. Tuesday: The last of days topping 70 degrees this week should occur across the region. There could be quite a bit of high level clouds around for parts of the morning, then high clouds will either thin or move offshore for the afternoon. Despite some late day mid level pressure falls, deep west flow and low level compression will allow highs to reach the lower to mid 70s. A dry cold front will pass through Tuesday Night with light W-NW breezes through the night and lows in the mid to upper 40s. Wednesday: A low level cold air advection pattern setting up in the wake of the cold front as an upper trough deepens across the eastern seaboard. Temps will be cooler despite abundant sunshine, highs expected to reach only the upper 50s northwest tier to a few mid 60s along South Coastal GA. It should be a fairly breezy day across the area with a 5 MB surface pressure gradient across the forecast area during the afternoon with a mixed atmosphere below a deep subsidence inversion. Cold air advection will continue Wednesday night, driving lows into the lower to mid 30s. Thursday: A dry air mass will blanket the region but temps will run at least 6-7 degrees below climo as low level thermal profiles remain chilly during the day. Highs will range from a few upper 40s well north to perhaps a few mid 50s near the Altamaha River area. There could be some high clouds later in the day but sunshine looks to prevail with light north winds. && .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... High pressure will settle over the Carolinas on Friday with dry weather and not too much moderation in the chilly temps. A coastal trough is expected to develop Friday night into Saturday as cold surface high pressure remains anchored across the Mid Atlantic region. Quite a bit of discrepancy lingers between the GFS and ECMWF regarding the strength of a short wave crossing the Lower Gulf Coast and thus we retained just chance POPs this weekend. More certain at any given time will be cloudy skies and below normal temps with persistent inland wedging and chilly northeast winds. High pressure should return for early next week with temps likely remaining below climo for the most part. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... VFR. Extended Aviation Outlook: VFR. Gusty surface winds from the NW are expected at times on Wednesday in the wake of a dry cold front. && .MARINE... Today: The waters will remain under elongated high pressure from southeast Canada to the Gulf of Mexico, with the anticyclonic flow around the high interrupted somewhat by an inverted trough along the edges of the shelf waters. This will keep winds more NE in closer to shore, and E or E-SE further out. Wind speeds on average will be no more than 10 or 12 kt, and seas at most are 1 to 2 ft. Tonight: Winds will turn move uniformly southerly, then southwesterly late as the coastal through lifts north and dissipates. Speeds will generally average 5-10 kt nearshore and closer to 10-15 kt over the Georgia offshore waters with seas 1-2 ft with a few areas of 3 ft seas possible in the northeast portions of the Charleston County nearshore waters, mainly 15-20 NM offshore. Tuesday through Wednesday: High pressure will give way to a cold front that moves through Tuesday night, and is followed by strong continental high pressure Wednesday. Cold advection, a tighter gradient and bouts of large pressure rises will generate strengthening winds behind the front, and Small Craft Advisories could become necessary late Tuesday night and Wednesday. Wednesday night through Friday: Small Craft Advisories will likely be ongoing at the start of the period, before at least some improvement occurs late Thursday into perhaps early in the day on Friday. Friday night and Saturday: A developing coastal trough and strong inland wedging will likely result in an uptick in northeast winds during this period. Speeds should increase to at least 15-20 kt with seas building to 4-5 ft. The pattern suggests a potential for pinching pressure gradients and SCAs cannot be ruled out during this time period. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...