Area Forecast Discussion
Issued by NWS Charleston, SC

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801 FXUS62 KCHS 251432 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1032 AM EDT Sun Sep 25 2016 .SYNOPSIS... High pressure will build into the area today but will weaken by late Monday ahead of an approaching cold front. The front will cross the forecast area and push off the coast by mid week and high pressure will build into the region by the end of the week. && .NEAR TERM /THROUGH TONIGHT/... Late this morning: The large cluster of offshore convection continues on its diminishing trend and morning stratus is scattering out in response to increasing surface heating. Overall, not much noteworthy change to the forecast for today which is in good shape. Water vapor imagery shows the broad upper low that stretches from the Florida panhandle to the area just off the South Carolina coast. Better moisture is wrapping around this feature, but the 12z sounding still shows considerable mid/upper level dry air and a precipitable water value well below 1.5 inches. High pressure will shift to over New England this afternoon and will attempt to ridge down into the forecast area. The prevailing low level flow will become more onshore through the day and help to increase the low level moisture profile. Model soundings are very unimpressive for today, featuring a prominent warm layer around 800 mb which should serve to cap convective development. We have still maintained the slight chance of showers closer to the Altamaha which will be within the ribbon of best moisture. It will be another warm day with temperatures in the upper 80s and even a few places hitting 90 inland. Tonight: The combination of high pressure moving off the mid Atlc coast and development of a weak wedge pattern inland suggests genesis of a weak coastal surface trough while a weak mid level disturbance is expected to lift north off the florida coast. Models show an increase in isentropic ascent and moisture late and we should see an uptick in coastal showers which could start translating more inland toward daybreak. We maintained 30-20 pop scheme from east to west after midnight. Lows in the upper 60s well inland to lower 70s closer to the coast. && .SHORT TERM /MONDAY THROUGH WEDNESDAY/... Heights aloft will be falling through the period as a large cut-off low slowly drops over the Great Lakes. Troughing overhead will spell increasingly unstable conditions Monday and Tuesday (compared to the weekend). At the surface, Monday will begin with onshore flow over the area around the southern flank of high pressure wedging down the eastern seaboard. This wedge of high pressure will weaken as the parent high pushes further off the New England coast during the day Monday, and our area will become entrenched in pre-frontal warm air advection, with an inverted trough off the coast. Thunder chances maintained over land Monday and Tuesday afternoons thanks to ample moisture and at least modest instability. Unimpressive lapse rates in the mid levels will keep the severe threat low both days. Wednesday`s convective coverage is a bit more uncertain as the axis of highest pre-frontal moisture shifts off the coast during the daytime. Still, have maintained low-end chance POPs Wednesday to account for possible convective enhancement immediately along the approaching front. Nighttime convection over the water is expected given the inverted trough that will be in place. The potential for this convection to bleed back over land justified keeping chance POPs in the forecast for mainly coastal areas through the overnight hours Monday and Tuesday. Despite ample cloud cover and rainfall, temps through the period will be a couple degrees above normal owing to persistent warm air advection. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... The cold front will move through the area Wednesday night, with global models showing the front over the coastal waters by sunrise Thursday morning. However, given the uncertainty in terms of the weakening parent low near the Great Lakes, the front may drag its feet getting through the area, moving over the waters and stalling out well to our south and west by midday Thursday. Either way, strong, dry continental high pressure will fill in behind the front and quiet weather will ensure for the second half of the week. Temps will be near to a few degrees below normal through the period. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... KCHS: VFR today and this evening then late tonight, small chances for low cigs and perhaps some developing showers in association with building moisture convergence and a weak mid level disturbance. KSAV: VFR conditions and dry weather look to be in order today. Tonight, there may be some deteriorating conditions late with small chances for reduced vsbys and slightly better chances for MVFR or even IFR cigs. Forecast soundings at this point giving some mixed signals and confidence is low. There may also be an increasing chance of showers late tonight as a surface coastal trough develops. Extended Aviation Outlook: Flight restrictions will be possible in showers and thunderstorms Monday through Wednesday. && .MARINE... Today: Onshore flow will gradually increase as high pressure builds north of the waters. Speeds should increase to near 15 kt later with seas 2 to 4 ft. A coastal trough is forecast to develop tonight and this could result in a pinching surface gradient over the waters by daybreak on monday. A bit too early to tell if and where the gradient will tighten a bit, we continued to indicate 15 kt and seas 3 to 4 ft for the most part. Monday through Thursday: The gradient will weaken through the day Monday as the high pressure wedge inland weakens. A coastal trough over the waters will keep flow mainly 10 kt or less through midweek, while keeping thunderstorm chances in the forecast, especially during the early morning hours. A cold front will move over the waters Thursday. Seas will generally range 2-4 feet through early in the week thanks to a combination of some lingering swell from Karl in the open Atlantic and local wind swell. Rip Currents: Onshore flow in the 10 to 15 kt range and residual swell from Karl will lead to a moderate risk of rip currents for all beaches Today. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM...CEB LONG TERM...CEB AVIATION...CEB MARINE...CEB

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