Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 261424 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1024 AM EDT Mon Sep 26 2016 .SYNOPSIS... High pressure north of the region will weaken today while a surface trough develops just offshore. A cold front will slowly approach through the first half of the week before stalling over the area Wednesday. Low pressure will form over the Mid-Atlantic Wednesday night, ushering the front off the coast Thursday morning. High pressure will build into the region by the end of the week. && .NEAR TERM /THROUGH TONIGHT/...
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Late this morning: The forecast for the day is quite complex and the features around this morning will provide plenty of complicating factors. Overnight convection over the coastal waters has continued to develop and is clearly associated with the upper level disturbance noted in water vapor imagery that extends south and off the coast. This same feature was positioned off the Florida coast yesterday and produced similar convection. So far, only isolated showers and storms have pushed onshore as the bulk of the activity has remained confined to the marine area. This convection has generated a considerable and thick area of mid/upper level clouds that has covered much of the forecast area. The degree to which this cloud cover limits insolation will have significant impacts on rain chances and temperatures this afternoon. Current thinking is that the cloud cover will keep things cooler and limit instability for most areas except along coastal South Carolina. Here, there are some differential heating boundaries and already some alignment of additional showers and storms back to the east. As such, the area of likely PoP`s have been shifted to mainly include Charleston County and the bordering regions of Dorchester and Berkeley counties. Elsewhere, have left rain chances in the chance range. Severe threat remains limited with pedestrian lapse rates and convective parameters. Given that precipitable water values are between 1.8 and 2 inches, we could see some locally heavy rainfall. Temperatures are tricky thanks to cloud cover and uncertainty about how quickly is thins through the day. Mid to upper 80s west of I-95, low to mid 80s east of I-95. Tonight: Rain chances should diminish across much of Southeast Georgia with a continuation of mostly cloudy skies. The South Carolina forecast is tricky with a wave of low pressure developing and tracking up the surface trough just off the coast. Models keep plenty of moisture and weak upper forcing forecast along the South Carolina coast even late tonight. We maintained chance pops in this region. It looks like too many clouds around for any significant fog chances, although the area of interest would be far inland Georgia where clouds could be thinner.
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&& .SHORT TERM /TUESDAY THROUGH THURSDAY/... The general trend through the period should be improving conditions, through there is still considerable model disagreement on some significant features that will determine how well the forecast verifies. Heights falls aloft thanks to a robust stacked/cutoff low moving over the midwest and warm air advection at the surface in a prefrontal regime locally will result in a chance of showers and thunderstorms for the entire area Tuesday. Though the entire day is not expected to be a washout due to underwhelming atmospheric moisture content (PW less than 1 standard deviation above normal), pinning down the timing of convection is a challenge as convective initiation could come from vort pulses moving around the southern periphery of the upper level low, subtle waves in the inverted trough along the coast, or the remnants of other convective complexes that will be moving across the region ahead of the front, so chance POPs are in the forecast for most of the day. Severe threat is low given modest instability. Wednesday should be active as well as the area remains in the warm sector of a large low-level cyclone over the Great Lakes/Midwest, though the axis of highest pre-frontal moisture will likely have shifted offshore by his time. This is where the certainty in the forecast takes a nose dive. Surface low pressure over the Midwest will stall and fill rapidly Wednesday as the cold front occludes and the baroclinic structure of the system weakens. The general pattern amongst global model ensemble members is that, given the troughing remaining aloft, low pressure will develop along the front-which will have essentially stalled over the area Wednesday evening-over the Carolinas and lift north along the front through the day Thursday. While significant disagreement exists amongst guidance in the strength, location, and timing of this feature, there is general agreement that the formation of this low will give the stalled front enough of a kick Thursday morning to get through the area and allow high pressure to fill in through the day Thursday. This general trend is represented in the official forecast, with the caveat that if the low is weaker or slower with developing over the Carolinas, the front will be slow to move through the area and at least the first part of the day Thursday could be more unsettled. && .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... High uncertainty continues into the long term. Cut-off upper low will continue to reside over the eastern U.S., though high pressure trying to build in at the surface should keep conditions dry locally, at least over land, Friday into the weekend. However, given the close proximity of the front which will remain stalled over or near the coastal waters and the area of destabilization under the core of the upper-level low, there is potential for at least nuisance showers mainly in the afternoon through the long term. Have opted for a dry forecast at this time given the high uncertainty, with temps near to a couple degrees below normal through the period. && .AVIATION /14Z MONDAY THROUGH FRIDAY/...
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Periodic showers are possible along the South Carolina Coast today as moisture advects onshore to the the north of a weak low pressure area trying to pinch off along the Gulf Stream. VCSH at KSAV this morning and afternoon with small chances for thunder. Late tonight, light north flow signals that we will need to watch for some low clouds or fog late. At this point, we think upper level cloudiness will preclude widespread IFR potential but will continue to monitor. Extended Aviation Outlook: Brief flight restrictions will be possible in showers and thunderstorms Tuesday and Wednesday.
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&& .MARINE... Today: High pressure to the north will gradual erode as it moves offshore. A coastal trough will sharpen but overall the pressure gradient and pinching gradients are forecast to weaken as the high loses influence. We think the models may be a bit quick which is a typical trait due to the localized moisture gradient complexities. Onshore flow 15-20 kt from Fort Pulaski up through Charleston County should decrease to around 15 kt later today and this steady flow will support choppy seas 3 to 4 ft with a few 5 ft seas possible well offshore. Tonight: A weak wave of low pres should traverse the offshore surface trough and turn the flow more north later tonight. Speeds should be less as the gradient continues to weaken with time. Seas 2 to 4 ft, highest offshore. Tuesday through Friday: A coastal trough/weak coastal low will persist over the waters Tuesday before weakening Wednesday as a cold front approaches from the west. This front will enter and stall somewhere near the waters Thursday morning, keeping conditions unsettled through most of the period. Winds will generally be less than 15 kt, with 2-3 ft seas, and locally higher winds and seas temporarily possible in showers and thunderstorms. && .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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