Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 280754 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 354 AM EDT Sun May 28 2017 .SYNOPSIS...
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High pressure will linger over the region today. A weak cold front will gradually sag south through the area Monday through Wednesday, then lift back north late week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Today: The ongoing area of convection (MCS) upstream of the forecast area continues to diminish in coverage and intensity as IR satellite imagery reveals warming cloud tops. However, it will be interesting to see how the residual outflow from this convection evolves through the morning and into the afternoon as it could eventually serve as a initiation mechanism later today. The pattern aloft will feature nearly zonal flow as the forecast area is situated between the ridge and anticyclone over the Gulf of Mexico and a upper low over the Upper Midwest and central Canada. However, this will not be benign zonal flow as models depict a series of potent shortwaves that will be poised to translate eastward through the day. The good news is that forcing associated with these embedded shortwaves appears to be solidly north of the forecast area. At the surface, high pressure to the south will result in westerly low level flow with a late day sea breeze that will bring more southwesterly flow along the coast. Despite the fact that the large scale forcing will pass by to the north, strong heating and the potential presence of the residual outflow boundary could help kick off isolated thunderstorms this afternoon. Model soundings show that any thunderstorms would mainly impact the South Carolina zones where a much weaker cap would be in place compared to southeast Georgia. If storms are able to initiate, the near storm environment is potentially quite interesting. Various model solutions show a well defined elevated mixed layer with steepening mid-level lapse rates in the -7 to -8 C range. DCAPE values could be as high as 1000-1200 J/kg, with hail CAPE approaching 1000 J/kg. Furthermore, a 50+ kt 500 mb jet is progged to extend west/east across the northern half of the forecast area, which would increase 0-6 km shear into the 40 kt range. With all this in place, the forecast area is in a marginal risk area in the SPC Day 1 outlook which seems quite reasonable. There are uncertainties for sure, including the impact of lingering debris clouds on surface temperatures and resulting buoyancy. Also, the fact that the main slot of forcing will be north of the area should temper expectations. The expectation is the coverage will be isolated at best, but the convective mode should favor discrete cells if they are able to develop. Damaging wind gusts and large hail would be the main impacts, but given the potential shear some rotating storms would be possible. Another source of uncertainty is the feeling that current model solutions are struggling with the current convection and any subsequent convection later in the day. So, the risk is not particularly high or widespread, but certainly something to watch. Tonight: If any storms develop in the afternoon, they should diminish quickly in the evening with the loss of heating. Through the remainder of the overnight, dry and mild conditions should prevail. Lows will likely not fall below 70 for most areas.
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&& .SHORT TERM /MONDAY THROUGH WEDNESDAY/...
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The upper ridge will gradually weaken and shift east Monday into Tuesday, allowing more shortwave energy to push into the area. A weak cold front will move into northern GA and far western NC/SC Monday afternoon, then approach the local area Tuesday afternoon. Monday looks rather toasty as warm low-level thicknesses reside over the area, ample sunshine occurs, and low-level flow is westerly. Highs expected to be in the mid 90s away from the coast. Model soundings show a decent mid-level cap until the afternoon which will probably preclude convective development until later in the day. There won`t be much forcing for convection on Monday though it`s possible that some activity will develop over the SC Midlands and push east-southeast late in the afternoon. Convective parameters would be conducive for some severe thunderstorms given extensive dry air off the surface and CAPEs of 1500-2000 J/kg. Tuesday is looking more active than Monday given the continued breakdown of the upper ridge and shortwave energy making its way into our inland zones during the day. The surface front pushing in late in the day will also provide a trigger for convection. We increased pops a bit during the afternoon. Convective parameters looks similar to Monday except the cap is weaker in the morning. There would be enough 0-6 km shear to support some multicellular organization, and the dry air aloft will be conducive for damaging winds. Slightly cooler temps expected Wednesday as the front remains stalled over the area and more cloud cover is present. There will be enough forcing for isolated to scattered convection mainly in the afternoon, though instability and shear will be a little lower than Mon-Tue.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
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A fairly active period is in store as a quasi-zonal flow exists aloft and weak shortwave perturbations continually traverse the area. With increasing moisture through late in the week and daytime temps in the upper 80s or low 90s, scattered to numerous showers and tstms expected to develop mainly during the afternoon and evenings.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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VFR conditions expected to prevail through 06z Monday. One thing to keep an eye on will be the slight chance of thunderstorms near KCHS tomorrow. An outflow boundary from ongoing thunderstorm activity could settle over the area during peak heating tomorrow and serve as an initiating feature. Coverage should be limited, if indeed there is any, so there is no mention in the TAF at this time. Extended Aviation Outlook: Mainly VFR conditions. Brief flight restrictions possible in mainly afternoon/evening showers/tstms starting Monday.
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&& .MARINE...
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Today through tonight: Overall the setup is quite similar to the last couple of afternoons. Morning westerly flow will shift to more southwesterly in the afternoon as the sea breeze develops. We will see enhanced flow mainly along the land/sea interface with a solid 15-20 knots along the Charleston County coast. Elsewhere 10-15 or a solid 15 knots is expected. The thinking is again that conditions in Charleston Harbor will stay just below Small Craft Advisory thresholds. Seas are expected to be 2-3 feet. A slightly stronger gradient will persist into Monday with a decent 15 kt sea breeze possible in the afternoon. Thereafter, fairly weak winds and small seas are expected as a front drops into the area and lingers through the week.
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&& .TIDES/COASTAL FLOODING...
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High tide will again be elevated this evening due to the new moon and perigee. Fortunately the wind directions are not very conducive for surge, but we could see very marginal coastal flooding along the South Carolina coast during the evening high tides Sunday.
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&& .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM...JRL LONG TERM...JRL AVIATION...BSH/JRL MARINE...BSH/JRL TIDES/COASTAL FLOODING...

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