Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 280011 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 811 PM EDT Thu Apr 27 2017 .SYNOPSIS... A weak disturbance is expected to affect the area tonight, followed by high pressure Friday into the weekend. A cold front will move through Monday night, then a stronger storm system will affect the area mid to late week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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We`ve recently trimmed back the PoP`s along our NW tier, and also adjusted sky cover grids based on recent trends. The general forecast though does not require any significant changes. Satellite pictures and radar trends clearly shows the initial large cluster of convection has dwindled, but a second cluster is developing over central and southwest GA where cooling cloud tops are noted. This appears to be in a region where the increasing CINH has not yet encompassed the area, unlike it is over our CWFA. The collision of the sea breeze, already through all of the SC zones, and passing near and west of US-301 in GA will occur after dark. That should limit the strength of convection, and with a steering flow from SW to NE, the bulk of the showers and t-storms will remain outside the local area. Still, isolated-scattered showers and t-storms will remain in the forecast along our extreme NW tier, through about midnight. There is still little if any risk for severe storms. Overnight we do find isentropic ascent and low level coastal convergence, maybe enough to generate a few showers or t-storms brushing Charleston County, but nil PoP`s elsewhere. Turbulent mixing within the boundary layer will negate the need for any mention of fog, although there is considerable moisture beneath a well pronounced nocturnal inversion, which will lead to a deck of stratus. These same conditions, plus plenty of convective debris clouds through at least the first half of the night will easily prevent temps from dropping any lower than the mid or upper 60s, which is about 10-12F above normal.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/... The Bermuda High will continue to expand into the forecast area through Sunday, with onshore flow around the High maintaining low- level moisture. This will promote a primarily rain-free but humid pattern over the weekend. Prognosed moisture convergence near the coast under varying levels of capping suggests a nonzero probability for an isolated shower or two to develop dependent on the strength of the seabreeze, but occurrence appears unlikely at this time with the most probable outcome being a diurnal cumulus field. Under this regime, temperatures will remain several degrees above normals, with maximum temperatures reaching the upper 80s and lower 90s away from the coast with minimum temperatures only in the lower 70s or upper 60s. Some uncertainty is introduced Sunday as high level clouds ahead of a developing low pressure system appear poised to move over the forecast area, but some model soundings indicate potential for low-level stratus-type development early Sunday morning. Prevalent cirrus cover would assist in mitigating this occurrence. While the net effect on sky cover is moot, this would serve to keep maximum temperatures a degree or two cooler than previous days. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... A mature surface cyclone will lift northeast into the Great Lakes region Sunday night and Monday, pushing a cold front through the local area Monday night. A decent channel of atmospheric moisture ahead of the front will allow scattered to numerous showers and thunderstorms to move through Monday afternoon and Monday night. A deeper upper trough moving through the central United States late week will bring a stronger low pressure system through the area, accompanied by numerous showers and tstms. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... Mainly VFR, although there are indications of flight restrictions for a few hours late tonight into Friday morning due to a low stratus deck. South winds will gradually fade this evening, then increase again Friday afternoon due to max heating and enhancement from the sea breeze circulation. Extended Aviation Outlook: Flight restrictions due to low ceilings will be possible at either terminal Friday and Saturday nights, possibly persisting into early the following morning. Ceiling and/or vsby restrictions will again be possible Monday into Monday night due to scattered showers and/or thunderstorms. && .MARINE... A cold front will stretch across the Appalachians this evening and overnight. The pressure gradient will equate to southerly winds increasing to 15-20 kt across most of the coastal waters this evening, then easing a bit after midnight. As a result, wave heights will build to 4-5 ft this evening before subsiding about a foot after midnight. Some 6 footers are possible beyond 20 nm, but confidence was not high enough to issue a Small Craft Advisory for AMZ374. Generally onshore flow averaging 10-15 kts will persist through the weekend, though winds nearest the coast may veer southwest or even west-southwest each night as a land breeze develops after sunset. Seas will generally remain 2-4 feet through the period. A cold front will approach the region Sunday into Monday, and flow will increase in advance of its arrival, building some 5-6 feet seas offshore as early as Sunday, with Small Craft Advisory conditions arriving over most of the waters Sunday night, persisting into Monday night. Rip Currents...The combination of swell impacting the beaches, an afternoon sea breeze, and astronomical influences will generate a Moderate Risk of Rip Currents along southeast South Carolina beaches Friday, with a low risk for our Georgia beaches. && .TIDES/COASTAL FLOODING... Coastal Flood Advisories are in effect this evening, thanks to previously elevated tides, moderate onshore winds and the recent Perigean Spring Tide cycle. Elevated tides are expected to continue through Saturday, so more advisories will likely be needed with each evening high tide. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Coastal Flood Advisory until 11 PM EDT this evening for GAZ117- 119-139-141. SC...Coastal Flood Advisory until 11 PM EDT this evening for SCZ048>051. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM...JRL AVIATION... MARINE... TIDES/COASTAL FLOODING...

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