Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 261433 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1033 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure over the Atlantic will extend westward into the forecast area today. A conveyor belt of deep tropical moisture on the east side of Tropical Cyclone Alberto will move overhead Sunday then linger, resulting in unsettled weather through the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... 10:30 AM: Very minor changes made to the forecast, mainly to the current temperature and dew points to mesh them with the ongoing forecast. Showers and thunderstorms are starting to form along the sea breeze and expect them to trend inland, into areas of higher instability. But the severe threat remains low. Depending on the convective coverage, we may need to adjust the hourly temperatures further. Rest of today: Deep layered ridging extends from offshore into eastern GA and the coastal Carolinas. Sufficient moisture and diurnal instability/thermodynamics look to exist for scattered showers and tstms along and inland from the sea breeze. Somewhat deeper convection is probable this afternoon over inland GA zones where over 2500 J/KG SB Capes are possible and environment lapse rates are just a bit steeper adjacent to the CSRA/Midlands. Moisture is also deeper in this region thus DCAPES should be unimpressive. The overall risk of severe storms remains low although a few stronger updrafts could pulse up at times. Tonight: Although even deeper tropical moisture will be approaching from the south overnight, most areas are expected to squeak out a dry night after early evening convection inland wanes. We will probably see patchy low clouds form again well inland and perhaps some fog adjacent to the upper Savannah River. Lows will remain quite consistent in the upper 60s to low 70s. && .SHORT TERM /SUNDAY THROUGH TUESDAY/... A plume of exceptionally deep tropical moisture (PWats 2-2.5 inches, 3 standard deviations above normal) advecting northward on the eastern side of Alberto will spread over the area Sunday as the tropical cyclone approaches the Gulf Coast. Guidance continues to consistently show a rainband-type structure containing widespread moderate to heavy rainfall forming along a surge of high theta-e air and a coupled jet structure that will begin to approach the area from the south Sunday morning, slowly working across the area Sunday afternoon through early Monday morning. We have collaborated with WPC to bring a slight chance of excessive rainfall across the area for Sunday and Monday. Much of the area will see 2 to 3 inches of rainfall during this time, with amounts in excess of 3 inches mainly focused across the Charleston Tri-County area. Given the relatively wet antecedent conditions currently in place (several inches of rain have fallen across most areas away from the immediate coast over the past week), localized flooding issues are possible. Additionally, any heavy rain falling near the coast around the time of high tide Sunday evening could result in significant issues as high tide is currently forecast to approach coastal flood level. A more diurnal rainfall pattern will ensue Monday evening, with storm coverage and intensity waning overnight, then becoming greater with the increase in thermodynamic instability during the day Tuesday. A few strong thunderstorms producing gusty winds are possible Monday and Tuesday afternoons as CAPE values climb to near 1500 J/kg for areas that receive some sunshine, but the overall severe risk is low in such a deeply moist environment lacking substantial shear/winds aloft. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... Active weather will persist through the middle-to-end of the week as deep tropical moisture remains in place. The lack of any larger scale forcing will result in a primarily diurnal trend for the POP forecast, with chance-to-likely most days and low-end-to-slight chance overnight. The flooding threat will decrease, though local issues cannot be ruled out given the ample moisture available to storms mainly Wednesday and Thursday afternoons. High pressure centered near the Bahamas will slowly assume control of the local pattern as we head into next weekend, bringing more settled conditions. Temps will trend from near to marginally above normal through the period. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... VFR conditions likely to persist through the 12Z TAF cycle into daybreak on Sunday. The main forecast quest is convective initiation and advancement inland with the sea breeze from late morning through early this afternoon. We maintained VCSH through early afternoon at both sites with thunder a possibility but too uncertain at this juncture. South to southeast breezes will strengthen a bit in the wake of the sea breeze this afternoon. Extended Aviation Outlook: Occasional to frequent ceiling and/or visibility restrictions can be expected at the terminals Sunday evening through Monday morning as an area of moderate to heavy rainfall crosses the area. VFR conditions will prevail thereafter, though continued unsettled conditions mean that brief periods of flight restrictions in showers and thunderstorms are possible mainly in the afternoons. && .MARINE... South to southeast winds remain light this morning, but should begin a gradual uptick with the onset of the sea breeze along with a slightly tighter low level pressure gradient later today and tonight. Speeds should increase closer to 15 kt both in the Intra-coastal and adjacent waters later today with 10-15 kt speeds tonight across the marine waters. Seas will range from 2 ft this morning to 2-4 ft by tonight, highest beyond 20 NM offshore. Persistent southerly winds will bring increasing windswell into the coastal waters Sunday into Monday. A Small Craft Advisory will likely be needed beginning midday Sunday as seas climb over 6 ft for the offshore Georgia waters, and possibly the nearshore South Carolina waters as well. Seas will return to the 2 to 4 foot range with moderate southwest winds mid-week as high pressure near the Bahamas takes control. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM...CEB AVIATION... MARINE...

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