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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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551 FXUS62 KCHS 132027 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 427 PM EDT Tue Aug 13 2024 .SYNOPSIS... A cold front will move slowly south into the area today, then pass through on Wednesday. High pressure will build from the north into late week. Another cold front may approach the area later this weekend or early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Rest of the afternoon and into the evening: Ongoing convection features a mixture of showers and thunderstorms that developed along the sea breeze in Charleston County, with additional thunderstorms moving in from upstream associated with Midlands convection. The coastal convection has exclusively been a heavy rainfall threat while the upstream convection organizing along an outflow boundary has the best severe potential. Instability across southeast South Carolina did feature MLCAPE on the order of 1,500-2,500 J/kg, but that has been significantly disrupted thanks to ongoing convection and resulting debris clouds. DCAPE values aren`t particularly impressive and the main severe threat will exist along outflow from the upstream convection. We might need another Severe Thunderstorm Warning or two this evening, but the overall threat is low. Locally heavy rainfall will continue to be an issue, and we have already seen a few automated gauges report 1.00-1.50 inches around the Charleston area. Ongoing convection should dissipate through the evening as it moves toward the coast. Overnight, there are some indications in model data that the subtle boundary will continue to linger along the coast and could provide enough low-level convergence for showers and storms over the coastal waters and perhaps along the Charleston County coast. We have kept a 20-30 percent chance along the coast as a result. Also, there could be some shallow ground fog around, especially in areas that receive rain in the afternoon and evening. But coverage isn`t expected to be enough to warrant carrying fog in the forecast. Lows are expected to be a bit lower than previous nights, with low to mid 70s more common away from the coast. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/... Wednesday: This will be the wettest day to an otherwise rather unusually dry period. A weak backdoor surface front will be pushing just south of the area by later in the morning, with surface ridging building over the mid Atlantic and northeast states, resulting in northeast low level northeast winds. The area will still be under the backside of an upper level trough, departing off of the mid Atlantic Coast. This feature, combined with modest deep layer moisture of PWs of 2+ inches, will result in scattered showers/thunderstorms, especially south of the Charleston Tri-County area. Highs noticeably cooler over the SC zones, in the mid to upper 80s, and around 90 to the lower 90s southern GA zones and near the Altamaha. Expect any isolated to scattered convection to diminish by the evening. Lows cooler than they have been in quite some time, in the upper 60s to around 70 inland, and lower to mid 70s near the coast. Thursday and Friday: Deep layer ridging shown by all models to build from the west and north, helping to drive drier air southward through the area. PWs falling below 1.5 inches and mean RH values generally below 50%, which is below normal for this time of year. Have no mentionable precipitation chances for both of these days, which will allow for some needed drying out of the many flooded/saturated regions across our area. Highs a tad below normal, in the mid 80s to near 90. Lows in the upper 60s inland/west of I-95 and lower 70s closer to the coast. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... The deep layer ridging over the southeast U.S. is shown by global models to weaken and push southward as a large/broad upper trough forms over the Great Lakes Saturday, pushing into southeast Canada and northern New England Sunday and Monday. This will translate to a gradual increase in precip./convection chances through the period. Have slight chance PoPs Saturday and into Sunday, then chance by Monday, which is around normal for this time of the year. Given the presence of a relatively strong upper level trough axis for mid August, will continue to monitor any potential threat for strong to severe storms, especially by later in the weekend. Temperatures warming slightly through the period, but guidance keeps highs around normal in the lower 90s and lows in the lower to mid 70s, except mid to upper 70s near the coast. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... At 18z, showers continue to develop along the Charleston County coast and could at times move close to KCHS and KJZI. The main forecast challenge for the TAF`s concerns a cluster of thunderstorms moving to the southeast out of the Midlands. This cluster of storms could reach KCHS and KJZI around 20-21z and bring a brief shot of IFR visibility and gusty winds. The chance of storms is much lower at KSAV. Thunderstorm activity should shift offshore during the evening hours. Then overnight, we could see additional showers around KCHS and KJZI as a weak boundary lingers in the area. We could also again see an area of stratus late tonight, but confidence in any ceilings developing at the TAF sites is low. Extended Aviation Outlook: Wednesday: Brief flight restrictions possible with scattered mainly afternoon and early evening showers and thunderstorms, especially KSAV. Thursday and Friday: VFR to prevail with much drier conditions expected, with less than 15% chance for afternoon showers/thunderstorms. Saturday and Sunday: Some moisture starts to return with isolated afternoon showers/thunderstorms Saturday, and scattered by Sunday with a higher chance for brief flight restrictions. && .MARINE... This evening and tonight: Thunderstorms will continue moving out into portions of the South Carolina coastal waters this evening. These storms could produce strong wind gusts, and possibly even a waterspout. We could need a few more Special Marine Warnings. Otherwise, a weak boundary will linger near the local waters through the night. Winds north of the boundary will turn northeasterly while winds to the south will continue to be southerly. Speeds should top out in the 10-15 knot range, mainly within the northeast flow in the Charleston County waters. Seas are expected to average 2-3 feet. Wednesday through Thursday: No highlights expected as a backdoor cold front pushes south of the waters through this period. Expect east-northeast winds of 10-15 knots with some gusts to near 20 knots possible. Seas generally 3 to 4 feet. Thursday night through Saturday: Seas expected to increase, especially beyond 20nm offshore as swells from Ernesto, which will remain well east of the area, arrive into the marine area. Guidance indicating seas building to 4 to 6 feet, possibly up to 7 feet by later Friday and into Saturday. Decreasing northeast winds through Friday, becoming south and southwest Saturday and Sunday. Winds during this period generally 10 to 15 knots or less and seas 3 to 5 feet. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM...RFM LONG TERM...RFM AVIATION...BSH/RFM MARINE...BSH/RFM