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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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570 FXUS62 KCHS 172041 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 441 PM EDT Sat Aug 17 2024 .SYNOPSIS... A cold front will move through our area Sunday night into Monday. High pressure will then gradually build in from the north through the remainder of next week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Water vapor images indicated that the leading edge of a H5 long wave trough was sweeping across the western Carolinas and northern Georgia this afternoon. Rounds of showers and thunderstorms may develop ahead of the trough tonight. The first round of convection should continue to drift over the inland counties of the forecast area this afternoon. This activity should dissipate around sunset this evening. High resolution guidance indicates that clusters of thunderstorms will develop ahead of the trough across the I-85 corridor of GA and SC this evening. Given a wide field of weak DCAPE across the area tonight. A weakening, second round, of convection and associated outflow boundaries may arrive over the forecast area late tonight. Coverage should remain limited to isolated over portions of the SC Lowcountry. Low temperatures are forecast to range from the low to mid 70s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... Sunday: The mid-levels will consist of a strong trough over the East Coast. An impulse of energy should round the base of the trough and move over our area during the evening hours. Further up, a jet stream initially to our north will become located over our area by the evening and overnight. At the surface, High pressure will initially be located off the Southeast Coast. It`s expected to move further offshore as time progresses. Additionally, a cold front will be located to the west of the Appalachians at daybreak, it`ll gradually shift to the east and southeast, becoming located over or just to the west of our area by daybreak Monday. Flow around the High and ahead of the front will usher moisture from the Gulf of Mexico into our area. PWATs should peak in the 2-2.25" range in the afternoon. The combination of 850 mb temperatures, low-level thickness values, and compressional heating ahead of the front will cause high temperatures to be several degrees above normal. They are forecasted to be in the lower to middle 90s, except cooler at the beaches. Heat indices should rise close to 108 degrees, mainly along the coast. But it`s marginal whether we`ll need Heat Advisories. The combination of the heat and moisture will generate a decent amount of instability. MLCAPEs should approach the 1,500-2,000 J/kg range with steepening lapse rates. 0-6 km bulk shear should be in the 20- 35 kt range. Combined with lift from the approaching front, convection is expected. Models and the long range CAMs disagree on the exact timing and some of the evolution. But we anticipated isolated to scattered convection across our area in the afternoon. Then, a cluster or line of storms originating well to our west and northwest should move through our area. It should be across our inland counties late in the afternoon, making its way to the coast in the evening. Severe storms with damaging winds are the main concern, so the SPC has our area under a Slight Risk, which seems reasonable. Heavy rainfall is also a concern given the saturated grounds. But these storms should be moving fast enough to limit enormous rainfall amounts. Either way, Flood Advisories cannot be ruled out. The convection will move offshore later in the evening and overnight as drier air builds in from the west. Lows will generally be in the 70s. Monday and Tuesday: The mid-levels will consist of a strong trough hovering over the East Coast. The front should be located off our coast Monday morning. However, it could stall and meander offshore through Tuesday. Meanwhile, High pressure should gradually build in from the northwest, with its periphery reaching into our region. The High should dominate the land forecast, leading to mainly dry conditions, except for maybe a stray shower along the immediate coast each afternoon. Highs will only be a couple of degrees cooler behind the front. Expect them to be in the lower to middle 90s on Monday and the upper 80s to lower 90s on Tuesday. Lows will be in the upper 60s to lower 70s. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... High pressure will build in from the north, causing an inland wedge pattern. Isolated to at most scattered showers and thunderstorms could develop during the afternoon and evening hours each day. However, modest subsidence will limit the overall coverage of convection. High temperatures will gradually lower each day, but will remain within a few degrees of normal. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... KCHS/KJZI/KSAV: VFR conditions expected through 18Z Sunday. Extended Aviation Outlook: A cold front will move through our area Sunday night into early Monday, bringing brief flight restrictions due to convection. VFR is expected to prevail beginning later Monday. && .MARINE... Tonight, the sfc pattern will support southwest winds between 10 to 15 kts across the marine zones. Seas should remain between 3 to 4 ft through most of tonight. Portions of the outer GA waters may see some 5 ft seas late tonight. Extended Marine: Winds and seas will increase ahead of an approaching cold front on Sunday. Seas should be around 6 ft across the GA waters beyond 20 nm, so we have a Small Craft Advisory in place. The front should move through the waters early Monday, then stall just to the east of our waters later Monday. This will cause conditions to improve. By Tuesday, winds will shift out of the northeast with speeds around 10-15 kt. A surge of winds is possible on Thursday as the pressure gradient becomes enhanced due to an inland wedge of High pressure to our north. Rip Currents: Today: The rip current risk remains in the high category for today as long periods swells from Hurricane Ernesto will persist. The rip currents will be especially strong at times given the power values noted on spectral plots from both the Edisto (41004) and Grays Reef buoys (41008). Sunday: Long period swell around 12 seconds from distant Hurricane Ernesto will continue to impact the coast. We have a High Risk of rip currents for all of our beaches. Monday: Long period swell around 11 seconds will gradually subside, but continue to bring impacts to the coast. We have a Moderate Risk of rip currents for all of our beaches. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through Sunday evening for GAZ117-119- 139-141. SC...High Risk for Rip Currents through Sunday evening for SCZ048>051. MARINE...Small Craft Advisory from 8 AM Sunday to 9 AM EDT Monday for AMZ374. && $$ NEAR TERM...NED SHORT TERM... LONG TERM... AVIATION...NED MARINE...NED