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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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666 FXUS62 KCHS 131150 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 750 AM 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, before giving way to a cold front early next week. && .NEAR TERM /THROUGH TONIGHT/... A bit of stratus has managed to expand across portions of southeast SC over the last few hours, which will erode by mid to late morning. Sky forecasts have been updated to reflect that. Otherwise, no changes were made to the going forecast. Previous discussion... Suppressed upper level heights extend down through the mid Atlantic and into the Carolinas with a mid level short-wave sliding southeastward out of the Ohio Valley toward the mid Atlantic region. Some semblance of a boundary extends off the Atlantic and cuts across the central part of the forecast area, with weak northeasterly low level flow across northern third of the CWA early this morning. Quiet weather inland for the time being with a touch of stratus lurking through the SC lowcountry (in the vicinity of the boundary) and a few showers outside the SC outer waters. Today: Weak/remnant boundary and axis of higher dewpoint/theta-E air will linger across the region through the day. The combination of daytime heating/destabilization in the presence of modest larger scale forcing for ascent along the southern periphery of the upper level trough and smaller scale boundaries...should kick off showers and thunderstorms along the coastal corridor and back through the SC midlands this afternoon into this evening. Forecast will feature scattered to numerous precip chances with the highest chances from the Midlands down through the SC Lowcountry. A slightly better setup for strong to severe storms anticipated this afternoon into this evening, particularly across southeast South Carolina. Afternoon heating will drive MLCAPE values into the 1500-2500 J/Kg range with an axis of higher dewpoint air and instability stretched out across the southeast SC counties into the Atlantic...but diminished instability down through southeast Georgia with slightly lower dewpoint air wrapping into that region. Belt of stronger mid-level flow (500 mb winds around 30 knots) will reside across the mid Atlantic into the Carolinas resulting in slightly elevated effective bulk shear values and 1-2 units of Supercell Composite Parameter, suggesting the potential for better organized updrafts and a marginal risk for severe storms across the SC counties. Meanwhile, one or more rounds of thunderstorms moving through the SC Lowcountry could produce locally heavy rainfall and potentially cause hydro issues given elevated rivers/creeks and wet soils. Tonight: Coverage of convection will diminish through the evening hours, although sitting along the southern flank of upper level troughing, high-res guidance does suggest that some shower activity might linger into the overnight. A bit cooler overnight than in recent times, with lows in the lower to middle 70s...upper 70s along the coast. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY/... Wednesday: The mid and upper trough is situated over the eastern states, as a 5940 meter anticyclone amplifies over the central region. The resulting northwest flow between these two large scale features will allow for a weak and slow moving cold front to drop through the local counties. The front will be near or just south of the area by late in the day. There remains sufficient moisture, instability, and even some shear to generate scattered PoPs, greatest over southeast Georgia with the interaction of the sea breeze with the front. There is also some additional forcing for ascent in response to a subtle short wave moving through. maybe that`s enough to produce some isolated strong or marginally severe storms with DCAPE in excess of 1000 J/kg. Max temperatures will be a bit lower given breezy onshore winds and a subtle change in air mass moving in. Thursday: The ridging aloft to our west is somewhat less amplified, but the east coast trough does persist, maintaining a northwest flow. the cold front will be to the south, as high pressure builds in from the north, and Tropical Cyclone Ernesto is east of the Bahamas. PWat is quite low for this time of year, down under 1.5 inches, and there is little in the way of instability. We`ll show slight chance PoPs over Georgia along the sea breeze, mainly to account for slightly better moisture. However, its is certainly possible that it`ll be rainfree all locations. It`ll certainly be a noticeable change in the air mass, with max temperatures mainly in the 80s, and even some dew points down into the 60s with a continued onshore flow. Friday: Flat ridging prevails aloft, while at the surface, high pressure persist overhead, and Tropical Cyclone Ernesto is heading toward Bermuda. We should be able to get by with yet another rainfree day, with limited moisture and the lack of forcing. Max temperatures will be very similar to those on Thursday, with many places again showing dew points down in the 60s. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... A full latitude trough takes shape across the area over the weekend into early next week, which not only lifts Ernesto off much further into the northern Atlantic, but also sends a cold front into closer proximity from the northwest at some points late in the forecast period. With warmer temperatures and a return of more typical moisture, convection will also return. We`ll go slight chance/chance for now, but given the potential for better shear, these values might be on the conservative side. temperatures will warm back into the 90s for all areas away from the beaches. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... KCHS/KJZI/KSAV: A touch of shallow fog/mist will impact the terminals until 13Z. Otherwise, VFR conditions through early afternoon. Scattered convection will develop after 18Z at all three sites as the interaction of the sea breeze and a weak cold front occurs in the area. Better chances for convection will occur at CHS and JZI and tempo groups for TSRA have been included with the 12Z terminal forecasts from 20Z through 23Z. Any convection that directly impacts a terminal could produce brief MVFR conditions and gusty winds. Coverage of convection diminishes this evening. However, there are signals that spotty shower activity could continue to impact CHS and JZI through the night. Extended Aviation Outlook: Wednesday: Brief flight restrictions possible at all sites, mainly with afternoon/early evening convection. The chances for more prolonged flight restrictions might be at KSAV with a cold front closer by. Thursday and Friday: Less than 15% chance for convection, with VFR expected to prevail as deep layer dry air tries to push southward through the region. Saturday: Maybe a chance of diurnal convection trying to return. && .MARINE... Weak surface boundary has already pressed into the SC coastal waters and will continue to slip southward through the day into tonight. Winds will (or have) turn north/northeastward behind the boundary through tonight but largely remain 10 knots or less. Seas will run 3 feet or less. Wednesday through Thursday: Fairly decent winds across the marine area as a cold front sags slowly southward Wednesday and gives way to high pressure from the north by Thursday. NE winds will reach a solid 15-20 kt and gusty, and although there could be some gusts around 25 kt at times, the duration and coverage at this stag e don`t look to be enough for a Small Craft Advisory. Friday through saturday: Tropical Cyclone Ernesto will be turning northward as it reaches our latitude, but is found out close to Bermuda during this time. Our local gradient steadily slackens as weak high pressure prevails. That said, some 6 foot seas associated with the tropical cyclone could make it into the outermost Georgia waters, and we might require a Small Craft Advisory if this occurs. Rip Currents: For now we`ll keep a Low Risk of rip currents at the local beaches Wednesday. However, if winds and/or swells are any higher than now forecast, the risk might be higher. There is likely an enhanced risk of rip currents late this week, as swells from distant Tropical Cyclone Ernesto impact the beaches. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory from noon today to 6 PM EDT this evening for GAZ117>119-139>141. SC...Heat Advisory from noon today to 6 PM EDT this evening for SCZ047>049-051. MARINE...None. && $$