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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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367 FXUS62 KCHS 131500 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1100 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/... Late this morning: Surface analysis shows that the weak boundary that has been lingering across the Southeast for the last few days is currently draped across the forecast area, running along the Charleston County coast, down through Beaufort and inland. We have already seen isolated convection this morning near the front, mainly to the northeast from around Georgetown up through the Grand Strand. This boundary should serve as the focus for convection through the rest of the day, bringing a threat for a few strong storms and locally heavy rainfall. Given the subtle nature of the boundary, hi-res models are struggling to paint a consistent picture for convective placement and overall coverage. Current thinking is that convection will first develop along the Charleston County coast, essentially beginning now and continuing into the early afternoon. Attention then turns to convection further inland, likely sourced from outflow associated with ongoing storms across the South Carolina Upstate. This activity should consolidate across the Midlands later this afternoon, and then drop southward into the Tri- County and the rest of the Lowcountry into the evening hours. The severe potential will be aided by some stronger mid-level flow on the southern extent of the belt of west-southwest flow across the Appalachians and into the Mid-Atlantic. This should provide around 25 knots of bulk shear and with DCAPE values approaching 800-1,000 J/kg, we will see a damaging wind gust potential. The locally heavy rainfall threat will also persist thanks to precipitable water values hanging around in the 1.80-2.00" range and the lingering on the ground vulnerability due to heavy rainfall last week. Convective coverage will be less the further southwest you go, so we are advertising less coverage for our southeast Georgia areas. Temperatures should be on track to reach the low to mid 90s for southeast South Carolina, and mostly mid 90s for southeast Georgia. We should again see an area of lower dew points across interior southeast Georgia, but dew points should pool along the Georgia coast and up into southeast South Carolina. Heat index values could reach 110 degrees along the Georgia coast, up through Beaufort and Edisto Beach. A Heat Advisory remains in effect. Elsewhere, heat index values around 105 will be more common. 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 FRIDAY/... 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 /FRIDAY 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 /15Z TUESDAY THROUGH SATURDAY/... KCHS/KJZI/KSAV: 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 until 6 PM EDT this evening for GAZ117>119- 139>141. SC...Heat Advisory until 6 PM EDT this evening for SCZ047>049-051. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM... LONG TERM... AVIATION... MARINE...