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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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328 FXUS62 KCHS 170734 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 334 AM EDT Sat Aug 17 2024 ...DANGEROUS AND POWERFUL LONG PERIOD SWELLS FROM DISTANT HURRICANE ERNESTO WILL CONTINUE AT THE BEACHES TODAY... .SYNOPSIS... High pressure will remain over the area through most of this weekend. A cold front will push across the area Sunday night into Monday. High pressure will then gradually build in from the north through the remainder of next week. && .NEAR TERM /THROUGH TONIGHT/... Through This Evening: Subtropical ridging will briefly build across the Southeast U.S. coast today in response to a potent mid-level trough digging across the Great Lakes into the Ohio Valley. At the surface, high pressure will maintain its influence on the region through the afternoon as a pronounced lee-side trough holds to the west. Most of the day looks warm and dry as dewpoints hold in the upper 60s to near 70. Isolated to scattered showers/tstms are expected to develop across central Georgia during the afternoon within a favorable juxtaposition of mixed-layered instability and a pooling of 850 hPa theta-e. Some of this activity could meander into interior Southeast Georgia during the late afternoon/early evening hours as the axis of higher 850 hPa theta-e nudges to the east. Slight chance pops were maintained across interior Southeast Georgia this afternoon/evening to account for this activity. Convection will quickly wane this evening with the loss of insolation and the onset of diurnal cooling. Both low-level thicknesses and 850 hPa temperatures will rise through the day, but their influence on afternoon highs will be tempered somewhat by the modest onshore flow. Highs look to peak into the lower-mid 90s away from the beaches. Overnight: The surface pattern will remain fairly static, but the upper air configuration is forecast to become increasing cyclonic as the broad mid-level trough becomes reinforced by a shortwave propagating southeast through the western Ohio Valley. There are signals in a few of the CAMs that a cluster of showers could move across the Charleston Tri-County prior to daybreak Sunday as a bit of channeled vorticity pushes through. Given HREF means for >0.01" is less than 12% and any activity would likely occur during/near the diurnal minimum, gridded pops will remain below mentionable thresholds. Lows will range from the lower 70s inland to the upper 70s/near 80 at the beaches and in Downtown Charleston. && .SHORT TERM /SUNDAY THROUGH TUESDAY/... Sunday into Sunday night: A large upper trough will dig into the Southeast along with a cold front that will slowly approach from the northwest through the day. The sea breeze is expected to drift inland during the mid to late afternoon, resulting in dewpoints to climb into the mid 70s. A broad area of deeper moisture will push across the area ahead of the front Sunday. Isolated showers and thunderstorms are forecast to develop between the sea breeze and the approaching cold front during the afternoon, with coverage increasing later in the evening. Temperatures should top out in the low to mid 90s across southeast South Carolina and mid to upper 90s across southeast Georgia. Some areas within the coastal corridor could see heat indices peak at 108 degrees prior to the onset of convection. Heat Advisories could be needed for a few counties along the coast. Overnight lows will bottom out in the low 70s inland and mid 70s along the coast. Interesting setup across the region Sunday evening. Models indicate that an MCS will develop across the western Carolinas Sunday afternoon, then tracking east. The MCS is expected to reach the forecast area Sunday evening, pushing over the Atlantic Sunday night. Forecast soundings show SBCAPE values ranging from 1500-2000 J/kg, with DCAPE values in excess of 1000 J/kg. Hodographs indicate a deep curved profile, with 0-6 km bulk shear around 30 kt and 0-3 km helicity ranging between 200-300 m2/s2. Strong updrafts look likely with modest mid-level lapse-rates around 7-8 C/km. This environment should support a few organized storm clusters, producing swaths of strong to severe wind gusts and/or large hail. It is possible that a few supercells may develop ahead of the MCS as well with notable supercell parameters. Monday and Tuesday: Aloft, a large trough extending from extreme eastern Canada down to the Florida Keys will sit nearly stationary, with the cold front currently timed to reach the Atlantic coast Monday morning. The forecast looks mostly dry behind the front both days, with the exception of an isolated shower/thunderstorm along the coast. Highs will only be a couple of degrees cooler behind the front, with little in the way of cold air advection. Low to mid 90s are forecast for most locations Monday. Overnight Monday, temperatures will drop into the upper 60s/lower 70s inland and mid 70s along the coast. High temperatures will be somewhat cooler Tuesday, reaching the upper 80s to lower 90s. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... The upper trough will lift northeast by mid next week and the main feature will be strong surface high pressure building in from the north, forming 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. Additionally, the sea breeze does not look particular strong with little land/sea temperature differential. High temperatures will gradually lower each day, but expected to stay within the near-normal range; in the mid to upper 80s. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... 17/06Z TAF Discussion: KCHS/KJZI/KSAV: VFR through 18/06z. Extended Aviation Outlook: A cold front is forecast to impact the region Sunday evening into early Monday morning, bringing the risk of brief flight restrictions within showers and thunderstorms. && .MARINE... Today: Long period swells from Hurricane Ernesto will dominate the waters again today. Combined seas will average 6 ft this morning over the Georgia offshore waters where a Small Craft Advisory has been extended until 11 am. Seas will subside a bit today with seas 3-4 ft within 20 NM and 4-5 ft in the Georgia waters out 20-60 NM. South to southwest winds will back a bit this afternoon with speeds holding below 15 kt. Dangerous wave actions will be found at inlets and in/near the entrances of bays, rivers and harbors today, especially as the out going tide combines with powerful, incoming long period hurricane swells. Mariners should use extreme caution. A Marine Weather Statement has been issued to address these conditions. Tonight: South to southwest winds will persist with the waters pinned between high pressure and lee-side troughing the west. The pressure gradient will tighten a bit overnight, but speeds look to hold 15 kt or less. Seas could build slightly with a bit more wind wave possible. Combined seas will average 3-4 ft within 20 NM and could build to 4-6 ft in the Georgia waters 20-60 NM. Another Small Craft Advisory could be needed for the Georgia waters beyond 20 NM. Sunday through Thursday: Winds and seas will increase ahead of an approaching cold front, which is expected to push into the local waters Monday and slowly push further into the Atlantic. Winds will surge from the SW to around 20 kts with gusts up to 25 kt Sunday evening into the overnight period. In addition, seas could build to around 6ft across portions of the outer GA waters Sunday. Small Craft Advisories could be needed for portions of the marine zones. Conditions will improve throughout the day Monday with seas dropping to 2-4 ft. By Tuesday, winds will shift out of the northeast with speeds around 10-15 kt. Another surge is possible Thursday as the pressure gradient tightening with inland high pressure wedging in from the 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 Southeast coast this weekend. A high risk of rip currents are forecast for all area beaches Sunday. High Surf: Wave breakers of 4-6 ft will occur along the Charleston County beaches today as powerful, long period swells from Hurricane Ernesto persist. There could be a few sets as high as 7-8 ft at Folly Beach this morning based on the latest NWPS breaking wave guidance. A High Surf Advisory has been posted for the Charleston County beaches through 8 PM. Elsewhere, breakers in the surf zone will average 3-4 ft with some 5-6 ft sets possible at Tybee Island, mainly this morning. Breaking waves of 5+ ft do not look frequent enough to hoist a High Surf Advisory for Tybee Island at this time. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through this evening for GAZ117-119-139- 141. SC...High Risk for Rip Currents through this evening for SCZ048>051. High Surf Advisory until 8 PM EDT this evening for SCZ050. MARINE...Small Craft Advisory until 11 AM EDT this morning for AMZ374. && $$