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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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353 FXUS62 KCHS 122028 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 428 PM EDT Mon Aug 12 2024 .SYNOPSIS... A stationary front will remain just to the west through Tuesday before dissipating. Weak high pressure will build in from the north during the middle of the week and prevail into the latter part of the week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Afternoon into the evening: Thunderstorms are developing in the anticipated area across the Tri-County and back to the southwest where the area is situated between the front and the sea breeze. Further inland, dew points are a bit lower, with some upper 60s at a few observation sites. MLCAPE values are between 1,500-2,500 J/kg with a stripe of DCAPE values on the order of 1,000-1,200 J/kg. With DCAPE this high there will continue to be a damaging wind threat, especially where updrafts are enhanced near boundary interactions. Overall, the severe threat isn`t particularly high but it isn`t out of the question for there to be a Severe Thunderstorm Warning or two. Locally heavy rainfall will also be a concern, especially since this area is still feeling the effects of rainfall and flooding from Debby. Convection should steadily diminish through the evening hours and we expect the bulk of the overnight to be dry. Lows are forecast to reach the mid 70s in most areas, with upper 70s along the coastal corridor. Some model guidance suggests there could be some shallow ground fog but no significant visibility reductions are expected. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/... Tuesday: The region will be caught between a broad upper trough over the mid Atlantic and Northeast states and a mid/upper level ridge centered over the Gulf of Mexico. A surface trough or weak backdoor front is expected to move southward toward the area during the day. Given PWs just above 2 inches, the presence of some upper forcing from an upper level short wave over North Carolina, and the approaching surface trough/backdoor front, all point toward above climo PoPs, especially for our SC zones. Have capped PoPs in the high end chance/low end likely for now over the northern 1/3 of the area, but they may need to be raised. Models also show adequate deep layer shear of 35-45 knots, but instability limited due to warm mid levels. Thus, there is only a small chance for isolated strong to severe storms, especially within boundary interactions later in the afternoon. Locally heavy rainfall possible, which could easily cause ponding of water, especially in/around urbanized areas. Highs ranging from around 90 northern 1/3 of the area to mid 90s southern GA/Altamaha region. Convection should decrease in coverage later Tuesday night, but kept slight chance/chance PoPs through about midnight. Lows in the lower to mid 70s. Wednesday and Thursday: The upper trough axis begins to shift off the mid Atlantic coast, while a surface trough/backdoor front continues to slowly push southward, likely reaching the southern forecast area by late afternoon. The front is expected to stall near or just south of our region later Wednesday and Thursday. For Wednesday, lingering deep layer moisture and instability, along with the potential for a weak upper shortwave on the backside of the upper trough, will keep at least chance PoPs for much/all of the area, especially GA zones. By Thursday, a deep layer ridge is shown by models to build west and north of the region, helping to push deep layer drier conditions southward. This will translate to much lower PoPs. Have kept slight chance for the extreme southern GA region, and below 15% elsewhere. Would not be surprised if the entire area stays convection free. Highs Wednesday in the upper 80s to around 90, then mid to upper 80s Thursday, which would be a welcome few degrees below normal if it verifies. As the drier air pushes southward, this will help to lower our surface dewpoint temperatures, especially by Thursday, where many will see dewpoints in the mid to upper 60s, especially northern and interior zones. These lower dewpoints will allow nighttime lows to get cooler than they have been for a while, in the upper 60s inland to the lower 70s coast Wednesday night. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... The deep layer ridge that was building from the west and northwest in the near term is shown by global models to flatten and push southward by later Thursday and early Friday and another upper level trough builds into the Great Lakes Saturday, then weakens as it moves into the northeast U.S. into Sunday. Friday looks dry with PoPs below mention/15% as the deep layer ridge hangs on for one more day. With more cyclonic upper level flow Saturday and Sunday, along with increasing deep layer moisture as the low level flow turns southerly, expect a return to at least slight chance PoPs both days. Will continue to monitor the potential for strong to severe storms this coming weekend given the presence of a full latitude upper trough and higher values of deep layer shear. High temperatures near to slightly below normal Saturday, then a tad warmer by Sunday. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... Isolated to scattered thunderstorms are possible this afternoon and evening, with the best coverage expected to be around KCHS where we have included VCTS from 1930-21z. Can`t rule out a thunderstorm near KSAV or KJZI, but will handle those with amendments as necessary. Overnight, there could be some shallow ground fog around, but no reduced visibility expected at the TAF sites. Extended Aviation Outlook: Tuesday: Some flight restrictions will be possible with scattered afternoon and early evening convection, especially KCHS and KJZI. Wednesday: Additional brief flight restrictions possible at all sites, mainly with afternoon/early evening convection. 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. && .MARINE... This evening and tonight: Sea breeze flow along the land/sea interface will persist into the evening mostly in the 10-15 knot range though a few gusts up to 20 knots are possible. Overnight, winds should be mostly southwesterly with speeds diminishing into the 5-10 knot range. Seas should average 2-3 feet. No highlights expected through the period. A weak surface trough/backdoor front is expected to approach the waters late Tuesday/Tuesday night, then move slowly southward, possibly reaching the southern/GA waters Wednesday night remaining just south of the waters through Friday before weakening. Southwest winds 10 knots or less Tuesday and seas 2 to 3 feet, veering to northeast from north to south beginning Tuesday night, with northeast winds expected all waters by later Wednesday through Friday. Winds Wednesday through Friday generally 10-15 knots with some gusts near 20 knots possible, especially SC waters. Seas building to 3 to 5 feet, possibly touching near 6 feet beyond 20 nm offshore by Thursday and Friday. By Saturday, high pressure builds briefly over the waters with lighter winds from the south, but seas may still hang on at 3 to 5 feet. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory until 6 PM EDT this evening for GAZ116>119- 137>141. SC...Heat Advisory until 6 PM EDT this evening for SCZ047>049-051. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM...RFM LONG TERM...RFM AVIATION...BSH/RFM MARINE...BSH/RFM