329 FXUS62 KCHS 060505 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 105 AM EDT Mon Sep 6 2021 .SYNOPSIS... High pressure will prevail through tonight. A cold front will approach the region on Monday, then stall and dissipate through the middle of the week. Another cold front is expected to bring impacts to the region towards the end of the work week. High pressure is forecast to settle over the region next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Early overnight SPC mesoanalysis reveals a tongue of 500-1000 J/Kg MLCAPE just off the Georgia and South Carolina coast, coincident with a narrow axis of low level moisture convergence with stronger southerly winds off the Atlantic impinging on the coast. Some cloud cover noted off the coast. But so far no sign of any convection. But, nearly all High Res CAM guidance insist on developing showers within the coastal waters within the next few hours, particularly off the SC coast and consistent with my inherited forecast. Will keep that idea going with just some cosmetic tweaks to timing and coverage. Previous forecast... The forecast revolves around three main concerns. The first is in regards to whether or not any showers that form over the marine area can push onshore. There is certainly better instability and moisture over the Atlantic, and with at least some low level convergence there could be a couple of showers brushing parts of the South Carolina shoreline. For now we kept PoP to 10%, which does not require a mention in the forecast. Would any occur though, after 2 am, and mainly across the Charleston coast. Second, the risk for fog actually looks a bit better than last night given higher dew points than 24 hours ago and thus more places likely to exceed their cross-over temps. Looks like the highest probabilities will occur near and west of I-95 after 3 or 4 am. However, condensation pressure deficits don't look as good as last night, and for that reason we feel that any dense fog will be limited in coverage. Finally, with a south to southwest synoptic flow and the higher dew points, min temps will be a bit warmer than last night. && .SHORT TERM /6 AM THIS MORNING THROUGH WEDNESDAY/... Labor Day: Short range agree that the forecast area will remain east of an approaching H5 trough on Monday. PW values ahead of the trough are forecast to increase to 1.8 to 2 inches. Forecast soundings indicate a strong inversion centered at H65, likely limiting CAPE values to around 500 J/kg. As the cold front approaches from the west, lift associated with the front may overcome the layer of CIN, supporting a few showers and isolated thunderstorms. PoPs will be limited to SCHC across the inland counties during the late afternoon and evening hours. Using a blend of MOS, high temperatures ahead of the late arriving cold front should range in the low to mid 90s. Tuesday: The mid-level pattern will feature a broad trough over the East Coast with a ridge across the Southern Great Plains and a weak ridge over the western Atlantic. The sfc front is expected to remain nearly stationary across the forecast area. Forecast soundings indicate that the mid-level inversion will weaken considerably from Monday. The pattern should support generally diurnal showers and thunderstorms, especially along a afternoon and evening sea breeze. High temperatures should range in the lower to mid 90s. Wednesday: The mid-level trough is expected to amplify across the East Coast of the U.S. A well defined cold front is expected to develop in the lee of the Appalachians during the afternoon. The combination of deep moisture and the approaching cold front should yield scattered to numerous showers and thunderstorms. Highs should range in the lower 90s. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... The longwave trough is expected to linger over the region Thursday into Friday morning. This feature allow should result in unsettled weather during the late work week. The NHC's 5-day Tropical Weather Outlook highlights a disturbance in the Gulf of Mexico with a 30 percent of tropical cyclone formation. Interestingly, the GFS show the system in the NHC outlook reaching north FL by Thursday night into Friday, with a large shield of rainfall north of the system over GA and SC. The forecast will feature a mention of showers and thunderstorm into the late work week. Generally dry conditions are forecast for the region this weekend. High temperatures should remain near normal. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... Scattered cloud cover will continue to brush the coastal areas and terminal sites for the overnight hours. Conditions will remain VFR overall. A few showers are possible just off the coast overnight, which might sneak into the JZI terminal area. But probabilities are too low to include the 06Z TAF forecasts. Cold front approaches the region during the day Monday with some showers/storms developing through central Georgia and South Carolina midlands through the afternoon. Some of that activity might sneak into the CHS and JZI areas late in the afternoon into the evening. But again, probabilities are too low to include in the forecast at this juncture. Extended Aviation Outlook: Brief flight restrictions are possible due to showers/thunderstorms Tuesday through Thursday, mainly during the afternoon and evening. && .MARINE... Tonight: A slackening pressure gradient will develop through the night, and then eventually land breeze influences will occur. This will allow for winds to veer to the S and SW early on, then to the SW and W late. Speeds on average won't be any more than 10 or 15 kt, with seas just 2 or 3 feet. Mariners can expect at least isolated showers on the South Carolina Atlantic waters. The pattern will support gusty SSW winds through the early week. Seas are forecast to range between 1-2 ft Monday and Tuesday, building to 3-4 ft on Wednesday. Long period swell from distant Hurricane Larry should start moving through the waters Tuesday, continuing through the end of the forecast period. Seas could build greater than 6 ft on Thursday and may remain into Friday night, which would necessitate Small Craft Advisories. But this will be highly dependent on the strength of the storm and where the largest swell is directed along the East Coast. The NHC's 5-day Tropical Weather Outlook highlights a disturbance in the Gulf of Mexico with a 30 percent of tropical cyclone formation. This system may track across northern FL Thursday night, pushing over the western Atlantic on Friday into Saturday. Rip Currents: Swell from distant Hurricane Larry will begin impacting the beaches Wednesday. The period should start around 16- 18 seconds, then gradually decrease through the end of the week. The exact height of the swell reaching our beaches will be highly dependent on the strength of the storm and where the largest swell is directed along the East Coast. Either way, this will lead to an enhanced risk of rip currents at all of our beaches. High Surf: Surf heights could become high enough to where we might need to consider High Surf Advisories towards the end of the week. && .TIDES/COASTAL FLOODING... The combination of a New Moon on Labor Day and a lunar perigee late this week will yield elevated astronomical tide levels. Onshore winds may develop this week, especially in the wake of afternoon and evening sea breezes. Minor coastal flooding will be possible during the evening high tides along the coast of Charleston and Colleton Counties through mid-week. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...TBA SHORT TERM...NED LONG TERM...NED AVIATION...NED/TBA MARINE...NED/TBA TIDES/COASTAL FLOODING...NED