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Saint Helena Island, South Carolina Weather Forecast Discussion

441
FXUS62 KCHS 300621
AFDCHS

Area Forecast Discussion National Weather Service Charleston SC 221 AM EDT Tue Sep 30 2025

.SYNOPSIS... Tropical Storm Imelda will move northeast away from the Southeast U.S. coast tonight through Wednesday. A cool, dry high pressure will then build in from Canada and allow for fall-like weather to take place towards the end of the week.

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.NEAR TERM /THROUGH TODAY/... The forecast area will remain between a sfc ridge across the western Carolinas and Imelda and Humberto over the western Atlantic. The pressure gradient across the forecast area will support gusty north- northeast winds through most of the near term period. Temperatures today will be limited by the gusty NNE winds and thick cloud cover, highs ranging from the upper 70s across the SC Lowcountry to the low 80s across SE GA. Light rain may occur from time to time within a plume of deep moisture across the Atlantic waters and coastal counties. QPF should remain less than a tenth of an inch.

Tonight, drier air will dive southward across the forecast area as the tropical cyclones track to the east. The arrival of the drier air should end and SCHC PoPs with decreasing cloud cover. Low temperatures are forecast to range from the low 60s inland to the mid 60s along the coast.

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.SHORT TERM /TONIGHT THROUGH FRIDAY/... As Imelda pushes farther away from the CONUS, strong Canadian high pressure will build in from the northwest and advect drier air into the region. This allow for a decrease in cloud coverage and breezy north-northeasterly winds to persist across the region (w/ winds highest near the coastline)as the local pressure gradient over the region tightens up. With the dry and cool airmass present, PWATs will fall well below normal on Thursday and Friday and this will support a more fall-like climate with temperatures in the mid to upper 70s. Additionally, dewpoints will drop into the upper 50s and low 60s! Thereafter, ensembles have been depicting an weak coastal trough forming in the Gulf and into the Deep South on Friday, and then becoming entrapped under this building high pressure from the northwest. This will trough will slowly allow for moisture to return to the region on Friday and expect chance for shower and thunderstorm activity to increase Friday afternoon (mainly along the immediate coastline).

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.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... There is some uncertainty this weekend depending on how quickly the moisture returns and how this weak coastal trough situated over the Deep South evolves throughout the period. Some model guidance has been hinting at the trough shifting more inland towards the region over the weekend, with some ensembles showing it dissipating before even reaching our area. Regardless, given the moisture returning, it`s possible to see isolated to scattered showers and thunderstorms develop along and east of I-95 each afternoon/evening through early next week. Temperatures should gradually warm back up to near normal on Sunday and Monday as upper-lvl heights increase and flow turns to the more to the east.

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.AVIATION /06Z TUESDAY THROUGH SATURDAY/... 6Z TAFs: Regional observations indicated widespread IFR to MVFR ceilings across the Southeast U.S. Based on simulated satellite products, restrictive ceilings should remain across the terminals until around sunset today. Each TAF will indicate IFR to MVFR ceilings until 1Z Wed. Otherwise, terminals will remain within a tight pressure gradient between Imelda and a ridge of high pressure over the western Carolinas. North-northeast winds should remain between 10 to 15 kts with gusts between 20-25 kts.

Extended Aviation Forecast: An extended period of sub-VFR ceilings could possibly continue through Tuesday night, along with occasional vsby reductions due to showers and thunderstorms. Also, expect breezy northeasterly winds through Wednesday.

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.MARINE... Today and tonight: The marine zones will remain between a ridge across the western Carolinas and Imelda and Humberto over the western Atlantic. Northeast winds will remain between 20 to 25 kts with gusts into the low 30s through this evening. Winds may begin to slowly weaken late tonight. Swell sourced from major hurricane Humberto and strengthening Imelda will build across the marine zones through today. By late this afternoon, seas should peak from 8 to 14 ft. Small Craft Advisories are in effect for all marine zones through the near term.

Wednesday through Sunday: Breezy north-northeasterly winds at 20 to 25 kt with gusts up to 30-33 kt will likely hold through the latter part of the week as Imelda and Hurricane Humberto meander over the western Atlantic and a strong high pressure builds down from Canada allowing for the pressure gradient to tighten up across the local waters. Additionally, swell from Imelda with long-period swell from Humberto will continue to build into the local waters through the weekend causing seas to slowly increase to 8 to 10 ft in the nearshore waters, and 10 to 12 ft in the outer Georgia waters (possibly 14-15 ft) on Wednesday and Thursday. By Friday, the swell should begin to taper off some, but seas will remain quite volatile into the weekend with waves +6 ft across all marine zones. Therefore, Small Craft Advisories (SCAs) have been issued for all marine zones through the end of the week w/ the Charleston Harbor SCA likely ending sooner) due the combination of high winds and seas.

Rip Currents/High Surf: A High Risk of rip currents is expected Tuesday (today) and Wednesday at all beaches due to large, long- period swells and strong winds. In addition, large breaking waves of 5+ feet are expected through much of the week and a High Surf Advisory remains in effect.

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.TIDES/COASTAL FLOODING... Strengthening and persistent north-northeasterly winds and swell wave run-up this week will result in increasing tidal departures along the southeast South Carolina and southeast Georgia coast. Minor coastal flooding is possible during the late afternoon/early evening high tide cycle, mainly for the Charleston and Colleton County coastlines, starting as early as this afternoon. The risk will continue into late week.

Heading into this weekend and early next week, astronomical tides will increase due to the upcoming Full Moon (Oct 7) and Perigee (Oct 8). This in combination with the anticipation of continued northeasterly flow will cause the possibility of coastal flooding to become more likely along the entire coastline, including both Downtown Charleston and Fort Pulaski. Saltwater inundation could then occur during both the morning and evening high tide cycles during this time. Although there is considerable uncertainty this far out, persistent favorable winds have the potential to produce up to major coastal flooding.

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.CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through this evening for GAZ117-119-139- 141. High Surf Advisory until 8 AM EDT Saturday for GAZ117-119-139- 141. SC...High Risk for Rip Currents through this evening for SCZ048>051. High Surf Advisory until 8 AM EDT Saturday for SCZ048>051. MARINE...Small Craft Advisory until 2 AM EDT Wednesday for AMZ330. Small Craft Advisory until 2 AM EDT Monday for AMZ350-352-354. Small Craft Advisory until 8 PM EDT Monday for AMZ374.

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NEAR TERM...NED SHORT TERM...Dennis LONG TERM...Dennis AVIATION...Dennis/NED MARINE...Dennis/NED

NWS CHS Office Area Forecast Discussion

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