FXUS62 KCHS 211620 AFDCHS
Area Forecast Discussion National Weather Service Charleston SC 1220 PM EDT Tue Jul 21 2026
.WHAT HAS CHANGED...
Short term update.
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.KEY MESSAGES...
- 1) Smoke from the Horse Island Wildfire will impact northeast
Berkeley County tonight into Tuesday.
- 2) Hot and humid conditions across coastal South Carolina and
Georgia through mid-week. Heat Advisories could be needed.
- 3) A slow-moving cold front may bring greater rainfall chances
late this week through this weekend.
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.DISCUSSION...
KEY MESSAGE 1: Smoke from the Horse Island Wildfire will impact northeast Berkeley County tonight into Tuesday.
A Dense Smoke Advisory has been requested by the U.S. Forest Service for northeast portions of Berkeley County in response to the Horse Island Wildfire. Smoke in the vicinity of the fire is significant and conditions will deteriorate overnight as the nocturnal inversion develops. The latest Hysplit output shows the worst of the smoke will remain across northeast Berkeley County and extend across the Santee River into a small area of southwest Georgetown County. Visibilities may drop below 1 mile at times overnight, especially along parts of Highway 45. Road closures are possible. Smoke will likely remain an issue into Tuesday although visibilities should improve some by mid-morning as dispersion increases and mixing heights rise. Another Dense Smoke Advisory may be needed Tuesday night as smoke settles during the overnight hours.
KEY MESSAGE 2: Hot and humid conditions across coastal South Carolina and Georgia through mid-week. Heat Advisories could be needed.
Hot and humid coniditons are expected to continue the next 48 to 72 hours as the forecast area remains on the western periphery of the upper ridge. Overall heat indices have come down a bit compared to the last few forecasts. This is likely a result of the moisture from now Tropical Storm Bertha remaining increasing further south and west and the ridge being a little stronger. With decreased moisture compared to previous forecasts, dewpoints are now expected to mix into the low 70s across inland areas keeping heat indices in the 100 to 105 range for most of the area today and Wednesday. While these values will still make things rather uncomfortable they are not quite advisory criteria. Closer to the coast, things will be determined by the movement of the afternoon seabreeze. As it moves slowly inland each afternoon, dewpoints will surge into the upper 70s behind the boundary leading to heat indices in the 105 to 108 range. However, this will likely only be for a couple hours during the late afternoons leading to lower confidence in reaching advisory criteria. On top of the shorter duration, isolated showers and thunderstorms during the early afternoon may limit how hot temperatures get during the afternoon leading to increased uncertainty. As a result we have continued to hold off on an heat advisories for coastal areas.
Despite not having any advisories, conditions will still remain rather uncomfortable and with dewpoints in the upper 70s overnight very little relief will occur each night. Given we have set record high minimum temperatures the last few nights, continued heat stress through Wednesday can be dangerous to vulnerable communities including elderly and those who do not have access to adequate cooling. The heat is expected to come to an end by the end of the week as increasing rain chances arrive with a slow moving front.
KEY MESSAGE 3: A slow-moving cold front may bring greater rainfall chances late this week through this weekend.
As a mid-lvl trough situates itself over the eastern CONUS on Wednesday, an associated front will slowly progress towards the forecast area. The front will likely reach the region on Thursday and then remain nearly stationary through the end of the week. This will yield increased chances of showers and thunderstorms on Thursday through Sunday. Forecast soundings continue to indicate very moist conditions across the region, with PWAT values ranging from 2.2 to 2.4 inches throughout most of the week. Given the deep moisture, synoptic scale forcing, and periods of moderate instability, the forecast will feature at least likely PoPs. The Weather Prediction Center (WPC) continues to highlight the region in a Marginal Risk of Excessive Rainfall on Thursday and Friday.
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.AVIATION /16Z TUESDAY THROUGH SATURDAY/...
21/12z Aviation Discussion:
KCHS/KJZI/KSAV:Mostly VFR conditions will continue throughout the period with a narrow band of MVFR ceilings located near JZI as the land breeze/seabreeze begins to setup this morning. Southerly winds around 10 knots will become gusty during the afternoon as the seabreeze slowly moves inland. Wind gusts up to 20 to 25 knots will be possible during the afternoon. An isolated shower or two could be possible around CHS/JZI around 14Z to 16Z as the seabreeze moves inland but minimal impact to aviation is expected. Isolated showers and storms will also be possible late in the forecast over eastern Georgia near SAV from about 23Z to 1Z.
Extended Aviation Outlook: Prevailing VFR through mid-week. Chances of flight restrictions increase later this week with a greater coverage of showers/thunderstorms.
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.MARINE...
Tuesday through Saturday: As a mid-lvl trough slowly situates itself across the region through the period, an associated cold front will approach the local waters and then stall out in the vicinity. This will yield breezy south-southeasterly flow on Tuesday through Wednesday. Also, southerly swell will briefly surge overnight on Tuesday yielding seas from 4 to 6 ft across the South Carolina waters. Otherwise, seas will mostly range from 2 to 4 ft throughout the period. Small Craft Advisories are in effect from Tuesday afternoon through Wednesday night. As the cold front inches closer on Thursday, the pressure gradient will slack back and allow for more tranquil marine conditions on Thursday through Saturday.
Rip Currents: A Moderate Risk of rip currents is in effect for the SC and GA beaches today and tomorrow (Wednesday), given enhanced winds along the coastline and wave periods up to 6 seconds. Breakers may exceed 4 ft on Wednesday, with wave periods between 6 to 7 seconds. It is possible that a portion of the SC coast may need a high risk if swell waves and/or wind speeds increase from the current forecast.
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.CLIMATE...
Record High Minimum Temperatures:
July 20: KCHS: 79/2025 KCXM: 83/2000 KSAV: 79/1942
July 21: KCHS: 80/1986 KCXM: 83/1998
July 22: KCHS: 81/2011 KCXM: 83/2011
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.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None. SC...None. MARINE...Small Craft Advisory until 8 AM EDT Wednesday for AMZ340.
Small Craft Advisory until 5 AM EDT Thursday for AMZ360-362. Small Craft Advisory until midnight EDT Wednesday night for AMZ364-384.
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