textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Smoke from the Horse Island Wildfire will impact portions of Berkeley County tonight into Wednesday.

- 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 will bring increasing chances for showers and thunderstorms late this week through the weekend, with the potential for locally heavy rainfall.

DISCUSSION

KEY MESSAGE 1: Smoke from the Horse Island Wildfire will impact portions of Berkeley County tonight into Wednesday.

The Horse Island Wildfire near Shulerville, SC has increased to 1200 acres. A Dense Smoke Advisory has been requested by the U.S. Forest Service for portions of Berkeley County in response to the wildfire smoke. The latest Hysplit output shows the worst of the smoke will remain across Berkeley County, between Jamestown and the Charleston border, extending northeast across the Santee River into portions of Georgetown and Williamsburg Counties. 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 Wednesday although visibilities should improve some by mid-morning as dispersion increases and mixing heights rise.

KEY MESSAGE 2: Hot and humid conditions across coastal South Carolina and Georgia through mid-week. Heat Advisories could be needed. Hot and humid conditions are expected to continue over 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 Heat Advisory criteria. Closer to the coast, conditions 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 Heat 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 the middle of the week can be dangerous to vulnerable communities including elderly and those who do not have access to adequate cooling. The extreme 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 will bring increasing chances for showers and thunderstorms late this week through the weekend, with the potential for locally heavy rainfall.

As a mid-level trough settles over the eastern CONUS on Wednesday, an associated cold front will slowly approach the region, reaching the area on Thursday before stalling through the end of the week. This pattern will support numerous showers and thunderstorms from Friday through Sunday. Forecast soundings continue to indicate a very moist airmass, with PWAT values increasing to around 2.2 inches on Friday and up to 2.4 inches this weekend. These conditions will favor efficient rainfall production and increase the potential for localized flooding. While most flooding impacts are expected to remain minor, repeated heavy downpours over urban areas such as the Charleston or Savannah Metro areas could warrant Flood Advisories or even Flash Flood Warnings.

Although still quite far away and subject to change, rainfall totals of a couple of inches are possible from Friday through Sunday. The highest amounts (~3-4 inches) are possible in proximity to the stalled frontal boundary, which at this time looks to be the LowCountry. Locally higher totals will be possible where thunderstorms repeatedly move over the same locations.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

18Z TAFs: VFR generally expected across the terminals through the TAF period. Thunderstorms may develop over southern GA late this afternoon into this evening. These thunderstorms may pass near or over KSAV between 20 to 24Z, highlighted with a TEMPO. Gusty south winds may persist through the rest of the daylight hours. Once mixing begins Wednesday morning, SW winds may develop gusts to around 20 kts.

Extended Aviation Outlook: Chances of flight restrictions increase later this week with a greater coverage of showers/thunderstorms.

MARINE

Tonight: The pressure gradient will tighten across the coastal waters as a cold front approaches from the north and a east-west sfc ridge remains to the south. This pattern should support increasing south winds tonight, with gusty conditions expected across the marine zones into Wednesday night. In addition, seas will build tonight, with 5 to 6 ft sea common across the waters into Wednesday morning. A Small Craft Advisory are in effect for all marine zones, including the Charleston Harbor.

Wednesday through Sunday: Breezy south to southeasterly winds ahead of an approaching cold front will maintain hazardous marine conditions through Wednesday. As the front stalls near the area Thursday and the pressure gradient relaxes, winds and seas will steadily diminish, with all Small Craft Advisories expected to end by Thursday morning. Benign marine conditions should then prevail through Sunday.

Rip Currents: A Moderate Risk of rip currents is expected to continue along the South Carolina and Georgia beaches Wednesday and Thursday. On Wednesday, increasing south to south-southwest winds will support breakers of 3 to 4 ft, with occasional breakers above 4 ft possible and dominant wave periods of 6 seconds. Although surf is expected to subside somewhat on Thursday, a longer-period swell of around 8 seconds should continue to support dangerous rip currents. If surf heights or wind speeds trend higher than currently forecast on Wednesday, portions of the SC/GA coast could require an upgrade to a High Risk.

CLIMATE

Record High Temperatures:

July 27: KSAV: 100/2010

Record High Minimum Temperatures:

July 21: KCHS: 80/1986 KCXM: 83/1998 KSAV: 81/1942

July 22: KCHS: 81/2011 KCXM: 83/2011

July 23: KCHS: 80/1991 KCXM: 83/1952

July 24: KCHS: 81/2024 KCXM: 84/1952

July 25: KCHS: 80/2011 KCXM: 82/2016 KSAV: 80/1893

July 26: KCHS: 78/2025

July 27: KSAV: 80/1885

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Dense Smoke Advisory from 8 PM this evening to 10 AM EDT Wednesday for SCZ045. 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.