textproduct: Charleston

WHAT HAS CHANGED

Removed key message about wildfire smoke and added information in the Marine discussion about the waterspout potential this morning.

KEY MESSAGES

- 1) Hot and humid conditions across coastal South Carolina and Georgia through Thursday. Heat Advisories could be needed.

- 2) 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: Hot and humid conditions across coastal South Carolina and Georgia through Thursday. Heat Advisories could be needed. Hot and humid conditions are expected to continue over the next 48 hours as the forecast area remains on the western periphery of the upper ridge. With decreased moisture compared to previous forecasts, dewpoints are expected to continue to mix into the low 70s across inland areas keeping heat indices in the 100 to 105 range for most of the area today. 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 long enought.Thursday will be much of the same with dewpoints mixing out inland and despite air temperatures heating up into likely the upper 90s, Heat indices likely top out once again in the 105 to 108 range. However, if moisture begins to come in a little more impressive and dewpoints do not mix into the low 70s then some considerations for an advisory would be needed.

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 2: 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-lvl trough situates itself across the Eastern Seaboard today and tomorrow (Thursday), an associated cold front will slowly progress towards the region, reaching the area on Thursday before stalling nearby through the weekend. 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. This combined with skinny CAPE profiles 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 and/or even Flash Flood Warnings.

Although still subject to change, rainfall totals of couple inches are possible Friday through Sunday, with the highest amounts of ~3-4 inches possible in the proximity of the frontal boundary. Additionally, locally higher totals will be possible where thunderstorms repeatedly train over the same location. The Weather Prediction Center (WPC) highlights portions of the region (mainly across the South Carolina Lowcountry) in a Slight Risk of Excessive Rainfall on Thursday, and then Marginal Risk of Excessive Rainfall for majority of the region on Friday and Saturday.

AVIATION /13Z WEDNESDAY THROUGH SUNDAY/

12Z TAFs: VFR conditions should prevail across the area through the morning. Some MVFR ceilings may be possible along coastal South Carolina over the next couple of hours possibly impacting JZI/CHS from roughly 14 to 16Z. An isolated shower or two could accompany the lower ceilings before clearing by 16 to 17Z. Winds will remain out of the south at around 10 knots increasing to around 15 knots behind the afternoon seabreeze. Gusts to 20 to 25 knots could be possible behind the seabreeze during the afternoon before diminishing around 02Z.

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

MARINE

This morning: Doppler radar and satellite detected a line of showers and towering cumulus across the Georgia and South Carolina coast this morning. The environment is favorable for the development of waterspouts within this line of showers. Waterspouts are most likely within breaks within the line of showers over water.

Today: Breezy south-southeasterly winds will continue across the local waters as a cold front approaches from the north and east-west surface ridge remains to the south. This will maintain hazardous conditions through Thursday morning. Small Craft Advisories are in effect for all marine zones through Thursday morning. As south- southwesterly swell tapers back, seas will range from 3 to 5 ft.

Thursday through Sunday: Small Craft Advisories are expected to end by Thursday morning. As the aforementioned front stalls in the vicinity, the pressure gradient will relax back and marine conditions should become rather benign through Sunday. Seas will range from 2 to 4 ft.

Rip Currents: A Moderate Risk of rip currents is expected to continue along the South Carolina and Georgia beaches through this evening. There is also a Moderate Risk tomorrow (Thursday), however it is just for the Charleston County beaches. 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 a little bit on Thursday, a longer-period swell of around 8 seconds should continue to support dangerous rip currents (esp. for the Charleston County beaches).

CLIMATE

Record High Temperatures:

July 23: KCHS: 99/2015

July 27: KSAV: 100/2010

Record High Minimum Temperatures:

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

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

July 24: KCHS: 81/2024

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

July 26: KCHS: 78/2025

July 27: KSAV: 80/1885

July 28: KCHS: 80/2025 KCXM: 84/2025 KSAV: 81/1878

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory until 5 AM EDT Thursday for AMZ360-362. Small Craft Advisory until midnight EDT tonight for AMZ364-384.