textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were updated.

KEY MESSAGES

- 1) Breezy and cooler weather will persist into the weekend.

- 2) The coastal flood risk ramps up through Friday due to persistent northeast winds and high astronomical tides.

- 3) High risk of dangerous rip currents Thursday across all South Carolina and Georgia beaches.

DISCUSSION

KEY MESSAGE 1: Breezy and cooler weather will persist into the weekend.

Through Tonight: Impressive cold air damming (CAD) for early Fall continues to strengthen this afternoon with surface cyclogenesis occurring well offshore of the Carolina coast. This is leading to breezy conditions at the beaches with temperatures across the Lowcountry and Southeast Georgia running well below normal for late September. This is certainly a welcome change from brutal heat that the region has experienced for most of the summer. As is typical for CAD events, the NBM is verifying horribly and was running ~10 degrees too warm as of 23/17z. This trend will persist into the rest of the afternoon with temperatures poised to only top out in mid 70s for areas well inland closer to the CSRA and the Southern Midlands to the lower 80s south of the I-16 corridor. A few showers embedded in the northeast low-level flow could brush portions of McIntosh County from off the Atlantic, otherwise dry conditions will prevail. Breezy conditions will linger into tonight at the beaches with gusts 25-30 mph at times. The inland wedge will tighten up even more tonight as low offshore deepens. Lows will range from the mid 60s well inland to the lower 70s at the beaches.

Thursday: The wedge will be at its strongest during this time as the developing nor'easter offshore moves north of Cape Hatteras and its broad, cyclonic flow helps to reinforce the CAD. Pinning down high temperatures for Thursday proved especially tricky where a passing shortwave rounding the base of the upper low combined with increased UVVs with the left exit region of the polar jet could produce scattered showers, mainly from late morning on. This could induce quite a bit of diabatic cooling given a cool/dry northeast flow that will be in place, especially over the Lowcountry. Highs were nudged closer to the lower side of the NBM interquartile range (IGR) and blended with some of the colder high-res direct model output. This yields highs from the lower 70s well inland to the mid-upper 70s elsewhere, warmest closer to the Altamaha River. Pops range from 30-50% across the Charleston Tri-County to 20% for far southern South Carolina with <10% over Southeast Georgia. Thursday night looks somewhat chilly with overnight lows expected to drop into the mid-upper 50s inland to the mid 60s at the beaches.

Friday into Sunday: The wedge will begin to weaken Friday and steadily break down through the weekend as the nor'easter moves closer to New England. Temperatures will gradually moderate through the period.

KEY MESSAGE 2: The coastal flood risk ramps up through Friday due to persistent northeast winds and high astronomical tides.

Breezy northeasterly flow combined with the upcoming full moon (9/26) will yield an increase in tidal departures across the region.

Charleston and Colleton County: With the ongoing environmental conditions, tidal levels could reach 7.1 ft MLLW in the Charleston Harbor for this evening's high tide (approx. at 628PM). Therefore, Coastal Flood Advisory has been issued from 5PM to 8PM this evening. Saltwater inundation will be possible 1-2 hours before and after high tide, and will likely result in some roads becoming impassable.

Coastal flooding concerns will then ramp up on Thursday and Friday, with the potential to reach moderate flood stage along the Charleston and Colleton County coastlines each evening. Additionally, guidance indicates the Friday morning high tide could see minor coastal flooding.

Lower South Carolina and Southeast Georgia: Tidal levels could reach 8.8 ft MLLW for this evening's high tide in Fort Pulaski, thus this keeps conditions just below minor flood stage. However, the Thursday and Friday evening's high tide forecast indicates potential to reach minor flood stage (9.5 ft MLLW). This would prompt a Coastal Flood Advisory for Lower South Carolina and Southeast Georgia for Thursday and Friday evening, however conditions will continue to monitored over the next 24 hours.

KEY MESSAGE 3: High risk of dangerous rip currents Thursday across all South Carolina and Georgia beaches.

Strengthening low pressure well off the mid-Atlantic coast will support breezy winds, building wave heights, and large swells with a 7-8 second period along the coast on Thursday. This setup will drive a High Risk for rip currents along the entire South Carolina and Georgia coastline on Thursday. An elevated risk for rip currents will persist Friday into the weekend due to incoming longer period (10-12 second) swell. Otherwise, latest guidance indicates breaking wave heights will largely stay 3-4 feet on Thursday, so no High Surf Advisory is planned at this time.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

23/18z TAFs: KCHS/KJZI/KSAV: Widespread MVFR cigs will prevail this afternoon. The risk for extensive IFR cigs will increase after 00z, especially at KCHS. KJZI and KSAV may be just on the eastern and southern periphery of the IFR cigs, but there is enough data to support showing IFR cigs at both of those terminals after 06z. Scattered showers may impact parts of the Lowcountry by late Thursday morning with impacts more likely after 18z. For now, a PROB30 for -SHRA with MVFR cigs was introduced 15-18z at KCHS.

Extended Aviation Outlook: Gusty winds will persist into Thursday, possibly Friday.

MARINE

Through Friday: Marine conditions will continue to deteriorate tonight into Thursday due to a tight pressure gradient between a wedge of high pressure inland and deepening low pressure off the mid- Atlantic coast. Small Craft Advisories are currently in effect for all waters. Winds are expected to peak Thursday afternoon into Thursday night when gusts to gale force (35 kt) are likely over the offshore coastal waters 20-60 nm out. The Gale Watches have been upgraded to Gale Warnings. Seas will peak in the 6-11 ft range, highest offshore. Conditions will begin to improve on Friday, but seas will be slower to subside and will remain supportive of Advisories through the daytime hours.

Saturday through Monday: Outside of lingering 6+ ft seas over the offshore GA waters on Saturday, no additional marine concerns are expected the remainder of the weekend through Monday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents from Thursday morning through Thursday evening for GAZ217-219-239-241. SC...High Risk for Rip Currents from Thursday morning through Thursday evening for SCZ148>151. Coastal Flood Advisory until 8 PM EDT this evening for SCZ149- 150. MARINE...Small Craft Advisory from 6 AM to 11 PM EDT Thursday for AMZ340. Small Craft Advisory until 8 PM EDT Friday for AMZ360-362-364. Gale Warning from 6 AM Thursday to 6 AM EDT Friday for AMZ380- 382. Small Craft Advisory until 2 PM EDT Thursday for AMZ384. Gale Warning from 2 PM Thursday to 6 AM EDT Friday for AMZ384.