textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were updated.

KEY MESSAGES

- 1) Cool weather will linger into the weekend. Near to slightly above normal temperatures return next week.

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

- 3) A High Risk of rip currents continues through this evening, followed by a Moderate Risk Friday. The rip current risk could remain elevated through the weekend.

DISCUSSION

KEY MESSAGE 1: Cool weather will linger into the weekend. Near to slightly above normal temperatures return next week.

An impressively strong CAD (for Early Fall) is ongoing across the region this afternoon as low pressure strengthens offshore of the North Carolina Outer Banks. The wedge appears to be nearing full maturity with inland pressure rises starting to wane. The combination of weak isentropic lift with a corridor of strong UVVs head of an approaching shortwave rounding the base of the powerful upper low centered over Eastern North Carolina will support the development of additional isolated to scattered showers across mainly the Charleston Tri-County into Colleton County over the next few hours with a few patches of light drizzle also possible. With low-level dry air still being funneled south within the CAD, diabatic effects will have an meaningful impact on temperatures for the remainder of the afternoon across mainly parts of the Lowcountry. Temperature falls into the lower-mid 60s were recently noted across parts of Berkeley County where shower activity is the greatest and shows the impact diabatic processes can have. The high temperature forecast for the rest of today was lowered yet again per ongoing short term trends with the largest adjustments made north of the I-26 corridor. Some locations across the interior portions of the Lowcountry into the Charleston Metro Area will likely hold in the 60s for the rest of the day. The NBM continues to perform poorly and is running a very warm bias (~5-10 deg)-- typical in CAD events.

Shower activity will quickly wane by late afternoon/early evening as the corridor of strongest forcing shifts offshore with the passing shortwave. Skies are expected to clear overnight as the developing nor'easter shifts farther to the north offshore of the Mid-Atlantic States and even deeper drier air to the north gradually erodes the low-level cloud deck from the top-down. With the lack of a strong downslope component expected off the Southern Appalachians, there are concerns guidance is too quick in scouring out the low-level moisture embedded in the CAD. The forecast will show improving sky cover through the night, but not exactly to the degree the NBM is forecasting. It will certainly be the coolest night so far since last Spring with lows expected to drop into the mid-upper 50s inland with lower 60s at the beaches.

The inland wedge will steadily break down on Friday into Saturday as the nor'easter moves closer to New England and secondary high pressure builds in from the northwest. Friday night looks noticeably cool with lows dropping into the 48-52 degree range inland to the mid 60s at the beaches. Temperatures will gradually moderate through the period with average to slightly above average temperature returning Monday into Tuesday. Dry weather will prevail for much of the extended period.

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

A nor'easter located offshore of the Mid- Atlantic will lead to the continuation of breezy northeasterly flow across the region. The combination of these breezy winds with the upcoming full moon (9/26) result an increase in minor to moderate coastal flooding with each high tide cycle through the weekend (esp. for the Charleston Harbor).

Charleston Harbor: With the ongoing environmental conditions, tidal levels could reach 7.5 ft MLLW in the Charleston Harbor for this evening's high tide (approx. at 710PM). Therefore, Coastal Flood Advisory has been issued from 5PM to 10PM this evening for Charleston and Colleton County. Saltwater inundation will be possible 2 to 3 hours before and after high tide, and will likely result in some roads becoming impassable.

Coastal flooding concerns will continue tomorrow (9/26) with the potential to reach moderate flood stage for Charleston and Colleton County with the morning high tide (~7AM) and the evening high tide (~8PM). Additionally, guidance indicates the Saturday morning's high tide could see minor coastal flooding.

Fort Pulaski: Tidal levels could reach 9.5 ft MLLW for this evening's high tide in Fort Pulaski (approx. at 718PM), thus this will yield minor coastal flooding impacts across lower South Carolina and southeast Georgia. This would prompt a Coastal Flood Advisory for this evening's high tide, however confidence levels remain low as there remains some uncertainty regarding if the tidal departures will reach 1.76 ft MLLW. Thus, we will hold off on issuing a Coastal Flood Advisory for lower South Carolina and southeast Georgia. Conditions will continue to be monitored through the afternoon and will issue a Coastal Flood Advisory if tidal departures start trending that way. Additionally, guidance indicates tomorrow evening's high tide could also see minor coastal flooding.

KEY MESSAGE 3: A High Risk of rip currents continues through this evening, followed by a Moderate Risk Friday. The rip current risk could remain elevated through the weekend.

A strengthening area of low pressure offshore will continue to drive strong gusty winds along the coast and the coastal waters through tonight and into Friday. Also, this low will help to push ~8 second swell along the coast which will increase to longer period swell (10- 12 seconds) on Friday. This setup will continue to support a High Risk of rip currents at all beaches through this evening, followed by a Moderate Risk at all beaches on Friday. An elevated risk could linger into Saturday thanks to lingering swell. A lesser risk if expected Sunday as the low moves away and its influence along the Southeast coast dissipates.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

24/18z TAFs: KCHS/KJZI/KSAV: MVFR cigs will give way to improving conditions overnight as the inland wedge begins to break down and low- level moisture scours out. Gusty winds will persist into the day Friday. Some very light showers/drizzle could impact KCHS and KJZI through late afternoon, but no meaningful reductions in vsbys are expected.

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

MARINE

Through Tonight: Overall conditions are expected to peak across the local waters this evening and into the early morning hours as an offshore low strengthens and lifts northward. Across the outer waters, gusts as high as 35-40 knots are likely and the Gale Warning remains in effect through the overnight. For the nearshore waters, gusts up to 30 knots will continue and a Small Craft Advisory continues. For Charleston Harbor, gusts up to around 25 knots continue to be the main concern through the late evening which will then diminish into the low 20 knot range through the overnight. The Small Craft Advisory continues there through the late evening. Seas will continue to build through the evening and are forecast to peak around 4-8 ft out to 20 nm and 8-11 ft beyond.

Friday through Saturday: Marine conditions are expected to gradually improve on Friday as the offshore low lift northward closer to the New England coast and and the pressure gradient starts to relax across the local waters. The Gale Warning for the outer GA waters will need to be replaced with a Small Craft Advisory, and the ongoing Small Craft Advisory for the nearshore waters will continue through Friday evening/night. By Saturday winds and seas will diminish further and the only remaining headline will likely be the Small Craft Advisory for the outer GA waters into Saturday night.

Sunday through Tuesday: Quiet conditions will return and winds and seas will be well below Small Craft Advisory thresholds.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents until 8 PM EDT this evening for GAZ217- 219-239-241. SC...High Risk for Rip Currents until 8 PM EDT this evening for SCZ148>151. Coastal Flood Advisory until 10 PM EDT this evening for SCZ149- 150. MARINE...Small Craft Advisory until 11 PM EDT this evening for AMZ340. Small Craft Advisory until 2 AM EDT Saturday for AMZ360-362. Small Craft Advisory until 8 AM EDT Saturday for AMZ364. Gale Warning until 6 AM EDT Friday for AMZ380-382-384.