textproduct: Charleston

WHAT HAS CHANGED

Increasing confident that this morning's high tide will remain below minor flood stage for the Charleston area. Increasing confidence in gusty northeast winds Saturday morning, bringing the potential for major coastal flooding to portions of the area. The risk for flash flooding and tornadoes remains unchanged for Saturday.

KEY MESSAGES

- 1) Moderate to Major coastal flooding expected with the Saturday morning high tide, with additional coastal flooding possible through the weekend.

- 2) Hurricane Isaias is expected to approach the northern Gulf Coast late today, before shifting across the Deep South on Saturday. Although the center of the storm will remain well west of the region, heavy rainfall is likely across Southeast Georgia and the South Carolina Lowcountry this weekend. Flash and urban flooding is possible.

- 3) A few tornadoes are possible across Southeast Georgia and Southeast South Carolina Saturday.

DISCUSSION

KEY MESSAGE 1: Moderate to Major coastal flooding expected with the Saturday morning high tide, with additional coastal flooding possible through the weekend.

Persistent northeast wind and Saturday's new moon will combine to bring elevated high tides into Sunday. Here is a breakdown of each high tide:

Friday AM: There is increasing confidence that this morning's high tide will peak just below minor flood stage in the Charleston Harbor again as departures have diminished further. A Coastal Flood Advisory is not expected at this time.

Friday PM: Northeast winds increase through the day, driving departures higher. However, the astro tide value will be lower than the morning high tide, potentially offsetting the increased tidal departure. Expect high tide to peak near minor flood stage for the Charleston and Colleton County Coastlines.

Saturday AM: The tide cycle of greatest concern as gusty northeast winds could drive departures significantly higher through the early morning hours. The astro tide value for the Charleston Harbor is into Action Stage by itself, so, conceptually, gusty northeast winds could be sufficient to push the high tide to the 8 ft MLLW major coastal flooding threshold for the Charleston and Colleton County coastlines. Additionally, the threat for heavy rainfall will increase through the morning, potentially exacerbating flooding problems for Downtown Charleston. Further south, Minor to moderate coastal flooding is expected along the Georgia and Beaufort County, SC coastlines.

Saturday PM: Northeast winds begin to diminish, but elevated tidal departures persist. Minor to moderate coastal flooding likely for the Charleston area, with lingering rainfall again bringing the threat of compounded flooding impacts. Further south, high tide is most likely to remain below minor flood stage for the Savannah/Beaufort area.

Sunday AM: Southerly winds will bring significantly diminished tidal departures, though minor coastal flooding remains possible for the Charleston area.

Rip Currents/High Surf/Beach Erosion: Increasing northeasterly winds and building wave heights bring a High Risk of rip currents along the southeast Georgia beaches today. Then, tomorrow, expect High Risk of rip currents across all area beaches. The elevated swell and flooding high tides may lead to areas of significant beach/dune erosion.

KEY MESSAGE 2: Hurricane Isaias is expected to approach the northern Gulf Coast late today, before shifting across the Deep South on Saturday. Although the center of the storm will remain well west of the region, heavy rainfall is likely across Southeast Georgia and the South Carolina Lowcountry this weekend. Flash and urban flooding is possible.

NHC expects Hurricane Isaias to approach the northern Gulf Coast late today, before shifting across the Deep South Saturday. As this process unfolds a deep plume of tropical moisture will be drawn northward across the Southeast United States starting this evening with rounds of showers and thunderstorms anticipated locally early Saturday into early Sunday prior to a cold front arriving from the west later Sunday morning.

For today, much of the area remains dry into mid-late morning hours while a wedge of high pressure holds across the Carolinas into Georgia inland. The exception will be for areas well south near the Altamaha River and coastal Georgia where moisture first arrives, with activity generally in the form of moderate showers. Expect activity to slowly expand north during afternoon into early evening hours as favorable isentropic ascent develops across the region and PWATs in excess of 2.0 inches arrive between the wedge front inland and a weak front/coastal trough approaching from the south.

Saturday into early Sunday: Numerous to widespread showers with embedded thunderstorms should be ongoing before daybreak Saturday as a weak warm front approaches the Altamaha River early and further north into Southeast Georgia prior to sunrise, before continuing north across the local area during the day. The position of the warm front will likely contribute to the bulk of enhanced precip activity for the day, with an uptick in rainfall rates while PWATs peak (2.25- 2.40 inches), potentially supporting minor and/or localized flash flooding within activity across saturated grounds, especially if coinciding with Saturday high tide cycles. The greatest risk for flooding remains across coastal counties of Southeast South Carolina, where a slightly longer duration of moisture convergence occurs with the nearby front/coastal trough. Given some tendency weighted towards this solution, we have refined rainfall totals to 1- 3 inches inland and across Southeast Georgia and 3-5 inches with isolated higher amounts possible across coastal counties of Southeast South Carolina. A Flood Watch could eventually be issued later this morning for parts of Southeast South Carolina for Saturday morning into Saturday night. The latest WPC Excessive Rainfall Outlook places Southeast South Carolina and Southeast Georgia in a Slight Risk for excessive rainfall on Saturday.

Sunday: Confidence in overall shower and thunderstorm activity is quite a bit less to start early week, highly dependent on the arrival of a h5 shortwave and cold front to start off next week. However, moisture supply is likely to remain within a southerly flow until fropa occurs. Although minor and/or flash flooding is not expected during this time period, brief heavy downpours are possible with activity shifting offshore late morning.

Key Message 3: A few tornadoes are possible across Southeast Georgia and Southeast South Carolina Saturday.

Latest guidance continues to support a potential for tornado activity within a warm sector well to the east of Isaias shifting across the Deep South and towards Tennessee. The threat for tornadoes increases locally during early morning Saturday, as winds veer from the east in advance of a warm front. Coastal areas of Southeast Georgia could see the first of this activity during predawn hours Saturday, when SBCAPE (~1000 J/kg), 0-3km SRH (~250 m2/s2), 0-6km bulk shear (~40kt) and a slight backing of the sfc wind occurs near the warm front shifting north. The latest HREF paintball plots also show a signal for this type of activity closer to the coast during early morning hours, and gradually shifting north into coastal Southeast South Carolina late morning. Despite widespread cloud cover and precip activity, instability levels should gradually increase across inland areas as a wedge of high pressure erodes and sfc winds gradually turn more southerly through late day. Although the risk for tornadoes appears less likely Saturday afternoon due to the departure of the warm front, there remains a risk for such activity within the warm sector displaying deep tropical moisture, with perhaps a secondary increase in tornado threat ahead of/near a cold front shifting through the region late Saturday night/early Sunday.

AVIATION /07Z FRIDAY THROUGH TUESDAY/

VFR conditions are expected at CHS/JZI/SAV terminals through 00Z Saturday. Gusty easterly winds and showers should begin impacting the SAV terminal between 00-06Z Saturday with previaling MVFR restrictions likely. A few thunderstorms could arrive at SAV Saturday night as well, producing MVFR/IFR conditions. At CHS/JZI, the risk for flight restrictions increases Saturday night as well, with PROB30 groups in the 00-04Z range at both sites, followed by prevailing MVFR conditions and showers after midnight.

Extended Aviation Outlook: Flight restrictions are likely at all terminals Saturday morning through Sunday with widespread showers and/or thunderstorms, some capable of heavy downpours and gusty winds.

MARINE

Friday: See NHC forecast for the latest on Hurricane Isaias. For our region, the pressure gradient will increase between a high pressure to our north and the hurricane to our southwest as it travels northwards. This is expected to strengthen the northeast winds Friday morning into the afternoon hours, with wave heights building as well. The wind speed forecast has trended slightly weaker, and persistent gale conditions are now unlikely across GA coastal waters where a Small Craft Advisory has been issued.

Friday night into Saturday: Combination of inland ridging and developing coastal troughing as a warm front moves up from the south will bring Gale conditions to the SC coastal waters. Strongest winds will be on the west side of a coastal boundary expected to linger offshore, with the GA waters remaining south of this coastal boundary and unlikely gust above 30 kt. Moderate southernly flow develops later Saturday as the warm front lift north of the area.

Lake Moultrie: Wind gusts across Lake Moultrie expected to reach 30 kts Saturday morning. A Lake Wind Advisory has been issued accordingly.

Sunday into Monday: The surface high pressure gets pushed off into the Atlantic as the remnant surface low pressure of Isaias is expected to push off into the northeast states before moving offshore. This will likely keep winds breezy on Sunday as southerly winds move in, with breezy westerly winds on Monday. Currently, expecting at least Small Craft Advisory conditions to persist into Sunday, with a non-zero chance for them to continue to portions of Monday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents from 8 AM EDT this morning through this evening for GAZ217-219-239-241. SC...Lake Wind Advisory from 5 AM to 2 PM EDT Saturday for SCZ045. MARINE...Small Craft Advisory from 3 AM Saturday to 1 AM EDT Sunday for AMZ340. Gale Warning from 10 PM this evening to 10 PM EDT Saturday for AMZ360. Gale Warning from 7 PM this evening to 10 PM EDT Saturday for AMZ362. Small Craft Advisory from 11 AM this morning to 1 AM EDT Sunday for AMZ364-384. Gale Warning from 10 PM this evening to 4 AM EDT Sunday for AMZ380. Gale Warning from 7 PM this evening to 4 AM EDT Sunday for AMZ382.