textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and Sections were updated.

KEY MESSAGES

- 1) The risk for coastal flooding and coastal hazards continues.

- 2) Seasonable temperatures and generally dry conditions are expected through Thursday.

- 3) Isaias is expected to strengthen into a strong hurricane before approaching the northern Gulf Coast on Friday. Although the center of the storm will remain well west of the region, heavy rainfall is possible across southeast Georgia and the South Carolina Lowcountry Friday into the weekend. Flash and urban flooding along with coastal flooding will be possible.

DISCUSSION

KEY MESSAGE 1: The risk for coastal flooding and coastal hazards continues.

For Charleston and Coastal Colleton Counties, gusty/persistent northeast winds associated with a wedge of high pressure inland and low pressure offshore in combination with high astronomical tide levels will likely support additional coastal flooding issues during high tide cycles through Thursday, and possibly into Friday.

The high tide during the afternoon/early evening hours later today will be the greatest concern when minor to potentially low-end moderate saltwater flooding is possible (~7.5 ft MLLW at the Charleston Harbor). Thus, have issued a Coastal Flood Advisory, which will be in effect from 4 PM to 8 PM.

For Beaufort, SC to McIntosh, GA Counties, the greatest risk for coastal flooding is also expected with the late afternoon/early evening high tide cycle today (~9.5 ft MLLW at Fort Pulaski, GA). Have opted to forgo issuing an advisory, as confidence in hitting flood stage still remains rather low. Will continue to monitor trends this afternoon/evening in the event that departures over-perform.

Rip Currents/High Surf/Beach Erosion: Gusty northeasterly winds will continue to promote a High Risk of Rip Currents along the Georgia beaches this afternoon, with a Moderate Risk of Rip Currents along our South Carolina beaches. Should see the pressure gradient relax heading into Thursday, resulting in only a Moderate Risk for our Georgia beaches through Friday.

KEY MESSAGE 2: Seasonable temperatures and generally dry conditions are expected through Thursday.

Quiet conditions continue to prevail across the region, with latest satellite imagery showing a fairly broad swath of mid-upper level clouds across much of our area. Given that this cloud cover is expected to linger for much of the evening and overnight period, should see afternoon highs remain limited into the upper 70s to low 80s. Similar to this morning, will also see tonight's lows fall into the 50s to lower 60s. Otherwise, expect weak ridging aloft and dry northeasterly surface flow to result in another dry day Thursday, with temperatures once again in the upper 70s to low 80s.

KEY MESSAGE 3: Isaias is expected to strengthen into a strong hurricane before approaching the northern Gulf Coast on Friday. Although the center of the storm will remain well west of the region, heavy rainfall is possible across southeast Georgia and the South Carolina Lowcountry Friday into the weekend. Flash and urban flooding along with coastal flooding will be possible.

NHC expects that Isaias will move northeast across the southern Gulf through much of Thursday. Once the storm passes north of the Yucatan Peninsula, the storm motion will turn north and is forecast to develop into a strong hurricane. Isaias is expected to approach the northern Gulf Coast on Friday. Guidance shows a large plume of tropical moisture surging northward across the Deep South and Southeast U.S. beginning on Friday, by late Friday night PW values may exceed 2.2 inches. Deep tropical moisture is expected to peak late Saturday into Saturday night, then drier air arriving with a cold front Sunday night.

NAM12 indicates that a ridge of high pressure will strengthen across the Carolinas and Georgia on Friday. The wedge should peak Friday night as the center of 1032mb high slides over New England and widespread rainfall cools the ridge over the Southeast U.S. On Friday, NAM12 H85 winds turn from the northeast and strengthen to 40 kts, developing a band of isentropic lift, advancing south to north across the forecast area. Flow will turn from the east Friday night into Saturday as Isaias begins to track north across the Deep South. In addition, the strong east flow should draw instability off the Atlantic, across the CWA. The pattern become more interesting on Saturday afternoon into Saturday evening as a coastal trough deepens and inland wedge begins to erode. Strong moisture convergence may support a concentrated area of rain showers and thunderstorms along the coast, this band could produce excessive rainfall rates. The GFS and ECMWF along with the AI versions indicate that the heaviest rainfall may occur along the SC coast. In addition, New Moon high tides with elevated astronomical tides will occur this weekend. Any elevated rainfall rates during high tides may lead to flooding of coastal areas, including Savannah and Charleston. Unfortunately, the greatest coastal flooding is expected with both high tides on Saturday, currently with 10 percent risk of major coastal flooding for the Charleston Harbor. The latest WPC Excessive Rainfall Outlook places much of Southeast Georgia in a Marginal Risk for excessive rainfall Friday and a Slight Risk for portions Southeast Georgia and Southeast South Carolina on Saturday.

Rainfall will continue Saturday night through Sunday, supported by the approach of H5 shortwave and sfc cold front. Rainfall rates should gradually decrease on Sunday.

Breezy conditions will likely develop across SE GA and SC on Saturday, especially as the wedge begins to erode. Wind gusts over inland GA and SC may range between 25 to 30 mph. Coastal counties, especially near the water's edge or elevated bridges, could see gusts of 40 mph at times.

Tornado Threat: The concern for tornadoes always increases with a landfalling hurricane in the region. CAD should dominate the region of Friday, keeping instability generally elevated. However, the wedge is expected to erode during the daylight hours on Saturday. In addition, winds will veer from the east as Isaias tracks north across the Deep South. Forecast sounding south and east of the wedge boundary indicate SRH 0-3 km in excess of 250 m2/s2 and SBCAPE around 1000 J/kg late Saturday afternoon and evening. This environment could support brief and fast-moving tornadoes or waterspouts. Interestingly, Google DeepMind based AI convective hazard forecast highlights portions of SE GA and coastal SC with a 15 percent risk of severe weather.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

18Z TAFs: VFR conditions continue, with gusts up to 20 kts possible through the afternoon. Otherwise, look for light N/NE winds to prevail for the remainder of the period.

Extended Aviation Outlook: VFR through late week. The risk for flight restrictions returns over the weekend with widespread showers and/or thunderstorms. Gusty east-northeast winds possible on Saturday.

MARINE

This Afternoon: Breezy winds continue offshore thanks to a pinched pressure gradient, though this should begin to relax as we head into the evening and overnight period. Wave heights should also gradually decrease during this time, allowing the Small Craft Advisories to expire from north to south along our inner South Carolina and Georgia waters. Despite headlines coming to an end, still expect gusts between 10 to 20 kts to prevail overnight.

Thursday: The last remaining 6 foot waves move out of the marine zone from Savannah to Altamaha Sound, GA 20-60nm offshore near daybreak, allowing for the final Small Craft Advisory to expire. While the surface high pressure continue to weaken, the pressure gradient remains elevated with the stalled front to our south, continuing wind gusts generally in the 15 to 20 knot range.

Friday through Sunday: See NHC forecast for the latest on Tropical Cyclone Isaias. The pressure gradient will increase between a coastal trough and high pressure inland. Winds should strengthen through most of this weekend. Small Craft Advisories are likely by Friday afternoon as 25+ knot gusts move in, with gale wind gusts possible by Saturday. Strengthening winds should drive sea higher, with 6-9 ft wave heights possible on Saturday. Seas may peak between 9-11 ft by Saturday evening. Elevated wind and sea conditions will likely persist through the rest of the weekend.

EQUIPMENT

The Metter, GA, NOAA Weather Radio Station (WWH-25) will be offline from 8 AM Wednesday, October 7, through the afternoon of Thursday, October 8. The transmitter is being relocated.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents until 8 PM EDT this evening for GAZ217- 219-239-241. SC...Coastal Flood Advisory until 8 PM EDT this evening for SCZ149- 150. MARINE...Small Craft Advisory until 5 PM EDT this afternoon for AMZ360. Small Craft Advisory until 8 PM EDT this evening for AMZ362. Small Craft Advisory until 2 AM EDT Thursday for AMZ364. Small Craft Advisory until 8 AM EDT Thursday for AMZ384.