textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) The risk for coastal flooding continues into the end of the week, primarily for Charleston and coastal Colleton counties.

- 2) Hurricane Isaias is expected to track across the central Gulf today, before approaching the northern Gulf Coast on Friday. 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 along with coastal flooding is possible.

DISCUSSION

KEY MESSAGE 1: The risk for coastal flooding continues into the end of the week, primarily for Charleston and coastal Colleton counties.

As the region remains between a surface high pressure to our north and a stationary front to our south, the gusty northeasterly winds persist, primarily over the coastal waters. Together with the elevated astronomical high tides, minor coastal flooding remains possible with each high tide along the Charleston and coastal Colleton counties through Thursday, potentially continuing into Friday.

Thursday morning's astronomical tide at the Charleston Harbor is for 6.2 ft MLLW, which would require a tidal departure of 0.8 ft MLLW to reach minor flood stage of 7.0 ft MLLW. While the departure will likely oscillate a smidge, as of 2 AM it is 0.91 ft MLLW and is expected to remain above 0.8 ft MLLW prior to high tide, likely resulting in minor coastal flooding in the 7.0-7.2 ft MLLW range. A Coastal Flood Advisory has been issued from 5 AM to 9 AM Thursday for Charleston and coastal Colleton county. Thursday evening and Friday morning will continue to see elevated high astronomical tides at 6.25 ft and 6.43 ft MLLW respectively, continuing chances for minor coastal flooding.

Friday evening will bring another chance for minor coastal flooding for Charleston and coastal Colleton as elevated high tides persist, though the weather setup becomes more complex. Rainfall chances will increase Friday afternoon as a tropical plume of moisture lifts northwards amidst positive vorticity advection, though there will be a bit of dry air in the low-level to saturate before rainfall reaches the surface, so if rain starts to accumulate prior to high tide the area will be more prone to coastal flooding. In addition, as surface high pressure continues to our north, the track of Hurricane Isaias to our west as it moves north will increase the surface pressure gradient across the region. How quickly the northeast wind strengthens as the system move north will further influence the potential for coastal flooding.

Rip Currents: Increasingly gusty northeasterly winds combined with increasing wave heights will create a High Risk of rip currents along the southeast Georgia beaches on Friday, with a Moderate Risk of rip currents along beaches from Edisto, SC to Savannah, GA. Expect High Risk of rip currents across all area beaches on Saturday.

KEY MESSAGE 2: Hurricane Isaias is expected to track across the central Gulf today, before approaching the northern Gulf Coast on Friday. 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 along with coastal flooding is possible.

NHC expects Hurricane Isaias to track across the central Gulf today, before approaching the northern Gulf Coast on Friday. Guidance continues to support a deep plume of tropical moisture drawn northward across the Deep South and Southeast United States as this process unfolds into Friday, with PWATs likely reaching 2.0-2.25 inches across the region by Friday night. Deep tropical moisture is then likely to peak across the local area Saturday into early Sunday (PWATs 2.25 to 2.5 inches), prior to a cold front arriving with drier air by Sunday evening.

Deep tropical moisture will set the stage for heavy rainfall locally this weekend as widespread isentropic lift occurs between a wedge of high pressure across the Carolinas/Georgia and a weak front/coastal trough starting Friday night. Widespread showers and embedded thunderstorms are possible Saturday into Sunday as the setup remains quite favorable for deep moisture transport feeding across the Southeast United States between a mid-lvl pattern featuring a closed low over the lower Mississippi River Valley and a ridge centered across the western Atlantic. Given the setup, an elevated risk for flash and urban flooding is anticipated locally, with the main period of concern occurring Saturday and Saturday night as PWATs approach ~2.5 inches and rounds of precip activity occur across already wet grounds. A Flood Watch could eventually be needed during this period. The latest WPC Excessive Rainfall Outlook places much of Southeast Georgia in a Marginal Risk for excessive rainfall Friday. On Saturday, a Slight Risk for excessive rainfall is in place for much of Southeast South Carolina and a Marginal Risk for Southeast Georgia.

Additional rains are possible heading into Sunday afternoon, but will be highly dependent on the speed of the h5 shortwave and sfc cold front arriving from the west to start off next week. In addition to heavy rainfall potential, 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.

Breezy conditions will also likely develop across Southeast Georgia Friday night into Saturday as the inland wedge begins to erode. Wind gusts upwards to 25-30 mph are possible across much of the local area with slightly higher gusts up to 35-40 mph across coastal counties near the beaches and/or on elevated bridges.

Tornado Threat: Latest guidance continues to support an increase in 0-3km SRH (~250 m2/s2) and SBCAPE (~1000 J/kg) locally as the high pressure wedge erodes during daylight hours Saturday. Forecast soundings indicate veering wind profiles at the time with a slightly backing easterly sfc wind, perhaps along a weak front shifting through the local area while Isaias tracks north across the Deep South. This environment could support brief and fast-moving tornadoes and/or waterspouts Saturday afternoon/evening.

AVIATION /07Z THURSDAY THROUGH MONDAY/

VFR conditions are expected at CHS/JZI/SAV terminals through 06Z Friday.

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

MARINE

Today: Between surface high pressure to our north and a stationary front to our south, breezy northeasterly winds will persist today. They will however be much weaker than recent, sustained in the 10-15 knot range with occasional gusts to 20 knots. Seas across the region are already reporting wave heights of 4 to 5 feet, so the two remaining Small Craft Advisories have been allowed to expire.

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. Small Craft Advisories will likely be needed for all waters as early as Friday afternoon across all near and offshore zones, though the Charleston Harbor may not see the breezy winds until overnight into Saturday.

Saturday: As Isaias moves northwards on Saturday, still expected to remain well west of the area in Alabama per NHC's latest forecast, the pressure gradient strengthens leading to a period where gale- force winds are possible. Latest NBM probabilities for gale-force winds (gusts >= 35 knots) peaks near 50%, so that's a trend we are monitoring. Rough seas are anticipated on Saturday, in the 5 to 8 ft range from 0-20 nm out and 8 to 10 ft in the waters from 20-60nm out.

Sunday into Monday: The surface high pressure gets pushed off into the Atlantic as the remnant surface low pressure 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.

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...None. SC...Coastal Flood Advisory until 9 AM EDT this morning for SCZ149- 150. MARINE...None.