textproduct: Charleston

WHAT HAS CHANGED

Key Message 1, the Marine, and Aviation Sections were updated.

KEY MESSAGES

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

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

- 3) The National Hurricane Center (NHC) has identified a disturbance in the southwestern Gulf with a high chance of tropical cyclone formation during the next 48 hours. Regardless of development, heavy rainfall could develop over the Southeast United States by this weekend.

DISCUSSION

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

Coastal Flooding: Strong northeast winds associated with a wedge of high pressure inland and low pressure offshore in combination with high astronomical tide levels will support coastal flooding issues during high tide cycles through Thursday and possibly into Friday. The tides of most concern will occur during the late afternoon/early evening hours.

Rip Currents/High Surf/Beach Erosion: The setup remains highly favorable rip currents into Wednesday with breezy/windy northeast winds expected. A high risk remains in place for all beaches through this evening. A high risk was highlighted for the Georgia beaches with a moderate risk for the South Carolina beaches for Wednesday. In addition, seas will build across nearshore waters, with 4-7 ft breakers occurring in the surf zone (largest along coastal Georgia) starting this afternoon and persisting into Wednesday. A High Surf Advisory remains in effect through early Wednesday afternoon along the entire coast and could eventually need to be extended longer along Georgia beaches into later Wednesday. Significant beach erosion is likely with guidance suggesting highest chances for impacts around the afternoon/early evening high tide cycle today.

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

Lake Winds: Observations at PNOS1 have been running in the 15-20 kt range out of the northeast for much of the day. The sensor tends to under sample winds in northeast wind regimes given the limited open fetch between the sensor and the southeast side of Lake Moultrie. Winds may be diminish a bit through mid-afternoon, but there are signals that a modest wind surge will work across the coastal waters, Charleston Harbor and Lake Moultrie during the early-mid evening hours as the thermal gradient tights a bit with the loss of heating. For this reason, a Lake Wind Advisory has been issued through 3 AM Wednesday for frequent gusts around 25 kt with waves 2 ft.

Wednesday: The forecast area will remain between a stationary front over FL and broad high pressure over the western Carolinas. This pattern will support steady northeast winds, speeds favoring values around 10 mph. Given dry and stable northeast winds, conditions across SE GA and the SC Lowcountry should remain dry. Latest HREF indicates a bit less cloud cover, especially across inland GA and SC. With weaker NE winds and increase insolation, high temperatures are forecast to range around 80 degrees. Wednesday night, a weak pressure gradient should provide light to calm winds. Scattered high clouds should limit radiational cooling conditions overnight. Using a blend of guidance, low temperatures are forecast to range from the low 50s inland to the low 60s along the coast.

Thursday: A weak cold front is forecast to slide over the Mid- Atlantic states by late afternoon. Winds across the CWA should remain fairly light and variable through the day. With dry conditions anticipated, high temperatures may increase by 1 to 2 degrees over values observed on Wednesday.

KEY MESSAGE 3: The National Hurricane Center (NHC) has identified a disturbance in the southwestern Gulf with a high chance of tropical cyclone formation during the next 48 hours. Regardless of development, heavy rainfall could develop over the Southeast United States by this weekend.

Late this week, the H5 pattern will feature a broad closed low over the Southern Plains and 595 DM ridge centered near Bermuda. This pattern will gradually lift tropical moisture northward across the Deep South and Southeast U.S. Friday through the weekend. NHC has highlighted a broad area of low pressure, invest AL92, located between Mexico and the Yucatan Peninsula in their Tropical Weather Outlook. Invest AL92 has a high, near 100 percent, chance of formation during the next day or so as it drifts eastward.

Friday into Saturday, moisture ahead of AL92 will push north across the Deep South and Southeast U.S. as wedge of high pressure builds across the Carolinas and Georgia. GFS forecast soundings and overall pattern suggest that some degree of instability will be present across the coastal counties of GA/SC. The instability combined with strengthening isentropic flow should support PoPs rising to categorical by Saturday afternoon.

Saturday night through Sunday: The latest GFS and ECMWF and especially the AI versions indicate a maximum of QPF within the coastal trough Saturday night into Sunday. This rainfall would be supported by strong isentropic lift and a building field of SBCAPE, supporting thunderstorms. This could produce a quick swath of significant rainfall, especially across the coastal counties. 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.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Some MVFR cigs will linger around KCHS/KJZI/KSAV to start the 00Z TAF period. Cigs should lift to VFR by midnight, remaining VFR through the remainder of the TAF period.

Extended Aviation Outlook: VFR through late week. The risk for flight restrictions returns over the weekend with increasing chances for showers and/or thunderstorms.

MARINE

Through Tonight: All Gale Warnings have transitioned over to Small Craft Advisories, as gusts are now below 34 knots.

Wednesday: The pressure pattern will continue to produce gusty northeast winds, with gusts between 25-30 kts across the nearshore and outer waters. Winds may gradually decrease later in the day. As a result, wave heights are forecast to begin the day between 5-8 ft, then decrease by a foot through the rest of the day. Small Craft criteria wind and sea should end Wednesday night.

Thursday into Thursday night: High pressure will relax the pressure gradient across the marine zones. Conditions should remain well below Small Craft criteria.

Friday through Saturday: See NHC forecast for the latest on invest AL92. The pressure gradient will increase between a coastal trough and high pressure inland. Winds should strengthen through the period, with gale wind gusts possible by Saturday. Strengthening winds should drive sea higher, with 6-9 ft wave heights possible on Saturday. Elevated conditions will likely persist through the rest of the weekend.

EQUIPMENT

The Metter, GA, NOAA Weather Radio Station (WWH-25) will be offline from mid-morning 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 through Wednesday evening for GAZ217-219- 239-241. High Surf Advisory until 2 PM EDT Wednesday for GAZ217-219-239- 241. SC...High Surf Advisory until 2 PM EDT Wednesday for SCZ148>151. Lake Wind Advisory until 3 AM EDT Wednesday for SCZ045. MARINE...Small Craft Advisory until 3 AM EDT Wednesday for AMZ340. Small Craft Advisory until 8 PM EDT Wednesday for AMZ360-362. Small Craft Advisory until 2 AM EDT Thursday for AMZ364. Small Craft Advisory until 8 AM EDT Thursday for AMZ384.