Hurricane Idalia will amplify what were already expected to be higher-than-usual high tides in Savannah, Ga., and Charleston, S.C., because the full moon is making its closest orbital pass to Earth, a rare phenomenon sometimes known as a ‘supermoon.’

A supermoon — this one is doubly rare because it a blue moon, or the second full moon in a month — makes for a spectacle in the night sky, an outsized glowing globe, but it also exerts a stronger gravitational pull, making tides higher, just as the storm is forecast to push a surge of water ashore.

Charleston Harbor is now forecast to reach 8.7 feet, the seventh highest tide on record, if it occurs, said Brian Haines, the meteorologist in charge at the Charleston Weather Office

Even if the tide reaches eight feet, it would land among the top 50 highest tides, and given the extensive historical record in Charleston, “it’s pretty impressive,” said Mr. Haines. At eight feet, major coastal flooding would occur, with widespread impacts to the downtown area, including closing numerous roads or making them impassable.

Mr. Haines noted that at Fort Pulaski near Savannah, officials were already experiencing a two-foot surge at low tide. The surge forecast for Savannah was a little more tricky because the storm’s center could pass near the city and coincide with high tide. If Idalia slows down slightly, the city could see more surge; if it speeds up, it could see less and reach its peak before high tide is expected.

On Wednesday morning, Darragh Simon skimmed the aisles for last minute items at Royall Ace Hardware in Mount Pleasant, S.C., a suburb of about 95,000 located to the east of Charleston Harbor. Awaiting the storm, she ran through what she called “her mental check list of items” — water, batteries, flashlights and candles at the ready.

Ms. Simon noticed the storm changing rapidly overnight, which put her on edge. “I’ve lived through a lot of hurricanes and rarely see one that comes on this strong this fast,” she said, even after losing her home and nearly all her belongings to Hurricane Hugo in 1989.

“Those were tough times,” Ms. Simon said. “I had a 1-year-old and 3-year-old living in an Airstream.”

While she plans to post up at her home in Isle of Palms, S.C., it will be in nervous anticipation.

Judson Jones is a meteorologist and reporter for The Times, covering extreme weather around the world. More about Judson Jones

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