South Carolina Gov. Henry McMaster said conditions are "very dangerous" as Ian comes ashore, but so far the amount of water in places like the hospital district of Charleston is lower than expected.

"But I would ask people: Don't quit yet because this is still coming. We're not out of the woods," McMaster said.

South Carolina has positioned 90 National Guardsmen for high-water vehicle support. They have received a few requests for support, including for swift water rescue teams, said Kim Stenson, the head of South Carolina Emergency Management.

Stenson said they expect calls for damage and flooding to increase throughout the day.

A tornado threat is increasing as Hurricane Ian gets closer to the coast of the Carolinas.

Myrtle Beach and Greenville, South Carolina; Wilmington, North Carolina and Virginia Beach, Virginia, are all under tornado watch through 10 p.m.

A tornado warning is also in effect south of Wilmington.

-ABC News' Melissa Griffin

President Joe Biden pledged the U.S. would do "whatever it takes," to help Florida recover from Hurricane Ian, stressing the effort would be done as "one nation and one America," during an event at the White House marking the Jewish New Year.

"We're working very closely with the governor and the entire Florida delegation, Democrat and Republican, making sure that we can do everything we can, including now search and rescue, recovery and rebuilding efforts, but it’s going to go on for a long while. And whatever it takes, we’re going to be there, as one nation, and one America. We're not going to walk away," Biden said.

Biden noted the time of celebration is also a "very difficult time for so many Jewish families in Florida," and those with loved ones in the state.

"[O]ur heart goes out to everyone there in the state experiencing what could be maybe one of the most devastating hurricanes in the history of that state," Biden said.

-ABC News' Molly Nagle

President Joe Biden spoke with South Carolina Gov. Henry McMaster Friday ahead of Hurricane Ian's expected landfall.

Biden told McMaster the administration will provide whatever assistance South Carolina needs. Biden issued an emergency declaration for South Carolina Thursday night.

-ABC News' Molly Nagle

Tropical Storm Ian is marching toward the Gulf Coast of the U.S., a region quite vulnerable to hurricanes.

The underwater geology of the Gulf of Mexico is what makes the Gulf Coast particularly unguarded to the massive influx of seawater, said Michael Brennan, acting deputy director of the National Hurricane Center, according to an article on Inside Science.

The symmetry of the Gulf of Mexico, with its shallow, sloping continental shelf, allows the storm surge to be pushed even higher onto land, Brennan told the outlet.

Because the land around the Gulf Coasts is typically flat, that exacerbates the flooding even more and allows the storm surge to travel farther inland, Brennan said.

"Of all of the hazards of hurricanes --- winds, rain, storm surges –-- surges are what can cause the largest loss of life," Brennan told Inside Science.

The Tampa Bay-St. Petersburg area, one of the regions bracing for Ian, is especially vulnerable to storm surge flooding but has avoided a direct hit from a strong hurricane since 1921.

Editor's Note: This post has been updated to reflect proper attribution to Inside Science.

-ABC News' Max Golembo