Extreme flash flooding is possible in the Blue Ridge Mountains in Georgia, North Carolina and South Carolina.

First, a storm system separate from Helene is hitting the south on Wednesday. Up to 6 inches of rain is possible over the next 24 hours, bringing a flash flood risk from Atlanta to Asheville, North Carolina.

Then, Thursday afternoon through Friday morning, Hurricane Helene will bring even more extreme rain and flooding to the southern Appalachians.

A rare "high risk" alert for heavy rain has been issued. Up to 15 inches of rain is possible in some spots.

-ABC News’ Max Golembo

Helene is forecast to bring 10 to 15 feet of dangerous storm surge to Florida’s Big Bend area, and 4 to 8 feet of storm surge to Tampa Bay.

In 2005, during Hurricane Katrina, at least 1,500 people died "directly, or indirectly, as a result of storm surge," according to the National Hurricane Center.

Click here to read how storm surge works and why it's so dangerous.

Tropical Storm Helene is nearing hurricane strength Wednesday morning as it churns just of the northeastern coast of the Yucatan Peninsula.

Helene is about 100 miles west-southwest from the western tip of Cuba.

-ABC News’ Max Golembo

Tropical Storm Helene was expected to "rapidly" strengthen and grow in size as it moved on Wednesday into the Gulf of Mexico, U.S. weather officials said.

The storm as of about 4 a.m. local time had maximum sustained winds of about 65 mph, the National Hurricane Center said. It was expected to intensify into a hurricane on Wednesday, before further strengthening into a "major" hurricane on Thursday, the center said.

Helene early Wednesday was traveling northwest at about 9 mph, the center said.

"On the forecast track, the center of Helene will pass near the northeastern coast of the Yucatan Peninsula this morning, move across the eastern Gulf of Mexico on later today and Thursday, and reach the Big Bend coast of Florida late Thursday," the center said.

-ABC News' Max Golembo and Kevin Shalvey