A flash flood emergency was declared in areas north of Orlando as the region was hit by about a foot of rain, the National Weather Service said.

The declaration covered Sanford, Lake Mary and Heathrow, the service said. Nearby Central Orange, Seminole and South Central Volusia were also under flash flood warnings.

“This is a particularly dangerous situation,” the service said. “Seek higher ground now! Life threatening flash flooding of low water crossings, small creeks and streams, urban areas, highways, streets and underpasses.”

Parts of the region have seen more than 16 inches of rain within 24 hours, with more than 9 inches in the last six hours, making the storm a 1,000-year flood event in the Orlando area and to the north.

-ABC News’ Max Golembo and Kenton Gewecke

More than 2.5 million customers were without power in Florida at about 5:30 a.m. on Thursday, the state’s providers said.

Ian’s winds slowed to 65 mph early on Thursday morning, downgrading the system to a tropical storm, the National Hurricane Center said.

“The Hurricane Warnings along the east and west coasts of the Florida peninsula have been changed to Tropical Storm Warnings,” the center said.

-ABC News’ Max Golembo

Florida Governor Ron DeSantis will be holding his next press conference on the latest developments of Hurricane Ian at 8:45 a.m. ET at the State Emergency Operations Center in Tallahassee.

President Biden, meanwhile, will visit FEMA headquarters in Washington, D.C. to receive an update on Hurricane Ian at 12 p.m. ET.

While Hurricane Ian is expected to become a tropical storm on Thursday, the storm has battered southwest Florida and has left more than 2 million people without power so far.

Federal Emergency Management Agency Administrator Deanne Criswell warned Tuesday, "Floridians are going to experience the impacts from the storm for a very long time."

"Our biggest concern as we wait for this storm to make landfall is storm surge," Criswell said. "In 2018, when Hurricane Michael impacted the Florida Panhandle, there were five recorded fatalities as a result of storm surge. So therefore, if people are told to evacuate by their local officials, please listen to them. The decision you choose to make may mean the difference between life and death."

President Joe Biden on Tuesday urged residents to "evacuate when ordered."

Biden said he spoke to the mayors of Tampa, St. Petersburg and Clearwater Tuesday morning and told them to "contact me directly" for "whatever they need."

Criswell said a search and rescue coordination group has been activated, including members from FEMA's urban search and rescue teams, the Coast Guard, the Department of Defense and the Department of the Interior.

The Red Cross has established 29 shelters and is prepared to open 60 more shelters if needed, she said.

Criswell stressed, "To those who may be watching at home, get ready and do not underestimate the potential that the storm can bring."