Tropical Storm Katrina threatens FloridaMIAMI, Florida (CNN) -- More than 5 million people living in metropolitan Miami and surrounding areas of southeast Florida were warned to prepare for Tropical Storm Katrina, which could strike the coast as a minimal hurricane late Thursday or early Friday.
On Wednesday night, forecasters issued a hurricane warning for a 170-mile stretch of coastline between Vero Beach and Florida City, which means hurricane-force winds of at least 74 mph are expected within 24 hours.
The warning area includes the major cities of Miami, Fort Lauderdale and Palm Beach, as well as Lake Okeechobee.
While Katrina was only packing sustained winds of 50 mph late Wednesday as it moved over the northwest Bahamas, Max Mayfield, director of the National Hurricane Center, told CNN the storm has a "very good chance" of strengthening to hurricane strength by the time it reaches Florida.
"People need to take it very seriously," Mayfield said.
He said one of the major concerns with Katrina will be flooding. Forecasters expect the storm to slow as it approaches the Sunshine State, creating the potential for a "significant heavy rainfall event" with up to a foot of rain.
Broward and Palm Beach counties announced they would begin evacuating barrier islands Thursday, with shelters opening to take in evacuees. Palm Beach officials declared a state of emergency Wednesday evening.
Schools will be closed Thursday in Miami-Dade and Broward counties, though they will remain open in Palm Beach County. Government offices will also be closed Thursday in Broward, where school was also canceled Friday.
Florida Power and Light, which serves 1.6 million customers in south Florida, activated its emergency center in Miami on Wednesday.
"Forecasts indicate we should expect heavy rainfall, potential flooding and a lot of tree debris," said Geisha Williams, FPL's vice president of distribution, in a statement. "Flooding may affect underground facilities, and outages might be prolonged as a result."
As of 11 p.m. Wednesday, Katrina's center was 60 miles southeast of Freeport, in the Bahamas, and about 135 miles east of the Florida coast. The storm was moving west at about 8 mph, according to the National Hurricane Center.