Hurricane Helene intensifies. Here's what people in Florida need to know.

MIAMI - Hurricane Helene is expected to slam into Florida's Big Bend coast on Thursday as a major hurricane, prompting a series of watches and warnings across the state as forecasters expect the storm to unleash life-threatening conditions.

All of South Florida and the Florida Keys are under a tropical storm warning as Helene makes its way north on a path through the Gulf of Mexico ahead of landfall, which is expected Thursday evening.

The storm is forecast to bring "catastrophic winds and storm surge to the northeastern Gulf coast," the National Hurricane Center said Thursday.

The storm was a Category 4 on Thursday evening as it approached Florida's Big Bend. "It will be a dangerous major hurricane," said NEXT Weather meteorologist KC Sherman.

A hurricane warning is in effect Thursday from Anclote River, near Tarpon Springs, to Mexico Beach in Florida's panhandle. A hurricane watch is in effect from Englewood to Anclote River and includes Tampa Bay.

The National Hurricane Center also issued a tropical storm warning for the Florida Keys and Florida's west coast from Flamingo, which is in the Everglades, to the Anclote River, including Tampa Bay. A tropical storm warning also covers the area west of Mexico Beach to the Walton/Bay county line, as well as Lake Okeechobee and Flamingo north to the Little River Inlet.

A Storm Surge Warning is in effect for Tampa Bay, Charlotte Harbor and Mexico Beach eastward and southward to Flamingo, the hurricane center said.

"Helene will remain a very large and growing storm, with the wind field expected to span over 400 miles at it's largest point," meteorologist Sherman said. "So it is important to remember that while center is projected to make landfall along the Big Bend, significant impacts will be felt up and down the west coast of Florida extending well outside of the center."

The hurricane center's forecast cone shows Helene moving north in the panhandle Thursday night. Tallahassee is in the center of the forecast path. From there it is forecast to pass into Alabama and Georgia.

"Dangerous" storm surge," flooding risks

"Life-threatening storm surge will remain a significant threat for the Big Bend of Florida all the way down the west coast of Florida," said Sherman.

Storm surge on Florida's Gulf Coast could reach 15 to 20 feet near the storm's landfall, according to the National Hurricane Center. Apalachicola to Carrabelle could see 10 to 15 feet, while Tampa Bay and the area from the Anclote River to the middle of Longboat Key could see 5 to 8 feet. Charlotte Harbor and Naples could see up to 5 feet of storm surge.

The hurricane center says Helene could drop as much as 12 inches of rain over the southeastern U.S. into the southern Appalachians, with some areas seeing  around 18 inches.

"This rainfall will likely result in catastrophic and potentially life-threatening flash and urban flooding, along with significant river flooding," the hurricane center said.

Tornados are also a threat.

"The risk for several tornadoes will gradually increase today through tonight," the hurricane center said in a Thursday advisory. "The greatest threat is expected from parts of northern Florida into southeast Georgia, the Midlands and Low Country of South Carolina, and southern North Carolina."

South Florida forecast

While South Florida will not be in the storm's direct path , the region will feel impacts.

The tri-county area started to feel the fringe impacts on Wednesday as gusty, tropical bands of rain moved through.

Thursday afternoon was expected to be very windy, with gusts up to 50 mph forecast for Miami-Dade and Broward Counties, and up to 60 mph in the Keys.

Bands of heavy rain will continue through Friday and up to 4 inches of rain are expected. There may be some locally heavier spots, particularly in areas that receive "training" rain bands that move over the same area for a prolonged period of time.

Saltwater flooding from storm surge will only be a threat in the Keys, where 1 to 3 feet of storm surge will be possible at high tide through Thursday. A coastal flood warning has been issued for the Keys.

Cities, residents prepare

Gov. DeSantis declared a state of emergency earlier this week for 61 of 67 counties in Florida. President Joe Biden approved a Florida emergency declaration on Tuesday, which authorizes the Federal Emergency Management Agency to coordinate all disaster relief efforts and includes assistance and reimbursement for mass care including evacuation and shelter support.

Federal authorities have positioned generators, food and water, along with search-and-rescue teams .

DeSantis' order also activated the Florida National Guard and Florida State Guard to assist in the storm's aftermath. About 18,000 utility line workers were being pre-positioned to help restore power when conditions were safe, and the governor said Wednesday that 12 health care facilities had been evacuated.

Several counties on Florida's west and northwestern coasts have issued evacuation orders, and many schools, offices and businesses were closed as Helene moved north through the Gulf on Thursday.

A mandatory evacuation order was issued at the University of Tampa. Other universities have been impacted, including the University of Florida, which canceled classes for Thursday.

Additionally, sandbags were made available for free in select cities, including Hollywood , Dania Beach , Fort Lauderdale , North Miami and North Miami Beach.

Hollywood and Miami Beach also allowed residents to park in public garages for free if they were concerned about flooding.

All public schools in Broward and Monroe Counties were closed on Thursday. Miami-Dade public schools remained open.