Hurricane Hermine swept onto Florida's Gulf coast early on Friday with winds and heavy rain causing flooding and cutting power to tens of thousands of people, with officials warning of "life threatening" conditions.
The Category One storm made landfall around 1:30 a.m. local time (0530 GMT) near St Marks, just south of Florida's capital Tallahassee, with 130 kilometre per hour winds extending 45 miles from the hurricane's eye, the Miami-based National Hurricane Centre (NHC) said.
"There is a danger of life-threatening inundation within the next 12 to 24 hours along the Gulf coast of Florida," it added.
The storm is moving north-northeast at 14 miles per hour.
Some 70,000 people in rain-drenched Tallahassee were already without power as local television stations broadcast video footage of buffeting winds and cars driving through flooded streets.
Authorities in several counties have issued mandatory evacuation orders for residents living on the coast and in low-lying regions.
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"This is life-threatening," Governor Rick Scott told journalists Thursday, urging residents to take warnings seriously. "We have a hurricane. You can rebuild a home. You can rebuild property. You cannot rebuild a life."
Hundreds of schools and government offices will be closed Friday as residents brace for the storm's full impact.
Hermine is the first hurricane to hit Florida in 11 years since Hurricane Wilma in 2005.
Pentagon spokesman Jeff Davis said 100 Florida National Guard personnel were activated, with 6,000 more on alert in the state and 34,000 members ready to deploy from elsewhere in the United States.
President Barack Obama has asked FEMA administrator Craig Fugate to keep him updated on the situation "and to alert him if there are any significant unmet needs", White House spokesman Josh Earnest said.
"Local, state and federal officials have been working diligently to prepare for these storms and have resources on hand to respond to them as necessary," he added.
Less severe but still dangerous tropical storm-force winds
are buffeting several hundred miles of Florida coast, from Tampa to the barrier islands south of Pensacola.