Tropical Storm Colin has picked up speed over the Gulf of Mexico as it heads towards Florida's northwest coast, unleashing thunderstorms and flooding while the state's governor activated the National Guard ahead of its landfall.
The storm, nearing the Big Bend area of the Florida coast as of 11pm local time on Monday, barrelled towards land at 37km/h, more quickly than it moved earlier in the day, the US National Hurricane Center in Miami said.
The combination of the storm surge and high tides threatened flooding in coastal areas across the US Southeast, with the storm expected to make landfall below Florida's Panhandle on Monday night.
A tropical storm warning was extended to the Outer Banks of North Carolina. On its forecast path, Colin would churn across southeastern Georgia early on Tuesday and later in the day menace the North and South Carolina coasts.
As Colin blasted 80km/h winds at Florida, tornado warnings were issued across the state. The storm was forecast to dump as much as 20cm of rain on parts of the state, the hurricane centre said.
Florida Governor Rick Scott, who had declared a state of emergency in 34 of the state's 67 counties, said more than 6000 Florida National Guard members were activated and ready for deployment.
Fast-moving squalls, tornadoes, flooding and property damage resulting from the fierce winds remained threats into the night, and far beyond the storm's immediate path, forecasters warned.
A statement from Scott's office warned residents to be wary of rip currents and the possibility of 3m waves along the Gulf Coast.
"It is critical that all Floridians use caution and remain alert," he said in the statement.
In the St Petersburg beach town of Gulfport, roads were already flooded. One resident used a kayak to float down a thoroughfare past a waterfront cafe that stayed open, allowing people used to severe weather to witness the storm.
More than 10,000 customers were without power ahead of the storm making landfall, utilities reported.
The storm also threatened crops in Florida, the country's biggest citrus producer, which sent US orange juice futures on Monday to their highest in more than two years.