US President Joe Biden declared a state of emergency in Florida due to Tropical Storm Helene, authorizing federal assistance.
Helene is expected to strengthen into a major hurricane over the Gulf of Mexico, potentially reaching Category 3 or 4 by Thursday.
Areas of Florida are under alert, with mandatory evacuation orders in place in some counties and voluntary evacuations in others.
Hurricane watches are in effect for Florida's Tampa Bay and from Englewood to Indian Pass, as well as for eastern Mexico and Cuba's Pinar del Río province.
A storm surge of up to 15ft is forecast for parts of Florida, with officials warning that 10-15ft of surge is "NOT survivable."
Helene is expected to produce 4-8in of rain in the south-eastern US, with isolated totals around 12in, potentially causing flash and urban flooding.
Florida Governor Ron DeSantis declared a state of emergency in 61 counties and activated the Florida state guard.
The NHC warned of "inland penetration of strong winds" and storm surge hazards extending far from the storm's center.
The Cayman Islands and Cuba are expecting heavy wind, rain, and waves, with schools and airports shut in the Cayman Islands.