Tropical Storm Elsa is lashing the Florida Keys, complicating the search for survivors in the apartment block collapse near Miami and prompting a hurricane watch for the state's upper Gulf Coast.
In addition to damaging winds and heavy rains, the Miami-based National Hurricane Centre warned of life-threatening storm surges, flooding and isolated tornadoes.
A hurricane watch was issued for a long stretch of coastline, from Egmont Key at the mouth of Tampa Bay to the Steinhatchee River in Florida's Big Bend area.
Bands of rain are expected to reach Surfside on Florida's Atlantic coast, soaking the rubble of Champlain Towers South, which collapsed on June 24, killing at least 28 people and leaving 117 people missing.
Search and rescue crews have worked through rain, but must pause when lightning threatens, and a garage area in the debris filled with water on Monday, officials said.
Elsa's maximum sustained winds strengthened to 95 km/h early on Tuesday.
A slow strengthening is forecast through Tuesday night and Elsa could be near hurricane strength before it makes landfall in Florida.
About 10cm to 15cm of rainfall with localised totals up to 20cm are expected through Wednesday across the Keys and into south-west and western portions of the Florida Peninsula.
The forecast included the possibility of tornadoes across south Florida and across the upper peninsula.
State governor Ron DeSantis expanded a state of emergency to cover a dozen counties where Elsa was expected to make a swift passage on Wednesday, and President Joe Biden approved an emergency declaration for the state ahead of the storm.
Forecasters predicted Elsa will hit coastal Georgia and South Carolina after Florida.
Georgia's coast is under a tropical storm watch, as is the South Carolina coast from the mouth of St Marys River to South Santee River.
Forecasters said tornadoes could strike in the eastern Carolinas and Virginia as Elsa moves north.
The storm surge could reach 1.5 metres over normally dry land in the Tampa Bay area if Elsa passes at high tide, forecasters said.
Elsa's westward shift spared the lower Florida Keys a direct hit, but the islands were still getting plenty of rain and wind on Tuesday.
Tropical storm warnings were posted from Craig Key westward to the Dry Tortugas and for the west coast of Florida from Flamingo northwards to the Ochlockonee River.
Cuban officials evacuated 180,000 people against the possibility of heavy flooding from a storm that already battered several Caribbean islands, killing at least three people, but Elsa spent Sunday and much of Monday sweeping parallel to Cuba's southern coast, sparing most of the island from significant effects.
Elsa was the first hurricane of the Atlantic season until Saturday morning and caused widespread damage on several eastern Caribbean islands on Friday.
As a tropical storm, it resulted in the deaths of one person on St Lucia and a 15-year-old boy and a 75-year-old woman in the Dominican Republic.