An alert from the National Hurricane Center said the storm had maximum sustained winds of 75mph (120kph) with higher gusts.
Forecasters in Miami issued a hurricane watch for a 120-mile stretch that includes the cities of Tampa and St Petersburg. The storm has been sitting in the Gulf of Mexico since crossing over South Florida on Sunday.
The latest hurricane watch extends from Anna Maria Island, which is south of St Petersburg, to Yankeetown.
The storm was about 130 miles west-southwest of Fort Myers, on Florida’s west coast, in the early hours of Wednesday and moving at 15 mph.
The last tropical storm that made landfall in the Florida Keys was Hurricane Gordon two years ago.
The hurricane center said that a “life-threatening storm surge" is possible early on Thursday, and forecasters advised residents to heed warnings from local officials. Tropical storm-force winds are expected in the area by late on Wednesday.
Forecasters say that the storm system will bring more rain over South Florida, which has already been inundated.
The storm first hit Nicaragua as a Category-4 hurricane and killed nearly 70 people from Mexico to Panama, before moving into the Gulf of Mexico early on Monday near where the Everglades meet the sea.
Eta was the 28th named storm of a packed Atlantic hurricane season, tying the 2005 record for named storms. And late Monday, the record was broken by the 29th storm – Theta.
AP contributed to this report
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