Irma's top sustained winds are 35 miles per hour (55 kph), and it is moving northwest at 15 miles per hour (24 kph).
The hurricane has been downgraded to a Category 1, but is set to cause further damage on its course for Georgia.
In Miami, which escaped the worst of Irma's winds but experienced heavy flooding, residents in the city's Little Haiti neighbourhood returned to the wreckage of trailer homes that were shredded by the storm.
Irma is then expected to remain a category 4 storm as it slides up the west coast of the Florida Peninsula before making landfall again near the Fort Myers area Sunday afternoon.
Later Sunday, the storm struck Tampa and St. Petersburg, the two largest cities around Tampa Bay on Florida's western coast.
The NHC said the storm "is still life-threatening with unsafe storm surge, wind and heavy rains". Flooding also was reported in Jacksonville on Florida's eastern Atlantic coast. Downed trees and debris littered the roads, he said, but the city did not experience any storm surge like it feared.
Florida's Division of Emergency Management said about noon on Monday that 6.52 million utility customers in the state were without power.
Christian relief groups are mobilizing to help in recovery efforts with Irma, much like they did with Hurricane Harvey and the damage it caused in Texas.
The hurricane was responsible for at least 26 deaths, including 10 in Cuba, as it passed through Caribbean island nations.
The evacuations, including from areas around Miami, have been made more hard by clogged highways, gasoline shortages and the challenge of moving old people around the top retirement destination.
The small French island of St. Barthelemy and the French-Dutch island divided between St. Martin and St. Maarten also were badly damaged.
Before hammering Cuba, the hurricane raged through the Caribbean and seriously damaged Puerto Rico as well as islands controlled by the United Kingdom, the Netherlands and France.
About a third of the state was without power Sunday evening - that's more than 3.4 million households.