In its update, the NHC said a hurricane watch - meaning a storm is imminent - was issued for west of the Aucilla River to Indian Pass, Fla.
Company officials estimate about 6 million people, representing 3.4 million customer accounts, will lose power in the Category 3 storm's wake.
"We're asking folks to be patient and remain sheltered in place", said St. Augustine Fire Chief Carlos Aviles.
Irma was ranked a category five, the rare top end of the scale of hurricane intensity, for days, and carried maximum sustained winds of up to 295km/h when it crashed into the island of Barbuda on Wednesday.
If Irma hits Florida at high tide, water levels there are expected to rise to 10 to 15 feet above ground from Cape Sable to Captiva. Gusty winds are likely across eastern Mississippi, Tennessee, and North Carolina from Monday afternoon through Tuesday night. And Irma, which is still making its way through the U.S. as a tropical storm, has already killed at least 38 people, including four in the U.S. Virgin Islands and three in Puerto Rico.
Irma battered Cuba on Saturday with deafening winds and relentless rain. As a result, severe flooding has ravaged many communities big and small, with streets and cars buried under the trail of excess water and debris that the storm has left in its path.
The westward swing away from Miami in the overnight forecast caught many people off guard along Florida's Gulf coast and triggered an abrupt shift in storm preparations.
North Carolina Gov. Roy Cooper also declared a state of emergency for all 100 counties in his state ahead of Irma. No injuries have been reported.
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Storm surge: Large portions of Marco Island were flooded by a 3- to 4-foot storm surge, said Chris Bowden, spokesman for the Marco Island Fire Department.
Storm surge is caused when the winds from the hurricane literally push the ocean onto the shore.
The most populous counties in the state Miami-Dade, Broward and Palm Beach would get spared of the worst effects of Irma if these predictions hold true.