Hurricane Irma gained strength as it broke loose from Cuba and was upgraded to a category 4 storm in the early morning hours Sunday.
One of the most powerful Atlantic hurricanes of the last century is roaring toward the Florida Keys with maximum sustained winds of 130 miles per hour, and the National Hurricane Center warned that it's bringing a "life-threatening storm surge" that will stretch up the west coast of Florida.
As of 2 a.m. Eastern Saturday, the eye of Irma was 70 miles from Key West, moving northwest toward the Keys at 6 miles per hour. It temporarily weakened to a category 3 storm while over Cuba before churning back out to sea.
The NHC issued storm surge warnings for the Keys, and for Tampa Bay on the state's west coast. Surge warnings also were in effect for the area from North Miami Beach southward around the Florida peninsula, as well as for the area between the South Santee River and Jupiter Inlet.
"A Storm Surge Warning means there is a danger of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours in the indicated locations," the NHC said.
The Tampa-St. Petersburg-Clearwater area, which is built around a huge bay that cuts into the mainland, has not taken a direct hit from a major hurricane in nearly a century.
Irma seen striking land as Category 4 storm
The National Weather Service said earlier on Saturday that "major hurricane force winds" were expected to batter the Keys on Sunday at daybreak, with wind speeds still well over 100 miles per hour making their way up the state's western coast.
Florida officials have issued mandatory evacuation orders that have sent millions of residents fleeing for safety.
On Friday, the National Weather Service warned via Twitter that "nowhere in the Florida Keys will be safe," and that residents should clear out while they were able.
On Friday night, new modeling from the NBC Weather Unit forecast direct hits on the Florida Keys, and worst-case scenarios unfolding for major cities like Naples, Fort Myers and also Tampa further up the coast.
Miami, Fort Lauderdale and Palm Beach County still face severe effects from Irma, but those cities are in less jeopardy than they were just a day ago.
"You need to leave — not tonight, not in an hour, right now," Gov. Rick Scott warned residents in the evacuation zones during a Saturday press conference.
Separately, FEMA's top official issued a stark warning for residents who chose to defy evacuation orders: They will be left to their own devices, with no immediate help.
"You're on your own until we can actually get in there, and it's safe for our teams to support local and state efforts," Brock Long told CNN. "The message has been clear — the Keys are going to be impacted, there is no safe area within the Keys, and you put your life in your own hands by not evacuating."
—CNBC's Javier David contributed to this report