The National Weather Service in Key West issued a stark warning Friday that "NOWHERE IN THE FLORIDA KEYS WILL BE SAFE" when Hurricane Irma brings its wrath to the tropical islands.

"THIS IS AS REAL AS IT GETS," the agency tweeted Friday. "YOU STILL HAVE TIME TO EVACUATE."

The string of islands were nearly empty Friday as the hurricane moved closer. Hurricane and storm surge warnings were in effect and mandatory evacuation orders were in place. At least 31,000 people have already left the area.

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By early Friday evening, Irma had maximum sustained winds of 155 mph -- just below the highest Category 5 status -- and is forecast to remain at about that strength when it comes ashore someplace south of Miami on Sunday morning.

Irma could bring at least 1 to 6 inches of flooding to the islands, which only stand about 6 feet above sea level. Throughout Key West, businesses were boarded up, hospitals were closed and the city officially shut down at 8 a.m. on Friday, CBS News' Elaine Quijano reports.

Irma left at least 20 people dead and thousands homeless as it passed through Caribbean islands.

The Federal Emergency Management Agency (FEMA) estimates about 2.5 million homes in Florida are in flood-hazard zones and many are in the densely populated Miami-Dade and Broward counties where more than 4.5 million people live. The zones include the cities of Miami and Fort Lauderdale.

Gov. Rick Scott ordered all Florida public schools statewide to close so the buildings can be used as shelters. In an interview with "CBS This Morning" on Friday, Scott urged resident Friday not to be complacent.

"Think about your life, think about your family's life. But five to 10 feet of storm surge in the southern part of the state -- think about that. That's going to cover homes. We're not a high state. We don't have mountains in the state," Scott said.