The hurricane center forecast moved the most likely path of the eye of the storm to the west in its 5 p.m. ET advisory.
"It looks like it's shifting, even though it may be just 20 miles, it puts Miami right in the worst possible position," CNN meteorologist Tom Sater said.
"Because when you look at the formidable storm, the strongest winds, the strongest storm surge, the bands of heavy rain are always in that north, northeastern quadrant.
But Sater cautioned that some models show the storm farther west and some farther east. It is still more than 600 miles from Florida.
"Regardless of which (Florida) coast you live on, be prepared to evacuate," Florida Gov. Rick Scott said Thursday at a news conference, with forecasters warning the storm could reach South Florida by Sunday.
Mandatory evacuations have been issued for many counties in South Florida, but Scott said others in Florida need to watch Irma's path and be ready to move.
All schools in Florida will close Friday
Gov. Rick Scott has directed the closure of all public schools, state colleges, state universities and state offices from Friday to Monday. Scott said the closures will ensure every space is available for sheltering and staging during Hurricane Irma.