Access to the Pinellas County barrier islands had already been restricted since Friday morning to residents, business owners and employees in advance of Hurricane Irma.
Starting Sunday at 6 a.m., no one will be allowed in - even if they live there.
"So once you come off, you're not going back in, and that's it because it's just going to be too unsafe out there," Sheriff Bob Gualtieri said Saturday night.
As part of Level A, Pinellas beach communities have been under a mandatory evacuation order since Friday morning. Level B zone evacuations were added Saturday morning.
Projections from the National Hurricane Center have tropical storm force winds arriving in Pinellas County around 2 p.m. Sunday, but that could change if Irma speeds up.
"That's why it's so important and imperative that people heed these warnings and heed the evacuation orders," Gualtieri said. "There's a chance that it could be earlier than 2 o'clock, that's why people need to really get into shelters and those who are in mandatory evacuation areas need to get out."
As of 9 p.m. Saturday, almost 16,000 people were in 17 Pinellas shelters, with John Sexton Elementary School in St. Petersburg being the only one at capacity.
Also, Gualtieri does not think they will shut down the Bayside Bridge, even if all the other bridges close because of high winds. He said that if there’s a problem on US 19, there’s no other major north-south route in that part of the county for emergency crews to use.