MIAMI — Mandatory evacuations of vast swaths of coastal South Florida began Wednesday as anxious residents continued to watch and wait — and watch, and wait — for the massive storm to roll closer.

"This storm is bigger, faster and stronger than Hurricane Andrew," cautioned Florida Gov. Rick Scott, who made Wednesday stops in Marathon, Doral and Naples to warn of potential widespread devastation.

Miami-Dade County ordered a mandatory evacuation, its first in 12 years, for more than 100,000 residents of mobile homes, barrier islands and low-lying mainland areas. The orders, which take effect at 7 a.m. Thursday, affect areas considered particularly vulnerable to storm surge, including Key Biscayne and Miami Beach.

"Irma remains a strong Category 5 hurricane," Miami-Dade Mayor Carlos Gimenez said at an evening news briefing in the county's emergency center in Doral. "Significant weakening is not expected."

The orders apply to county Zone A, which covers Key Biscayne and coastal areas of Southeast Miami-Dade and north of Miami, and to only the barrier islands of Zone B: Bal Harbour, Bay Harbor Islands, Golden Beach, Indian Creek Village, Miami Beach, North Bay Village, Sunny Isles Beach and Surfside.

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Not included were Brickell Avenue and other mainland areas in Zone B, though Gimenez warned that future orders may be broader and affect those neighborhoods and more zones.

Miami-Dade had held off from ordering evacuations earlier Wednesday, citing an Irma slowdown. Broward County didn't wait; it asked residents in mobile homes and coastal and low-lying areas to go beginning at noon Thursday.

"We're not going to knock on doors," Sheriff Scott Israel said. "We're not arresting people and we're not pulling people out of their homes. We're asking you to leave so you don't become a victim."

Monroe County, comprising the Florida Keys, where a single road leads to the mainland, began requiring visitors to leave Wednesday, and about 2,500 people had done so by the evening, according to Scott. Keys residents have been ordered to get out Thursday.

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"If you stay, you think you're a tough guy, then you're on your own," Monroe County Administrator Roman Gastesi said. "Don't expect us to come get you."

Highway tolls were suspended statewide starting Tuesday afternoon. South Florida schools will be closed Thursday and Friday.

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So many people tried to check Miami-Dade's evacuation-zone map Tuesday that the website crashed for several hours, requiring the county to increase its capacity Wednesday, Gimenez said.

Two Miami-Dade cities didn't wait for county orders. Leaders of Miami and Miami Beach encouraged families earlier Wednesday to get out while they still can. Miami suspects two dozen construction cranes that dot the city's skyline and are designed to withstand 145 mph winds might not hold up to a direct hit from Irma.

"We strongly urge all residents in the evacuation areas to evacuate," Miami City Manager Daniel Alfonso said. "There will be no possibility of assisting you if you're in an emergency in the middle of the storm if you're in an area of evacuation."

Miami faced an unusual logistical situation, with Mayor Tomás Regalado and Police Chief Rodolfo in Argentina for a business conference and unable to return before early Friday. There was some internal turmoil over whether Miami should open its emergency operations center, which it did Wednesday. Alfonso and Fire Chief Joseph Zahralban said the city's 80-member urban search-and-rescue team is ready to go, along with four vacuum trucks already sucking out storm drains to prevent unnecessary flooding during the storm. The 1,300-member police department is said to go on longer, emergency shifts Friday.

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First-responder Miami-Dade and Miami teams that had deployed to Texas to help recover from Hurricane Harvey returned Wednesday afternoon.

In Miami Beach on Wednesday, a day after Mayor Philip Levine started asking people to leave voluntarily, some weren't heeding his suggestion — yet.

"I'm going to wait until tomorrow to see what we're going to do," said Manish Baker, a mid-Beach resident snacking at The Frieze Ice Cream Factory just south of Lincoln Road Wednesday and arguing — lovingly — with his wife over the merits of staying versus leaving.

Shelley Baker said friends had left earlier in the day for Atlanta and Savannah.

"We've lived here since 1984, and we don't like to panic," she said. "You hope and pray for the best. But the panic is just the worst."

By 5 p.m. Wednesday, Miami-Dade had opened four emergency shelters at North Miami Beach Senior High School, South Miami Senior High School, Felix Varela Senior High School in West Kendall and a pet-friendly facility at 10901 Coral Way. Four more will open by 9 a.m. Thursday. In total, the eight shelters can accommodate up to 16,000 people, according to Gimenez, who said the county expects 8-10 percent of people in evacuation zones to seek refuge there.

Broward will open 14 shelters Thursday.

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Across the state, shoppers reported a mad rush for supplies, especially plywood, bottled water and gas. Gimenez implored Miami-Dade residents to consider filling jugs with tap water.

Scott called restoring fuel supplies a "top priority" he addressed with the Federal Emergency Management Administration and with Alabama, Georgia and South Carolina. The governor asked neighboring states to follow Florida's lead and lift weight and driver restrictions to get more fuel trucks in. But the "bottleneck," he said, comes from trucks unable to refill quickly enough to supply stations.

"We're moving as much fuel through the system as fast as possible," he said, blaming the spotty shortages on an unusually large surge in demand. "I've said it many times, and I can't say it enough: Please only take what you need."

To help with preparations, Scott ordered another 900 National Guard members to duty on Wednesday. By Friday, all 7,000 of the state's Guardsmen will report to duty.

The state's Division of Food, Nutrition and Wellness said it has prepared millions of food servings so storm victims can quickly receive aid after the storm, according to Agriculture Commissioner Adam Putnam. The list includes 6.3 million servings of meat, eggs, nut butters and beans, 3.8 million servings of canned and frozen vegetables, and 2.6 million servings of cheese.

Scott spoke to President Donald Trump and Vice President Mike Pence on Wednesday. Trump also spoke to Republican Sen. Marco Rubio, who along with Democratic Sen. Bill Nelson joined the governor at the Doral emergency operations center Wednesday morning. Both promised assistance from the feds.

"It's not going to be a repeat of what happened with the federal government 25 years ago with Andrew," Nelson said. "You're not going to see what happened with Hurricane Katrina."