Hundreds of thousands of people on Florida’s Gulf Coast clogged highways and drained gas pumps on Tuesday as they headed for higher ground, in an exodus that could be one of the largest evacuations in state history ahead of Hurricane Milton.

The monster storm exploded Monday into one of the most powerful hurricanes on record. Everyone from local officials to President Biden urged vulnerable residents to flee as it bore down on the vulnerable Tampa Bay region, a metro area of about three million people that hasn’t taken a direct hit from a hurricane in more than a century.

At least 11 counties, with a combined population of more than six million people, have issued mandatory evacuation orders for coastal and low-lying areas.

Milton, the strongest storm in the Gulf of Mexico since 2005, was expected to make landfall on Wednesday, bringing life-threatening hazards to the state for the second time in less than two weeks after Helene swept through. “We must be prepared for a major, major impact to the west coast of Florida,” Gov. Ron DeSantis said.

Where it’s going: Milton brought strong winds and storm surge to the northern coast of Mexico’s Yucatán Peninsula overnight. Though its wind speeds had come down some — Milton was a Category 4 hurricane by late morning, with 150 miles-per-hour winds — it has expanded in size. Forecasters at the National Hurricane Center expect it to remain a major hurricane as it approaches Florida. Track the storm.

Tampa’s high risk: The densely populated Tampa Bay region, which is highly vulnerable to storm surge, hasn’t had a direct hit from a major hurricane in more than a century. Mayor Jane Castor of Tampa told residents to prepare for “an event like none other” and said on CNN that if they chose to stay in a mandatory evacuation zone, “you’re going to die.”

Florida prepares: Mr. DeSantis urged residents to evacuate and finish their emergency preparations on Tuesday. “There’s no guarantee what the weather is going to be like Wednesday morning,” he said. Highways were snarled on Monday, and traffic was picking up again on Tuesday. Walt Disney World and Universal Orlando remained open, though other Florida parks announced closures.

Politics put aside: Governor DeSantis said he had spoken with President Biden late Monday about what the state needed to prepare. “Everything that we’ve asked for, the administration has approved,” Mr. DeSantis said. From the White House on Tuesday morning, President Biden called Hurricane Milton “a matter of life and death’’ and urged Floridians in the storm’s path to “evacuate now, now, now.”

FEMA shortages: Stretched thin by Hurricane Helene’s devastation in southern Appalachia, as well as by floods, wildfires, tornadoes and other disasters elsewhere, the Federal Emergency Management Agency now has less than 10 percent of its personnel available to deploy. Here’s how that could affect the agency.

President Biden called Hurricane Milton a potentially “devastating” storm that was “a matter of life and death,’’ as he urged Floridians in the storm’s path on Tuesday to “evacuate now, now, now.”

Speaking in the Roosevelt Room of the White House, he repeated what the White House said last night: that he had spoken with Gov. Ron DeSantis of Florida and given the governor his personal phone number to call if he needed more help.

At a Walmart Supercenter in Tampa, the parking lot is full, water purchases are limited and canned food options are dwindling. But some shoppers say they are taking the situation in stride. “I’m not concerned,” said Jim Stephenson, 57, who said he snagged some beef stew and chili as a precaution.

Tevis McKinstry, who for 30 years had always chosen to stay nearby when hurricanes headed her way in Florida. This week is no different — she said she would leave her Fort Myers home and stay with her daughter in a second-floor condo two miles away.

Electric utilities are warning residents to prepare for lengthy outages. Duke Energy Florida said more than a million customers might lose power because of storm damage. Mary Lou Carn of Tampa Electric said her company had brought in workers from as far away as Texas, Missouri and Minnesota to help with repair work.

Milton has replaced its eyewall, and forecasters said it did not appear to have weakened much in the process, and may have even rebounded a bit. It is still a Category 4 hurricane with 150 mile-an-hour winds, and is likely to remain a major hurricane as it expands in size on its way to Florida.

Mayor Andy Ross of Temple Terrace, north of Tampa, said that days of “nonstop, terrifying messages” about the storm had put a heavy mental strain on residents. “This is really stressful for a lot of people," Ross said, "so look out for each other.”

Officials in Hillsborough County, which includes Tampa, urged residents to get to a safe place by 7 a.m. Wednesday. Warning that the window for evacuation was narrowing, the county's emergency management director, Timothy Dudley, Jr., said: “Don’t panic, you have time — get somewhere safe and we’ll see you on the other end.”

President Biden will postpone a diplomatic trip to Germany and Angola to help oversee his administration’s response to Hurricane Milton, according to a statement from Karine Jean-Pierre, the White House press secretary. Biden had been set to travel to Berlin on Thursday.

Milton's projected impacts are now expected to extend all the way across the Florida peninsula. On Tuesday morning, a hurricane warning alert blasted across phones in St. Johns County, nearly 200 miles from the Tampa Bay area, where landfall is expected.

Though the Gulf Coast will face the immediate impact of the storm, Florida’s other coast will have to grapple with considerable storm surge and other effects when the storm meets the Atlantic.

DeSantis, who is not known for getting personal in his remarks, notes that he grew up in Pinellas County, which is low-lying and vulnerable to Hurricane Milton’s storm surge. “It’s a type of storm that could potentially have an indelible impact on a community,” he says. “There’s other storms like a Hurricane Andrew, like a Hurricane Michael in Panama City, like a Hurricane Ian in Lee County, that just leave a mark.”

DeSantis mentions that past major storms were forecast to hit Tampa and then swerved south, which is why many residents of Southwest Florida have been evacuating toward Miami. “Places like Southwest Florida, they have muscle memory,” DeSantis said. “Charley was supposed to hit Tampa Bay. Ian was supposed to hit Tampa Bay.”

Governor DeSantis said he spoke with President Biden last night, and said he was planning on asking the federal government for help with debris removal. “Everything that we’ve asked for, the administration has approved,” Mr. DeSantis said.

The potential path of Milton continues to be a “worst-case scenario” for the Tampa Bay region southward to Sarasota, forecasters said Tuesday morning. While confidence was increasing amongst forecasters that landfall would likely occur in this region, they also added a reminder that changes in Milton's potential path are possible and that the hazards will stretch far from the center of the hurricane as it grows larger.

Jared Perdue, secretary of the Florida Department of Transportation, said the state opened shoulders of highways for emergency use on Monday, to help ease evacuation traffic. These returned to normal at around 1 a.m. Tuesday but traffic was already starting to pick up again, he said, and emergency use of interstate shoulders will be in effect again along evacuation routes. He urged Floridians to use other roads besides highways to evacuate.

Perdue said the ports on western side of Florida have ceased waterside operations. Landside operations continue, including the movement of fuel to gas stations.

DeSantis says Florida will open four mass shelters, including one that could shelter some 10,000 people, if necessary. They will be located near Interstate 75 north of Tampa and near Interstate 4 east of Tampa, he says.

Milton will skirt past the northern side of the Yucatán Peninsula today as it begins to turn more northeast toward the west coast of the Florida Peninsula. The most likely scenario is that Milton will make landfall in Florida on Wednesday night.

Governor DeSantis urged Floridians to evacuate and finish their emergency plans on Tuesday. “There’s no guarantee what the weather is going to be like Wednesday morning,” he said. He noted major roads were clogged on Monday as people started evacuating.

Mr. DeSantis added that there was no gas shortage, but there was high demand with many people filling their tanks. The state is deploying fuel trucks to try to keep stations supplied, knowing that fuel shipments will stop, at least for a while, at the Port of Tampa, which typically supplies gas to much of western Central Florida.

“Basically the entire peninsula portion of Florida is under either a watch or a warning,” Gov. Ron DeSantis said at a news briefing about Milton. “It’s possible that it could hit north of Tampa Bay, it could hit Tampa Bay, it could hit south,” he said of the center of the storm, urging residents not to focus solely on that.

Governor DeSantis added that the storm surge from Hurricane Milton could peak at 10 to 15 feet around the Tampa Bay area: “We saw what even four feet did with Hurricane Helene in some of these communities, so this is something that’s really, really significant.”

Mandatory evacuations were in place for low-lying areas in nearly a dozen Florida counties, including parts of the cities of Tampa, St. Petersburg and Clearwater. Many of the orders also apply to anyone countywide living in mobile homes and recreational vehicles.

Milton became a Category 4 storm Tuesday morning and was forecast to grow again to a Category 5 later in the day, according to the National Hurricane Center. The latest forecast predicted the storm would hit Florida as a major hurricane late Wednesday or early Thursday.

Most flights in and out of the Tampa area will be grounded in the coming days. Tampa International Airport said it would close starting 9 a.m. Eastern on Tuesday.

St. Pete-Clearwater International Airport said it would close after the last flight departed on Tuesday, and it canceled all flights on Wednesday and Thursday.

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Meteorologists were glued to their computers on Monday morning, watching virtual data as the Air Force Reserve Hurricane Hunter airplane made pass after pass through the eye of Hurricane Milton. Every time it did, it found the storm’s pressure had dropped and the eyewall wind speeds had increased, indicating that it was becoming more intense by the minute.

The hurricane went from a Category 1 storm at midnight to a Category 5 hurricane by noon. And it didn’t stop there.

By 8 p.m. on Monday, the storm’s maximum sustained wind speeds had increased to 180 miles per hour, making Milton one of the strongest Atlantic hurricanes ever. Based on wind speed, it joined a handful of other hurricanes to rival the strongest Atlantic storm ever recorded: a 1980 hurricane named Allen, which had a peak wind speed of 190 m.p.h. before it made landfall along the United States-Mexico border.

As a small, compact system, however, Milton was more similar to Hurricane Wilma in 2005, which holds the record for the lowest pressure in a hurricane, another measure of a storm’s intensity.

Its small size, an excess of extremely warm waters in the Gulf of Mexico and calm atmospheric conditions allowed Milton to “explosively” intensify, as hurricane center forecasters noted Monday afternoon.

The standard meteorological definition of “rapid intensification” is 30 knots in 24 hours, or roughly 35 miles per hour daily. Milton increased by more than double this definition on Monday, at a pace similar to that of Wilma and another record storm, Hurricane Felix in 2007.

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