The Category 4 Hurricane Milton was expected to grow larger on Tuesday as it threatened Mexico’s Yucatan Peninsula on its way to Florida, where more than a million people were ordered to evacuate from its path.

The densely populated west coast of Florida, still reeling from the devastating Hurricane Helene less than two weeks ago, braced for landfall on Wednesday.

The US National Hurricane Centre (NHC) projected the storm was likely to hit near the Tampa Bay metropolitan area, home to more than 3 million people and where some evacuees rushed to dispose of mounds of debris left behind by Helene on their way out of town.

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With maximum sustained winds of 155 mph (250 kph), Milton was downgraded from a category 5 to a category 4 hurricane on the five-step Saffir-Simpson scale, according to the US NHC’s latest advisory early on Tuesday.

While fluctuations in intensity are expected, Milton is forecast to remain an extremely dangerous hurricane through landfall in Florida, according to the NHC. That means catastrophic damage will occur, including power outages expected to last days.

Fed by warm waters in the Gulf of Mexico, Milton became the third-fastest intensifying storm on record in the Atlantic Ocean, the NHC said, as it surged from a tropical storm to a Category 5 hurricane in less than 24 hours.

Its path from west to east was also unusual, as Gulf hurricanes typically form in the Caribbean Sea and make landfall after travelling west and turning north.

“It is exceedingly rare for a hurricane to form in the western Gulf, track eastward, and make landfall on the western coast of Florida,” said Jonathan Lin, an atmospheric scientist at Cornell University.

“This has big implications since the track of the storm plays a role in determining where the storm surge will be the largest.”

The NHC forecast storm surges of 3 to 4.5 metres along a stretch of coastline north and south of Tampa Bay.

Jamie Rhome, deputy director of the NHC, said Milton was expected to grow in size before making landfall on Wednesday, putting hundreds of miles of coastline within the storm surge danger zone.

Milton was likely to remain a hurricane for its entire journey across the Florida peninsula, Rhome told a Monday news briefing.

As of 10am CDT on Tuesday, the eye of the storm was 105 kilometres north-northeast of Progreso, a Mexican port near the Yucatan state capital of Merida, and 840km southwest of Tampa, moving east at nine mph (15 kph).

Milton was expected to pound the northern edge of the Yucatan Peninsula in the early hours of Tuesday.

The area is home to the picturesque colonial-era city of Merida, population 1.2 million, several Maya ruins popular with tourists and the port of Progreso.

In Florida, counties along the western coast ordered people in low-lying areas to take shelter on higher ground.

Pinellas County, which includes St. Petersburg, said it ordered the evacuation of more than 500,000 people. Lee County said 416,000 people lived in its mandatory evacuation zones.

At least six other coastal counties ordered evacuations including Hillsborough County, which includes the city of Tampa.

With one final day for people to evacuate on Tuesday, local officials raised concerns about traffic jams and long lines at gas stations.

Relief efforts remain ongoing throughout much of the US Southeast in the wake of Helene, a Category-4 hurricane that made landfall in Florida on September 26, killed more than 200 people and caused billions of dollars in damage across six states.