Hurricane Milton is expected to sweep past Mexico’s Yucatán peninsula, bringing sustained winds of nearly 155 mph (250km/h), as the category 4 storm heads towards Florida’s dangerously exposed Tampa Bay.

The storm’s trajectory suggested it would pass the Mexican city of Mérida, home to 1.2 million people, in the early hours of Tuesday morning before swerving north towards the US. Mexican officials have been bussing people out of low-lying coastal areas.

Milton is projected to hit the south-west coast of Florida by Wednesday evening local time, the US National Weather Service said in its latest update, and could cause destruction in areas already reeling from Hurricane Helene’s devastation nearly two weeks ago.

Almost all of Florida’s west coast was under a hurricane warning, with more than a million people told to evacuate, fleeing potentially catastrophic damage and power outages that could last days. With one day left for people to leave, local officials raised concerns about traffic jams and long queues at fuel stations.

US forecasters and officials fear Milton could make landfall in the Tampa Bay region, home to more than 3 million people. Tampa has not had a direct hit by a major hurricane since 1921 and could see waters rise by 15ft (4.5 metres).

Hurricane damage modellers have for years warned that the Tampa Bay area is particularly vulnerable to rising seas caused by storm surges, owing to its wide and shallow seabed, which can push water upwards.

The mayor of Tampa, which is low-lying and has a population of 3.3 million, issued a stark warning to residents as Hurricane Milton dashed across the Gulf of Mexico.

“If you choose to stay … you are going to die,” said the mayor, Jane Castor.

Castor delivered the blunt assessment to CNN on Monday while also describing Milton as a “literally catastrophic” hurricane projected to push up to 15ft of Gulf water inland – an amount that officials say is deadly.

Hurricane Helene, which made landfall in late September, caused more than 200 deaths and catastrophic damage stretching from Florida to the Appalachian mountains. There are fears that mounds of building rubble left in Helene’s wake could turn into dangerous debris if caught up in Milton’s floods and winds.

The National Weather Service downgraded Milton early on Tuesday to a category 4 hurricane but forecasters said it still posed an extremely serious threat.

“While fluctuations in intensity are expected, Milton is forecast to remain an extremely dangerous hurricane through landfall in Florida,” the agency said.

The slight weakening from category 5 status attained Monday occurred after Milton’s barometic pressure rose slightly to 924m from 879m. That happened as Milton appeared to be undergoing an eye wall replacement, which can briefly raise barometric pressure and reduce its intensity.

However, the phenomenon tends to make a hurricane wider, increasing its windfield. Projections expect the hurricane to restrengthen to a category 5 then weaken as it approaches Florida, though the storm’s effects are still going to be potent.

Milton is due to become the 10th major hurricane – category 3 or higher – to make landfall along the US’s Gulf coast since 2017, gaining power from the warm seas in the gulf. Milton was the third fastest-intensifying storm on record in the Atlantic Ocean, the agency said.

Weather and climate experts attribute such a high rate of powerful, destructive storms to the climate crisis, spurred by the burning of fossil fuels.

Before Milton’s arrival, the governor of Florida, Ron DeSantis, declared a state of emergency for 51 of its 67 counties. “What you don’t want to do is stay in an area where you have 10, 15ft of storm surge,” he told Fox News on Monday.

DeSantis also told Floridians to make sure they had a week’s food and water and were braced for more evacuation orders.

The governor is pro-fossil fuels and has criticised climate action as being led by “radical green zealots”.

Reuters and the Associated Press contributed to this report