Not even two weeks after Hurricane Helene swamped the Florida coastline, Milton heads towards the state as a category five hurricane, strengthening at a rapid pace.

The system is threatening the densely populated Tampa metropolitan area — which has a population of more than 3.3 million people — with a potential direct hit, and menacing the same stretch of coastline that was battered by Helene.

On the way to Florida, it is also posing an immediate threat to Mexico's Yucatan peninsula, which includes three Mexican states and parts of the Central American nations of Belize and Guatemala.

How powerful might Milton be?

Milton was classified as a tropical storm on Sunday afternoon, local time, but in less than 24 hours it "explosively" morphed into a category five — the third-fastest intensifying storm on record in the Atlantic Ocean, according to the United States National Hurricane Center.

Category five is the strongest level of tropical storm on the Saffir-Simpson scale.

For comparison, Helene, which hit Florida last month as one of the deadliest hurricanes in the US in decades, made landfall as a category four storm.

US authorities have ordered mandatory evacuations in areas along Florida's west coast, including for 500,000 residents of Pinellas County.

"There's no other way to put this. Over the next few days, Tampa Bay is faced with the biggest hurricane it's had for more than 100 years," Pinellas County emergency management director Cathie Perkins told a press conference.

Florida Governor Ron DeSantis issued a state-of-emergency declaration for 51 counties. "It's going to be powerful, so please take the appropriate precautions," he said. "This has the potential to have a lot of damage."

President Joe Biden also declared an emergency for Florida, allowing federal disaster-relief operations to commence.

Milton is the season's ninth hurricane, and the second Atlantic storm to reach category five this season after Beryl, which in July became the earliest storm to reach that distinction.

When will Milton make landfall?

According to the National Hurricane Center's Live Hurricane Tracker, Milton will make landfall on the west coast of Florida, somewhere around the Tampa-St. Petersburg area, on Wednesday.

It is expected to weaken slightly to a category three storm when it hits the shore in the Tampa Bay region, which has not endured a head-on hit by a major hurricane in more than a century.

After making landfall, Milton was projected to cross the Florida peninsula and still emerge as a hurricane on the Atlantic coast.

Though it is likely to weaken by the time it makes landfall in Florida on Wednesday, counties along the state's western coast are bracing for potentially catastrophic winds, rain and a storm surge of 2.4 to 3.7 metres.

"Damaging hurricane-force winds and a life-threatening storm surge with destructive waves are expected across portions of the northern coast of the Yucatan Peninsula through tonight," the hurricane centre said. 

"Milton is expected to grow in size and remain an extremely dangerous hurricane when it approaches the west coast of Florida on Wednesday."

It could retain hurricane strength as it churns across central Florida toward the Atlantic Ocean.

Where is the storm now?

Milton intensified quickly on Monday over the eastern Gulf of Mexico.

As of 4pm central time in the US on Monday, the storm was about 125 kilometres west-north-west of Progreso, and 1,085km south-west of Tampa, the hurricane centre said.

Mr DeSantis said the hurricane was already far stronger than what was predicted two days ago, with maximum sustained winds of 285 kilometres per hour.

The Tampa Bay area is still rebounding from Helene and its powerful surge.

The hurricane centre stressed the storm would cross Florida as a hurricane and "grow in size", regardless of what category it was.

With Milton bearing down on Florida, forecasts of a supercharged Atlantic hurricane season were starting to look more on target than they did at the beginning of September, typically the peak time for the formation of named storms.

How bad is damage expected to be?

The entire Gulf Coast of Florida is especially vulnerable to storm surge.

Hurricane Helene came ashore some 240km away from Tampa in the Florida Panhandle and still managed to cause drowning deaths in the Tampa area due to surges of about 1.5 to 2.5m above normal tide levels.

Forecasters warned of a possible storm surge of 2.4 to 3.6m in Tampa Bay. 

That is the highest ever predicted for the region and nearly double the levels reached two weeks ago during Helene, said National Hurricane Center spokeswoman Maria Torres.

The storm could also bring widespread flooding. Thirteen to 25 centimetres of rain was forecast for mainland Florida and the Keys, with as much as 38cm expected in some places.

Relief efforts are already underway throughout the south-eastern US in the wake of Helene, which killed more than 200 people across six states. 

It was the deadliest named storm to hit the US mainland since Hurricane Katrina killed nearly 1,400 people in 2005.

What if I have travel plans to that part of Florida?

Tampa International Airport said it would stop flights on Tuesday morning. 

"We will suspend flight operations at 9 a.m. Tuesday and reopen when safe to do so," it posted on X.

St. Pete-Clearwater International Airport said it is in a mandatory evacuation zone and will close after the last flight leaves on Tuesday.

Southwest Florida International Airport in Fort Myers will also be closed on Wednesday and Thursday.

Further inland in Orlando, Orlando International Airport will cease operations before the hurricane reaches the Gulf Coast, Orlando Sanford International Airport will remain operational, and the Melbourne Orlando International Airport will close from Wednesday afternoon to Friday morning.

Most flight disruptions are expected to be on Wednesday and Thursday.

Miami International Airport — the United States's second busiest — has not announced a halt to operations so far. It is located on Florida's east coast, away from where Milton will make landfall.

How is Mexico preparing?

Mexican officials are organising buses to evacuate people from the low-lying coastal city of Progreso on the Yucatan peninsula after Mexico's National Meteorological Service said Hurricane Milton "may hit between Celestun and Progreso" late Monday or early Tuesday.

Celestun, on the western corner of the peninsula, is a low-lying nature reserve home to tens of thousands of flamingos. Progreso, to the east, is a shipping and cruise ship port with a population of about 40,000.

"We are facing a hurricane that is not following the typical trajectory of those that normally have hit us," said Yucatan state Governor Joaquín Díaz, who ordered the cancellation of all non-essential activities except grocery stores, hospitals, pharmacies and gas stations.

ABC with wires