Hurricane Irma slammed into Florida early on Sunday, belting the southern US state with wind gusts of up to 240km per hour while threatening massive tidal surges.

Here are some fast facts on the most powerful storm to hit Florida in decades.

Where has it been so far?

Hurricane Irma began life as a tropical wave near the Cape Verde Islands on August 30. It then made its way across the Atlantic and caused catastrophic damage in the northern Leeward Islands before heading for the far north of Cuba.

How dangerous is this storm?

At its peak, its winds were around 298km per hour. It is the strongest hurricane ever recorded in the Atlantic basin outside the Caribbean and Gulf of Mexico. Irma was the first Category 5 storm to affect the northern Leewards.

After dropping to a Category 3 storm on Sunday as it passed along Cuba's northern archipelago, the storm has since risen to Category 4 again as it crossed warm waters between Cuba and the Florida Keys.

Irma is packing winds of up to 215km/h and gusting up to 240km/h. It is moving northwest at only 9km/h. That is no more than a gentle jogging pace.

Is it good news that the storm has slowed down?

No, not really. The large storm surge, which will be three to four metres high, and the accompanying heavy rain will be there for longer and will exacerbate the life-threatening floods. Some parts could receive as much as 500mm-600mm of rain over the next few days.

Where is it going?

Current forecasts are now taking the storm along the western coast of Florida. Initial runs of the computer model expected Irma to pass over the eastern side of the state. Subsequent runs have shifted further west, primarily because it stayed south and it clung onto the Cuban islands for longer.

Is it good news that it stayed to the south?

It was certainly good news for the Bahamas. It has also taken the sting out of the storm as far as passing over eastern Florida is concerned. However, that is the "dirty" side of the storm where the strongest winds will still affect the large conurbations of Miami and Orlando.

The storm surge of 3-4 metres is now forecast to pass up the western side of the state with big waves on top of that.

When will the storm clear Florida?

The eye wall is currently hitting the Florida Keys as a Category 4 storm. Irma is expected to pass close to, if not make landfall, near Fort Myres at about 00:00 GMT on Monday.

This is a massive storm with hurricane force winds extending about 130km from the centre of the storm. Tropical storm force winds are currently spread around 350km from the eye. Irma is staggering along so slowly that it may take two days to clear into Alabama still as a Category 1 hurricane.