Before crashing into Florida, Hurricane Irma set all sorts of records for brute strength as it flattened Caribbean islands and swamped the Florida Keys.

Irma's assault - so soon after Harvey's deluge of Houston - marked the first time the US was hit by two Category 4 storms in the same year. But it could have been worse.

In what way was Florida 'lucky'?

Irma hit the Sunshine State as a big wide beast, though not quite the monster it once was shaping up to be. Earlier, it was the most powerful recorded storm in the open Atlantic. But as the once-Category five storm neared the US mainland, it lost some oomph after running into the northern coast of Cuba. And on top of that, Irma avoided what could have been its most destructive paths along the Florida Peninsula - over Miami and the heavily developed Atlantic seaboard.

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How much did it drop off?

Winds dropped to a quite potent 185 km/h by the time Irma made landfall on Marco Island, on the Florida Peninsula, still a major and dangerous hurricane yet not near its 297 km/h former self when it set a record last Wednesday for the most powerful storm in the open Atlantic. Still, at about 640km wide, it raked much of the state with devastating storm surge, destructive winds and drenching rains.

How much of a difference do those changes make?

"There's a huge difference between a (Category) three and five when it makes landfall," said private meteorologist Ryan Maue of WeatherBell Analytics. "Barbuda is an example of that. It was wiped. This is obviously not the worst case scenario for Florida overall."

Had the centre of Irma hit Florida 32-50km to the east "it would have been much worse".

Why didn't it?

Florida can thank Cuba, where it did hit as a Category five storm, said Maue and Jeff Masters, meteorology director for Weather Underground.

"We got super lucky that this storm didn't go 32km north of Cuba over the weekend instead of going over the coast of Cuba because this would have been a Category five hurricane," Masters said.

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The storm briefly trekked over Cuba's low populated coast on Saturday. That weakened Irma enough that when upper level winds from the west eroded some of the storm's top and also blew in dry air, it had the combined of effect of making Irma more ragged, Masters said. It was at that, he said, that Irma's southwest eyewall sort of came apart, no longer a perfect circle on satellite imagery.

What else weakened it?

Slightly weakened from Cuba, the storm got caught up in competing weather systems a little longer, delaying its northward right turn into Florida.

And that delay pushed the track further west, making it more of a threat to Florida's west coast than its east.

What did that mean for damage on the ground?

Florida's west coast has about US$1 trillion in property at risk to a storm, compared to US$1.5 trillion on the east, according to insurance computer modeling firm AIR Worldwide. The company estimates insured losses for Irma will be between US$15 billion and US$50 billion. Even another 32-50km would have put the nasty and stronger northeast quadrant of stronger winds, storm surge, rainfall and tornadoes more directly in Miami, Maue said. And even hitting the Tampa region from land to the south instead of from open water will reduce storm surge ever so slightly, Masters said.

Flood waters rise around signs at the Haulover Marine Centre at Haulover Park as Hurricane Irma passes by in North Miami Beach, Florida. Photo / AP
Flood waters rise around signs at the Haulover Marine Centre at Haulover Park as Hurricane Irma passes by in North Miami Beach, Florida. Photo / AP

Where will Irma stand among damaging storms?

Masters predicts that when Irma is done it will go down as one of the five costliest hurricanes in US history, but not up with the top three of 2005's Hurricane Katrina, 2012's Sandy and this year's Harvey. Still, he guessed, it will be grouped with two other South Florida storms: 1992's Andrew and 2005's Wilma.

How does the hurricane's intensity compare?

Irma's two US landfalls were unusually powerful. When Irma passed over the Florida Keys its central pressure was the seventh lowest for a smack into the US. Only the 1935 Labour Day storm, 1969's Camille, Katrina, Andrew, an 1886 Indianola, Texas, storm, and a 1919 Florida Keys storm were more intense based on atmospheric pressure. Irma ties with the killer 1928 Lake Okeechobee hurricane.

Irma's second landfall on Marco Island, taken by itself, still would rank among the top couple of dozen landfalls in intensity, slightly weaker than Harvey.

What other records did Irma set?

Irma's 297 km/h winds were the highest on record for the open Atlantic ocean, outside the Gulf of Mexico and Caribbean sea. Only one other storm in the entire Atlantic basin - 1980's Allen - was stronger. It spent three consecutive days as a top-of-the-scale Category five hurricane, the longest in the satellite era. It generated the second most Accumulated Cyclone Energy - a key measurement that combines strength and duration - in the satellite era.

Irma was also the strongest storm to hit the Leeward Islands and the first Category five hurricane to hit Cuba, which regularly gets assaulted by hurricanes, in nearly 100 years.

- AP