Hurricane Irma shaping to be a perfect - but awful - storm

One of the most powerful hurricanes to develop in the Atlantic basin is shaping up to be a perfect but "potentially catastrophic" storm as it remains on course to hit the US state of Florida by Sunday.

Phil Klotzbach, an atmospheric research scientist at Colorado State University said the descriptor of hurricane Irma as perfect was "fair" given its huge size and shape.

"Storms don't get much more impressive on satellite than Irma right now," Dr Klotzback told Fairfax Media. "That's the hurricane paradox - so amazingly beautiful from afar and so terrorising if you're in one."

Irma has already begun to challenge the record books as it intensified on Tuesday, local time, into a category 5 hurricane with sustained winds reaching 185 miles per hour (297 km/h).

The storm is the strongest Atlantic storm to exist outside of the Caribbean and Gulf of Mexico, and is within about 10 mph of the most potent cyclones - both typhoons in the north-west Pacific - ever recorded.

The speeds are at the top end of the Saffir-Simpson hurricane wind scale, and are above the 180-mph range that has previously prompted calls for a category 6 rating.

The distinction, though, would likely be lost on populations lying in the wake of the storm given the potential for severe damage to buildings -and their occupants - from sustained wind speeds that exceed 250 km/h.

On Wednesday, it was the Caribbean island of Barbuda which was hit first. The US National Weather Service was warning of "life-threatening wind, storm surge and rainfall hazards" for northern Leeward Islands and the US territory of Puerto Rico.

The hurricane could become "extremely dangerous" for the Bahamas and Cuba later in the week.

Current projections indicating Irma will hit the Florida Keys and part of the Florida Peninsula by the weekend, the NWS said. (See NWS chart below as of 11 pm, local time Tuesday (1pm AEST).)

Unprecedented

For the US, the prospect of another big hurricane could hardly come at a worse time as authorities continue recovery efforts of massive flooding to Houston and nearby regions of Texas and Louisiana from category 4-strength Harvey.

President Donald Trump on Tuesday declared an emergency for Florida "to lessen or avert the threat of a catastrophe in all 67 counties in the state".

US President Donald Trump declared an emergency for Florida.

US President Donald Trump declared an emergency for Florida. Photo: Bloomberg

Low-lying coastal regions will not only be exposed to severe winds but also the expected storm surge in areas already subject to regular floods as sea levels continue to rise with climate change.

While uncertainty remains about the course of Irma and its strength, if the storm hit the continental US as a category 4 or higher event, it would be unprecedented, Dr Klotzbach said.

"We've actually never had two Cat 4s make landfall in the United States in the same year," he said.

Beyond theory

Meteorologists were among those surprised by the ferocity of Irma, which preliminary readings suggested it had exceeded its "maximum potential intensity" given the conditions.

Kevin Walsh, a cyclone researcher at the University of Melbourne's School of Earth Sciences, cautioned that the potential intensity theory is "incomplete in the sense that it might not be 100 per cent accurate under all circumstances".

Dr Klotzbach said that several other storms had exceeded their maximum potential intensity: "It's not common but it does happen".

Climate signals

Professor Walsh said that with the right conditions it was possible for a basin to spawn several cyclones simultaneously.

He noted US research showing the Atlantic has had as many as four hurricanes spinning at the same time twice - in 1893 and 1998.

The severity of the twin hurricanes is likely to fan further debate about the role of climate change.

Research published in 2015 by researchers including Professor Walsh and Dr Klotzbach found "climate models mostly continue to predict future decreases in global [tropical cyclone] numbers, projected increases in the intensities of the strongest storms and increased rainfall rates.

"Sea level rise will likely contribute toward increased storm surge risk," the researchers found.

Dr Klotzbach said that the US had seen a long-term decline in the number of hurricanes making landfall in the US (see chart below).

But that trend was nothing to be complacent about.

"Really the key point here is that in general, we've been riding an amazing streak of luck the past 11 years," he said. "We went 31 major hurricanes in the Atlantic without a US landfall. The odds of that were well over 1:1000."

World Meteorological Organization experts, meanwhile, have also weighed into the debate, saying that will there is "no clear evidence" that climate change has made slow moving hurricanes such as Harvey more or less likely in the Houston region, some "ingredients" of the event "may have linkages to climate change".

"[T]he rainfall rates associated with Harvey were likely made more intense by anthropogenic climate change," the WMO said. "This is a consequence of the tropical atmosphere generally holding more water vapor due to climate warming", or about 7 per cent more water vapor per degree celsius of sea surface temperature increase.

This story Hurricane Irma shaping to be a perfect - but awful - storm first appeared on The Sydney Morning Herald.