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ABC News
weather reporter Tyne Logan

How the record-breaking Tropical Cyclone Ilsa became so strong

The final remnants of the ex-Tropical Cyclone Ilsa have dissipated across the country, with most communities in Western Australia's north-west emerging largely unscathed.  

But the powerful tropical cyclone recorded peak gusts of up to 289 kilometres per hour — becoming so strong that it broke the Bureau of Meteorology's equipment.

The category-five system also set a preliminary Australian record with 10-minute sustained wind speeds of 218kph as it ripped directly over the top of both Rowley Shoals and Bedout Island.

The now-flattened Pardoo Roadhouse took a direct hit from the system. It shows just how destructive the winds were for those in its path.

So how did it become so strong?

BOM meteorologist Jessica Lingard said in the case of Ilsa, it was not a question of what was there to build it up, but rather what was not there to stop it.

Destruction at Pardoo homestead after Tropical Cyclone Ilsa. (Supplied: Jack Poplawski)

Nothing to tear Ilsa apart

Tropical cyclones require sea-surface temperatures to be above 26.5 degrees Celsius in order to form. This warm, moist air generates the massive amounts of energy that fuel a cyclone.

There also needs to be a relatively low wind shear in the atmosphere for the system to be able to organise itself vertically — the structure it needs to thrive.

Wind shear refers to a change in wind speed or direction with height.

A high wind shear can cause a cyclone to crumble.

"It basically just tears the system apart," Ms Lingard said.

Ms Lingard said any cyclone could reach a category five given the right opportunity.

And in the case of Ilsa it had it all — until it hit the coast.

"There's really nothing stopping a system from reaching a category five every single time," she said.

"Usually the things that prevent a system from developing is entering unfavourable atmospheric conditions like high shear, or it hits the mainland, or it moves over waters that aren't warm enough.

"So really it's not about what helped it get really strong, it's what wasn't there to stop it from getting strong."

Ilsa was the only severe cyclone to cross the Australian coast this year, but this cyclone season has seen cyclones of category five strength repeatedly.

Five of six cyclones to form in Australian waters in from 2022 to 2023 have reached "severe" intensity and four of six have reached category five, including Tropical Cyclone Freddy.

Ms Lingard said it should also be noted that while Ilsa was no doubt a very powerful storm, it was likely there had been cyclones of the past with higher wind gusts that simply didn't track directly over a BOM observation site.

The ingredients of Ilsa

Tropical Cyclone Ilsa formed over waters up to 31C – well above the threshold needed to fuel a cyclone.

It moseyed over waters to the north of Australia for several days as a tropical low, struggling to form to cyclone intensity under an atmosphere of high wind shear.

Sea surface temperatures were up to 31C off the Kimberley coast when Tropical Cyclone Ilsa formed (ABC: Chris Lewis)

But once it reached waters further south, to the north-west of Broome, conditions in the atmosphere changed and the wind shear dropped off, allowing Ilsa to spin up rapidly.

"And it had the moisture feed from the warm waters underneath it, and we were able to see it develop quite quickly," Ms Lingard said.

The cyclone became so strong it broke some of the BOM's equipment.

"I don't know whether it is the anemometer (a device that measures wind speed) that failed or whether it's just the communications that have failed," Ms Lingard said.

"But, either way … we've lost communication with both [Rowley Shoals and Bedout Island] and we'll send technicians out when the weather clears to investigate what actually happened.

"Anything over 200kph is pushing the limits of the equipment, especially when you're talking about two stations who spend their days and nights getting bashed by salt water."

Low wind shear for 60 hours

BOM tropical cyclone meteorologist Joe Courtney said any system that spent more than 48 hours in a low wind shear environment was likely to become a severe cyclone.

Ilsa spent well over 60 hours in a low wind-shear environment.

"It was only moving in the order of 10kph for most of that time so it wasn't racing through that favourable zone, which can be another reason [it got so strong]," he said.

An SES worker cleans up debris near Pardoo Roadhouse after Tropical Cyclone Ilsa tore through the area. (ABC News: Cason Ho)

More unusually, it maintained strength well into its journey across land, crossing Marble Bar as a category three and Telfer as a category two.

Typically tropical cyclones decay rapidly after crossing the coast, with inland parts of the Pilbara only built to withstand weakened cyclones.

"In this case, it went over very flat terrain so the frictional effects weren't great, which allowed it to go for a bit longer," he said.

"And the wind shear in the first part of Friday as it went inland remained low.

"It was also still accessing deep tropical moisture from off the Kimberley, and that was also a feature of ex-Tropical Cyclone Ellie too."

Late-season cyclones stronger

The time of year was also in favour of Ilsa's strength.

Statistics show cyclones are more likely to become severe in March and April, rather than earlier in the season.

Pardoo Roadhouse copped the full brunt of Tropical Cyclone Ilsa, but damage elsewhere appears to have been minimal. (ABC News: Cason Ho)

Mr Courtney said this was because the monsoon trough was closer to the equator toward the end of the season, allowing cyclones more time to build.

"A lot of January cyclones form further south and so they will hit land soon after developing," he said.

"To get to a category five, you need a solid five days of development time."

Mr Courtney said later in the season waters were also warmer, generally.

Research published in the American Meteorological Society Journal in 2021 found evidence that tropical storms in the late hurricane season had a better chance of intensifying than early-season storms.

Mr Courtney said this would be broadly applicable to the southern hemisphere too, although the North Atlantic was more prone to water temperature changes throughout the season compared to the Kimberley coast.

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