Hurricane Ida is looking eerily like a dangerous sequel to 2005’s Hurricane Katrina — the costliest storm in American history. But there’s a few still-to-come twists that could make Ida nastier in some ways, but not quite as horrific in others.

Ida is forecast to make landfall on the same calendar date, August 29, as Katrina did 16 years ago, striking the same general part of Louisiana with about the same wind speed, after rapidly strengthening by going over a similar patch of deep warm water that supercharges hurricanes.

What could be different is crucial though — direction and size.

Katrina hit Louisiana from due south, while Ida is coming to the same part of the state from south-east. A day-and-a-half before landfall Ida’s hurricane-force winds extended 13 miles (21 kilometres) from the centre compared to 106 miles (170 kilometres) for the much more massive Katrina at the same time before landfall.

“This has the potential to be more of a natural disaster, whereas the big issue in Katrina was more of a man-made one” because of levee failures, said University of Miami hurricane researcher Brian McNoldy. Levee failures pushed Katrina’s death toll to 1,833 and its overall damage to about US$176 billion in current dollars, and experts don’t expect Ida to come near those totals.

DIFFERENT DIRECTION

Ida is coming to the same general place from a slightly different direction. Several hurricane experts fear that difference in angle may put New Orleans more in the dangerous storm quadrant — the right front part of a hurricane — than it was in Katrina, when the city was more devastated by levee failure than storm surge. Katrina’s north-east quadrant pushed 28-foot (8.5-metre) storm surges in Mississippi not New Orleans.

Ida’s “angle is potentially even worse”, McNoldy said. Because it is smaller “it’s not going to, as easily, create a huge storm surge… but the angle that this is coming in, I think is more conducive to pushing water into the lake (Pontchartrain).”

That north-western path of Ida not only puts New Orleans more in the bullseye than it did in Katrina, but it also more targets Baton Rouge and crucial industrial areas, said meteorologist Jeff Masters, who flew hurricane missions for the Government and founded Weather Underground. He said Ida is forecast to move through “the just absolute worst place for a hurricane”.

“It is forecast to track over the industrial corridor between Baton Rouge and New Orleans, which is one of the key infrastructure regions of the US, critical to the economy. There’s hundreds of major industry sites there, I mean petrochemical sites, three of the 15 largest ports in America, a nuclear power plant,” Masters said. “You’re probably going to shut down the Mississippi River for barge traffic for multiple weeks.”

“It’s not just the coastal impact. It’s not just New Orleans,” said meteorologist Steve Bowen, head of global catastrophe insight at the risk and consulting firm Aon. “We’re certainly looking at potential losses well into the billions.”

SIZE MATTERS

The difference is size is not just physically huge, but it matters for damage. Storms that are bigger in width have larger storm surge because of the broader push of the water.

Ida “is not going to generate the huge storm surge like Katrina did, it’ll have more focused storm surge like (1969’s) Camille,” Masters said.

But larger in size storms are often weaker, Bowen said. There’s a trade off of intense damage in a smaller area versus less damage, but still bad, in a wider area. Bowen and Princeton University’s Gabriel Vecchi said they don’t know which scenario would be worse in this case.

RAPID INTENSIFICATION

Ida is about to hit an eddy of what’s called the Loop Current, which is a deep patch of incredibly warm water. It takes warm water off the Yucatan Peninsula, does a loop in the Gulf of Mexico, and spins up the eastern edge of Florida into the Gulf Stream. Water above 79 degrees (26 degrees Celsius) is hurricane fuel.

Normally when a storm intensifies or stalls it takes up all of the region’s warm water and then hits colder water that starts to weaken the storm or at least keeps it from further strengthening. But these warm water spots keep fuelling a storm. Katrina powered up this way and Ida is forecast to do the same. The eddy that Ida is going to pass over has necessary warm water going more than 500 feet (150 metres) deep, “just a hot tub”, McNoldy said. That means lots of rapid intensification.

“Running over these Loop Current (eddys) is a very big deal. It’s really dangerous,” said climate and hurricane scientist Kossin of The Climate Service. “It could be explosive.”

In the past 40 years more hurricanes are rapidly intensifying more often, and climate change seems to be, at least, partly to be blamed, Kossin and Vecchi said. Hurricane Grace already rapidly intensified this year and last year Hanna, Laura, Sally, Teddy, Gamma, and Delta all rapidly intensified.

“It has a human fingerprint on it,” said Kossin, who with Vecchi, was part of a 2019 study on recent rapid intensifications.

NEW EYEWALL

After a hurricane rapidly intensifies it becomes so strong and its eye so small that it often can’t quite keep going that way, so it forms an outer eyewall and the inside eyewall collapses, Kossin said. That’s called eyewall replacement.

When a new eyewall forms, often a storm becomes larger in size but a bit weaker, Kossin said. So key for Ida is when, and if, that happens. It happened for Katrina, which steadily weakened in the 12 hours before it made landfall.

However, many of the other forces like crosswinds that made Katrina weaken at the last minute aren’t there for Ida, McNoldy said.