Wild hurricane season for Atlantic Seaboard: Irma to come calling

By PBS Newshour, adapted by Newsela staff 09/11/2017

Hurricane Harvey devastated Texas and Louisiana two weeks ago. It caused at least 60 deaths and billions of dollars in damages. Then, Hurricane Irma came.

Last week, Irma moved toward the Caribbean and on a projected path towards southern Florida, with wind speeds more than 156 miles per hour. That would make it a highly dangerous Category 5 storm. However, on Friday it downgraded to a Category 4 but remained powerful. On Sunday it was downgraded again to a Category 2 before hitting Florida's coast.

Since we started keeping records in 1851, “The U.S. has never been hit by two Category 4 or stronger hurricanes in the same season,” said Jeff Masters. He's a meteorologist and co-founder of Weather Underground. Now, "that could happen.”

Here’s what to expect from the storm — plus an explainer on why this hurricane season is churning out so many extreme storms.

Will South Florida Be Hit Hard?

Hurricane Irma already hit the Carribean hard. On Friday, it hit the islands of Turks and Caicos. It was on track towards Cuba before making landfall in southern Florida. What happened in Cuba would largely determine what happens to the rest of south Florida.

If the storm squeaked through these islands, then it would likely be a Category 4 or 5 hurricane when it hit. A Category 5 hurricane has only hit the nation three times since 1851.

Irma did hit Cuba, though, ravaging the small Caribbean country.

Major Hurricanes Could Form In Atlantic

Masters said, “the Atlantic is primed for making major hurricanes” now for three major reasons.

First, and most important, is weak wind shear, says Philip Klotzbach, who studies the atmosphere at Colorado State University. Hurricanes lose power when winds near the ocean surface blow at one speed and direction while winds in the upper atmosphere blow another. This difference — or shear — causes a hurricane to tilt, like a spinning top. This lessens the force of the winds coming from it.

The other two ingredients: warm Atlantic ocean temperatures and high levels of moisture in the air. Irma is brewing where sea temperatures hover around 85 degrees Fahrenheit, which makes hurricanes stronger.

Irma is following close behind Harvey because of a change in air pressure along the East Coast, Klotzbach said. For about 10 years, a ridge of low pressure on the East Coast has pushed away hurricanes, Klotzbach said. But this invisible ridge has pushed westward this year, leaving the eastern seaboard, the Caribbean, and the Gulf of Mexico exposed to hurricanes.

Above-Average Season Was Predicted

In early July, Klotzbach’s team at Tropical Meteorology Project predicted the Atlantic Ocean would experience an above-average hurricane season. It's because of the El Niño-Southern Oscillation (ENSO), which was absent this year.

El Niño creates extremely warm temperatures in the Pacific Ocean. This pushes strong winds into the Atlantic, creates high levels of wind shear that stop hurricanes from forming. When Pacific sea temperatures reverse and cool, it creates La Niña, which tends to drive hurricanes.

Here’s the strange thing: ENSO is neutral this year, so you might expect an average year of Atlantic storms.

But 2017 has behaved much like 2005. Back then, ENSO was neutral, but the U.S. still experienced many colossal storms, including Hurricane Katrina.

Harvey and Irma appear to be replicating that legacy. Harvey dumped a record 24 trillion tons of water onto Texas and Louisiana. Meanwhile, when Irma’s wind speeds reached 180 miles per hour Tuesday, it became one of the strongest Atlantic storms in history.

“We’ve only had 16 other hurricanes on record that have been as strong as Irma is right now,” Klotzbach said.

Florida residents are worried about a hurricane similar to Hurricane Donna which happened in 1960.

Donna Was Devastating

Residents of the Florida Keys experienced 13-foot storm surges before Donna slashed up the East Coast. It became the only storm to produce hurricane-force winds in Florida, the Mid-Atlantic states, and New England. If a hurricane like this hit Florida today, it could cause between $50 billion to $100 billion in damages.

Klotzbach urged people to pay attention to hurricane watches or warnings for their communities. He doesn't expect Irma to dump as much rain as Harvey, but its storm surge could still have a huge impact on south Florida, he said. Heavy rains, storm surge, and flooding will likely hit Georgia and the Carolinas after it hits Florida.

“Follow the advice of the local emergency management. They’ll tell you whether you should evacuate or shelter in place,” Klotzbach said.

A fake news story garnered more than 2 million shares on Facebook last week. It asked if Hurricane Irma and its 180-mile-per-hour winds could be considered a Category 6 storm?

No, said Masters.

There will never be a Category 6 storm, Masters says, "because Category 5 is already catastrophic." But climate change is expected to make the winds and rains of major storms stronger over the next century.

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