Two University of Arizona researchers predicted a busy and intense hurricane season, although most other forecast models said it was likely going to be an average period.

And while it’s not unusual to have an active season, what has been unpredictable is that the United States would be hit by two major hurricanes in fairly rapid succession — Harvey in Texas and Irma in Florida, they said.

And the researchers said global warming’s effect on tropical weather may, in fact, mean that fewer but more intense hurricanes could be in our long-term future.

This July, Xubin Zeng’s hurricane forecasting model, which he developed with two students in 2015, predicted 11 total hurricanes, plus or minus two, reaching Category 1 levels or higher.

Zeng is the Agnese N. Haury Chair in Environment, director of the UA’s Climate Dynamics and Hydrometeorology Center and a professor of atmospheric sciences.

Other forecasting models across the country predicted an average season, about seven hurricanes, or slightly below average, said Thomas Galarneau, assistant professor in the UA’s department of hydrology and atmospheric sciences.

“We were really nervous,” Zeng said, but his team stuck to its prediction despite being outliers. “Months later, we didn’t change, but other centers changed their models,” which moved their predictions closer to Zeng’s.

Tropical depressions, storms and hurricanes are named in alphabetical order, restarting at the beginning of each season with the letter A. The Atlantic hurricane season runs from June 1 to Nov. 30.

On Aug. 25, Harvey was the first hurricane to make landfall on the United States this year where it brought historic rain and flooding to the Texas coast.

But as the storm finally dissipated, Hurricane Irma pounded the Caribbean and then swept through Florida, bringing devastation from the Florida Keys all the way to Jacksonville.

Hurricanes Jose and Katia formed soon after. Katia made landfall in Mexico, while Jose has stayed out at sea.

Many feared that these seemingly unusual and unrelenting forces of nature were omens or the direct cause of a changing climate, but Zeng and Galarneau have a different response to these events.

“These situations where you have multiple hurricanes in the Atlantic at the same time actually happen pretty frequently,” Galarneau said.

What does make these hurricanes unusual is that two Category 4 Atlantic hurricanes made landfall in the United States in the same season.

Then, the fact that they devastated high population centers brought the storms national attention.

“I realize that many people talk about hurricanes and climate change because they want to see the government take action, and from that perspective, it’s good,” Zeng said.

But both he and Galarneau stressed the reality is more subtle than that. It’s really difficult to claim that a single hurricane’s activity is the result of climate change.

“Everyone knows for hurricanes to develop you need a few ingredients, like cooking,” Zeng said. Those ingredients include a warm ocean, which allows moisture and rainfall to build.

It’s more accurate to say that as the average global temperature gets warmer, and so does the ocean, hurricanes on average will get less frequent but more intense, said Galarneau, citing an ever-growing and evolving body of climate research.

“The climate-change signature in Harvey is there,” Galarneau said. But part of the reason the flooding in Houston was so dramatic was the design of the city, specifically the impenetrable pavement.

“In the future, there might be studies that can tell us how much was caused by climate change, from a statistical perspective,” he said.

There have been six hurricanes so far this season, three major.

“We forecasted six major hurricanes; the average was two. Now we already have three,” Galarneau said in a statement.

For now, there’s still a lot of time in the hurricane season, and their forecast so far is still on track.

Contact Mikayla Mace at mmace@tucson.com or 573-4158. On Twitter: @mikaylagram