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Science behind NOAA outlook for 2013 hurricane season

A NOAA satellite image on Sept. 26, 2004, shows Hurricane Jeanne slicing across Central Florida with a girth of 400 miles.

Associated Press

A NOAA satellite image on Sept. 26, 2004, shows Hurricane Jeanne slicing across Central Florida with a girth of 400 miles.

Scientists with the National Oceanic and Atmospheric Administration say the Atlantic region could see an "active or extremely active" hurricane season in 2013.

But they say that every year, right?

NOAA forecasters annually estimate how many storms might emerge during hurricane season and how strong they could be. This year, NOAA believes there is a 70 percent chance that between 13 and 20 named systems (storms with wind speeds above 39 mph) will develop from June 1 to Nov. 30.

If that seems like an old prediction, it's probably because forecasters have been calling for active seasons a lot lately, said Gerry Bell, lead seasonal hurricane forecaster at NOAA's Climate Prediction Center.

"That's because there are more hurricanes threatening," Bell said.

Since 1995, the Atlantic region has been in the warm phase of a cycle that historically has created decades of increased tropical storm activity at a time, he said. Scientists call it the AMO, short for Atlantic Multidecadal Oscillation.

Meteorologists look to established indicators like ocean temperatures and wind strength to read the AMO and forecast the hurricane season. Warm phases generate more cyclones and can last 25 to 40 years, followed by equally long cooler periods of decreased activity.

Bell said in the 18 years since the latest warm phase began, the Atlantic region has experienced 12 hurricane seasons with above-average activity.

In some ways, generating the annual outlook is just guesswork. But the NOAA specialists behind it are not in the business of wild speculation.

"There's really a strong scientific basis for making the seasonal outlook," Bell said.

Signs of an active season this year include warm water in the Atlantic Ocean, a strong West African monsoon, and less than normal rainfall in the Amazon Basin, according to Bell.

Conversely, during the last down period for storms from 1971 to 1994, rainfall was heavier in the Amazon Basin. During those decades, Bell said, the Atlantic experienced only two above-average hurricane seasons.

Bell said the lack of an El Niño in the west, which would hamper the formation of Atlantic hurricanes, could also support high storm activity in 2013.

Yet, despite all the data, the outlook is inexact and has obvious limits.

Forecasters cannot predict how many hurricanes will reach land or where they will hit. Bell said landfall is only predictable up to about a week in advance because it depends on local weather patterns when a storm approaches.

Even if the Atlantic plays host to 20 named storms in the coming months, it's possible that none would hit Tampa Bay and the season would seem inactive for local residents. When the last hurricane to strike the area hit in 1921, the Atlantic was actually in a period of low activity.

"The utility of the seasonal forecast even for the insurance companies or FEMA is still pretty limited I think because you can't definitely say with the seasonal outlook what that means for landfall," said Chris Landsea, science and operations officer at the National Hurricane Center in Miami.

The long-term outlook is more a measure for science than public safety, said Pinellas County emergency management spokesman Tom Iovino.

"It really doesn't weigh on our decisions in terms of preparedness," he said.

The forecast does, however, generate hype each year that forecasters and emergency planners say can help grab residents' attention.

"The seasonal forecast gets us talking about hurricanes and reminds us that the season is upon us," said Rick Knabb, director of the National Hurricane Center.

Around Tampa Bay, that is especially important. Decades have passed since the last direct hit, and it's easy to become complacent about preparedness.

But Knabb said a storm will come. He created a list of American cities that were overdue for a hurricane in 2011. Tampa was number five on that list.

"It really is just a matter of when, not if, a major hurricane will impact the Tampa Bay area," Knabb said.

Zachary T. Sampson can be reached at zsampson@tampabay.com or (727) 893-8804. Twitter: @zacksampson.

Science behind NOAA outlook for 2013 hurricane season 05/31/13 [Last modified: Friday, May 31, 2013 10:21pm]

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