FORT LAUDERDALE — During busy storm seasons, the U.S. coastline frequently comes under attack.
Blame the atmospheric steering currents that tend to push Atlantic systems in this general direction. That was seen in bustling 2004 and 2005 — five hurricanes made landfall in each of those years.
On the other hand, active seasons don’t always translate to disaster. In 1958, a year climatologically similar to this one, seven hurricanes, five major, developed, and not one hit the United States.
“Some active seasons, you have more landfalls than others. That depends on what the winds are doing over the Southeast U.S.,” said Gerry Bell, lead hurricane forecaster for the National Oceanic and Atmospheric Administration.
This storm season is expected to see up to 14 hurricanes and, with 1,200 miles of coastline, Florida is by far the most vulnerable state. Historically, 40 percent of all U.S. hurricanes and major hurricanes hit the Sunshine State.
“If we get 10 hurricanes this year, I’d be surprised if one of them didn’t hit Florida,” said Jeff Masters, chief meteorologist of Weather Underground, an online weather site.
In an average year, six hurricanes form and two strike the United States. But when seven or more hurricanes develop, the U.S. coast becomes increasingly vulnerable, experts say.
This year, climatologists Phil Klotzbach and William Gray of Colorado State University calculate the odds of at least one major hurricane, with winds greater than 110 mph, striking the U.S. coast at 76 percent.
They put the chance of a major hurricane striking Florida at 21 percent. In both instances, those odds are significantly higher than average.
“We’re forecasting about twice the average activity at this point, and it certainly could be more active than that,” Klotzbach said.
At the same time, they put the odds of a major hurricane striking Miami-Dade County at 5.2 percent; 3.8 percent in Palm Beach County and 2.4 percent in Broward County. The percentages are based largely on the length of each county’s coastline.
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Those numbers don’t take into account that hurricanes are enormous systems that can cause widespread damage, even without a direct hit.
Other forecast teams also say the United States faces tropical trouble. Tropical Storm Risk, a London-based forecast firm, predicts at least two hurricanes will strike the U.S. In April, AccuWeather predicted five hurricanes will hit.
While it doesn’t develop odds of a strike, NOAA says on average, a hurricane strikes South Florida once every four years. The last hurricane to hit this region — and the state — was Wilma, in 2005.
Why does the United States become a target in active years?
Storms tend to form in the deep tropics, meaning at relatively low latitudes, in an area where the waters are extremely warm and the wind shear is light. That gives them a better chance of crossing the entire Atlantic, Bell said.
“And that increases the threat of landfalls for the Caribbean, Hispaniola, the Bahamas and the U.S. East and Gulf coasts,” he said.
Then, as storms approach the Caribbean, they have a natural tendency to turn to the north, the result of the Coriolis effect, the force created by the Earth’s rotation.
Where a storm starts to make this turn is critical to whether it will strike the Caribbean, Central America or the U.S. coastline — or curve harmlessly out to sea, Bell said.
One of the main steering mechanisms is the Bermuda High, an area of high pressure generally centered in the Western Atlantic. If it extends farther to the west, storms will start their turn farther west, increasing the threat to the Caribbean and the U.S. coastline.
The Bermuda High helped guide two hurricanes toward almost the exact same spot along Florida’s shoreline: Frances and Jeanne hit near Stuart in 2004.
Because the Bermuda High is constantly moving and fluctuating in strength, Bell said it’s too early to say what effect it will have on storms in August and September, the heart of the season.
If a storm approaches the U.S. shoreline, it likely will come under the influence of weather patterns over the Southeast. Notably, low-pressure “troughs” tend to push storms out to sea; high-pressure “ridges” can act to pull systems closer to shore, Bell said.
Why do forecasters feel confident this will be an active season?
Primarily because the waters of the tropical Atlantic are abnormally warm. Ocean temperatures in June already matched those during the peak of the season last year. Warm water fuels tropical systems.
Additionally, El Nino, the atmospheric condition that suppresses storm formation, has dissipated and La Nina, its polar opposite, could emerge by July.
Bell said no matter how many hurricanes form, residents need to prepare.
“You need to be hurricane ready every year,” he said. “They strike in less active seasons and very active seasons.”
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