NOAAHurricane Irene, the most devastating storm of 2011, spins off the Mid-Atlantic coast in August.
NOAAHurricane Irene, the most devastating storm of 2011, spins off the Mid-Atlantic coast in August.
A typical season, based on the years 1981-2010, sees six hurricanes.The Atlantic season runs from June 1 to Nov. 30. This forecast, out Thursday from the National Oceanic and Atmospheric Administration (NOAA), covers any storms that form in the Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico.Overall, NOAA predicts that nine to 15 named tropical storms are likely. Tropical storms have top wind speeds of 39 mph or higher. Once a storm's winds reach 74 mph, it becomes a hurricane.According to NOAA, two current climate factors will limit hurricane development, if they persist:
The first is stronger-than-average wind shear over the Atlantic, which, if it persists, can tear apart burgeoning hurricanes before they start, says forecaster Todd Kimberlain of the National Hurricane Center.Wind shear is when winds are roaring from different directions in different layers of the atmosphere. Winds recently have come from the east at low levels, Kimberlain says, while they've been from the west at upper levels, about 40,000 feet above the surface.The second factor is cooler-than-average sea-surface temperatures in the eastern Atlantic Ocean. Warm water, usually of 80 degrees or above, helps fuel hurricanes. "Another potentially competing climate factor would be El Niño, if it develops by late summer to early fall," says Gerry Bell, lead seasonal hurricane forecaster at NOAA's Climate Prediction Center.El Niño is a warming of tropical Pacific ocean water. The opposite pattern, La Niña, has been diminishing in recent months."In that case, conditions could be less conducive for hurricane formation and intensification during the peak months (August-October) of the season, possibly shifting the activity toward the lower end of the predicted range," he says.This month, two of the biggest private weather forecasting companies, AccuWeather and The Weather Channel, predicted an average or slightly below-average hurricane season. AccuWeather said 12 named tropical storms will form, five of them hurricanes; The Weather Channel forecasts 11 tropical storms — of which six will be hurricanes.Last month, the meteorologists at Colorado State University estimated 10 tropical storms, of which four would be hurricanes. Colorado State University meteorologist William Gray was the first scientist to make seasonal hurricane forecasts in the 1980s.Since 2000, NOAA's tropical storm and hurricane forecasts have been more right than wrong, but not by much: NOAA's prediction has been accurate in seven out of the past 12 years, according to a USA TODAY analysis.NOAA's prediction was too low in four years and too high in just one year: 2006. Ten of the 12 years have seen above-average activity for tropical storms and hurricanes.So far this year, one tropical storm has formed in the Atlantic: Alberto, which spun off the Southeast coast earlier this week. Does the early start portend an active season? No, says the hurricane center's Kimberlain. "There is little, if any, relationship between the early occurrence of a storm at higher latitudes and the type of activity we will ultimately observe later in the season." Forecasters also released their prediction for the Eastern Pacific basin, where 12 to 18 named storms are expected. An average Eastern Pacific hurricane season produces 15 named storms. Eastern Pacific storms and hurricanes primarily stay out to sea and seldom affect the USA, although some storms do hit the west coast of Mexico.Two Eastern Pacific storms have formed this year: Tropical Storm Aletta, which spun harmlessly out to sea, and Hurricane Bud, which could affect the west coast of Mexico by this weekend.For more information about reprints & permissions, visit our FAQ's. To report corrections and clarifications, contact Standards Editor Brent Jones. For publication consideration in the newspaper, send comments to letters@usatoday.com. Include name, phone number, city and state for verification. To view our corrections, go to corrections.usatoday.com. 