U.S. government forecasters announced Thursday they expect three to six major hurricanes from an above average Atlantic storm season.
No major hurricane has made a U.S. landfall in five years, but the forecasters warned U.S. coastal residents that odds are diminished that they can't expect a sixth straight year without a major landfall on either the Atlantic or Gulf coasts.
As many as 18 named tropical storms may develop during the six-month Atlantic hurricane season that begins June 1, according to forecasters at the National Ocean and Atmospheric Administration. Six to 10 of those storms could strengthen into hurricanes with top winds of at least 74 mph, the agency said. Three to six could become major hurricanes, with maximum winds of 111 mph and up.
Last year's hurricane season was one of the busiest on record with 19 named storms, including 12 hurricanes. The 2011 season was not expected to be as extreme, partly because ocean temperatures were only two degrees warmer than normal, instead of four degrees warmer as they were last year, said NOAA administrator Jane Lubchenco.
"We still expect that to support an above average hurricane season," Lubchenco said.
Also, a Pacific Ocean weather phenomenon known as "La Nina" was expected to dissipate early in the summer before the season's peak, typically from August to October, Lubchenco said.
La Nina is an unusual cooling of the Pacific waters near the equator. When its in effect, wind shear over the Caribbean Sea and tropical Atlantic decreases, meaning that tropical storms have a chance to develop and strengthen before being ripped apart.
Forecasters say La Nina helped make the 2010 season so active. The opposite El Nino phenomenon, which warms Pacific waters near the equator and increases wind shear over the Atlantic, helps suppress storm development.
Nevertheless, atmospheric and marine conditions indicating a high-activity era that began in 1995 continues and lingering La Nina impacts such as reduced wind shear are conducive to a busy storm season, said Gerry Bell, lead seasonal hurricane forecaster at NOAA's Climate Prediction Center in Washington.
"We don't think La Nina will be a player for much of the season, but that's really secondary," Bell said. "Conditions are already starting to be in place and we expect them to develop, and that's why we expect an active season to be likely."
No major hurricane has made a U.S. landfall since Category 3 Hurricane Wilma struck Florida in 2005, though Hurricane Ike caused extensive damage in September 2008 when it roared ashore in Galveston, Texas, as a strong Category 2 storm with top winds around 109 mph. After peaking as a Category 4 storm near the Turks and Caicos Islands, Ike caused $10 billion in damage in Texas, Louisiana and Arkansas, making it the third-costliest storm after Hurricanes Katrina in 2005 and Andrew in 1992, according to the National Hurricane Center.
Scientists said coastal residents can't expect their luck to hold.
"The US was lucky last year. Despite an above normal season we did not have significant damage from these storms on U.S. land. The winds that steer where storms go kept them away from our coastlines," Lubchenco said. "We cannot count on having the same luck this year."
Federal Emergency Management Agency Administrator Craig Fugate urged residents from Texas to Maine to develop disaster plans and determine whether they live in evacuation zones.
"Far too many people will not be prepared and will try to get ready in the last minutes when a hurricane is threating their community and not have enough time," Fugate said.
Forecasters name tropical storms when their top winds reach 39 mph. The first named storm of the 2011 season will be Arlene.
The seasonal average is 11 named storms, six hurricanes and two major hurricanes.
In April, Colorado State University researchers predicted 16 named storms would form this season, with five strengthening into major hurricanes.
Hurricane season ends Nov. 30.
NOAA's National Hurricane Center: http://www.hurricanes.gov