The probability of an above average Atlantic hurricane season is even greater than three months ago, U.S. government forecasters said Thursday.
The National Oceanic and Atmospheric Administration slightly upgraded its May outlook, calling for 14 to 19 named tropical storms, up from a range of 14 to 18.
That includes the five tropical storms that have formed since the six-month hurricane season started June 1. It typically ends around Nov. 30 and the peak period for hurricanes runs from August through October.
"We expect considerable activity," said Gerry Bell, lead seasonal hurricane forecaster at NOAA's Climate Prediction Center in Washington.
"There is absolutely no reason that people should be complacent," Bell said. "Now is the time people really need to make sure they have their hurricane preparedness plans in place."
Tropical storms get named when their top winds reach 39 mph or higher. NOAA now expects seven to 10 named storms to strengthen into hurricanes with top winds of 74 mph or higher, and three to five of those hurricanes could become major storms with winds blowing 111 mph or more.
In May, forecasters called for six to 10 hurricanes this season. The seasonal average is 11 named storms, six hurricanes and two major hurricanes.
Key climate factors predicted in May continue to boost forecasters' expectations for an above-average season, Bell said.
"The atmosphere and Atlantic Ocean are primed for high hurricane activity during August through October," Bell said. "Storms through October will form more frequently and become more intense than we've seen so far this season."
"We're now entering the peak months of what's expected to be an active season," he added.
What's changed since May is that the climate factors are even stronger, allowing NOAA to forecast an 85 percent chance of an above normal season, up from 65 percent in May. An above normal season this year would make 12 in the last 17 years.
Atmospheric and marine conditions indicate a high-activity era that began in 1995 continues, and ocean temperatures are the third warmest on record, he said.
The La Nina weather phenomenon also may redevelop this fall, Bell said.
La Nina is an unusual cooling of the Pacific waters near the equator. It cuts wind shear over the Caribbean Sea and tropical Atlantic, which gives tropical storms a chance to develop and strengthen before being ripped apart.
Forecasters say La Nina helped make the 2010 season one of the busiest on record with 19 named storms, including 12 hurricanes. The opposite El Nino phenomenon, which warms Pacific waters near the equator and increases wind shear over the Atlantic, helps suppress storm development.
"The numbers in May reflected the possibility that El Nino could develop. El Nino has not developed," Bell said.
Five tropical storms have developed so far this season.
The Mexican government reported 22 deaths after Tropical Storm Arlene came ashore June 30 with heavy rains that caused flooding and mudslides. Last week, Tropical Storm Don fizzled to a tropical depression just before crossing the Texas coastline.
The last hurricane to make landfall in the United States was Ike in 2008. Though not considered a major hurricane, 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 hurricane center.
The last major hurricane to strike the U.S. was Category 3 Hurricane Wilma, which made landfall in Florida in 2005.
"We've been quite lucky in recent years, but that's no reason to be complacent," said Steve Woodward, the Federal Emergency Management Agency's deputy assistant administrator for response. "As spring and summer have taught us, with tornadoes and flooding and the heat wave, disasters can strike practically anytime and anywhere."