By TAMARA LUSH
ST. PETERSBURG, Fla. - The tough task of guessing what hurricane season will look like could be even more difficult this year for forecasters, who won't be able to rely on the relatively predictable forces known as El Nino and La Nina.
So far, the National Hurricane Center in Miami is predicting that the season that begins on Wednesday will be busier than normal, with as many as 18 named tropical storms, three to six of them major hurricanes.
El Nino and La Nina - warming and cooling trends in the ocean that can either rev up hurricanes or suppress them - are expected to be essentially neutral, complicating any predictions. The last time temperatures were neutral was 2005, when hurricanes Katrina and Rita hammered the Gulf Coast with lethal results.
"With a strong La Nina or El Nino year, the forecast is much easier," said Dan Kottlowski, senior meteorologist at AccuWeather.com. "Since we don't have a strong signal toward El Nino or La Nina, there's somewhat more uncertainty in trying to determine how strong this season will be."
The La Nina effect is a cooling of Pacific Ocean waters near the equator. It decreases wind shear in the Atlantic and can give storms extra giddyap as they form. It has been linked to above-average hurricane seasons in the Atlantic. But it appears to be weakening.
The opposite phenomenon, El Nino, warms Pacific waters, increases wind shear and can blow storms apart. But El Nino isn't happening this season.
La Nina helped make last year the third-most active hurricane season on record, said meteorologist Jeff Masters, who writes a popular weather blog. Last year, there were 19 named storms, 12 of which became hurricanes, including Earl, which sideswiped North Carolina just before Labor Day weekend and was the first hurricane to threaten New England since 1991.
The seasonal average is 11 named storms, including six hurricanes, two them major.
Meteorologists say La Nina also contributed to this past winter's barrage of blizzards in the northern United States, heavy summer flooding in Australia and recent tornadoes in the Southeastern U.S. But those events are no indication of what hurricane season might be like.
Even though La Nina's cooling effect is expected to end by June or July, the federal Climate Prediction Center says it could continue to affect weather for months.
To be sure, there were other important factors that caused last year's tropical storms to form and strengthen: record warm Atlantic waters, low barometric pressure in the Caribbean Sea and favorable winds coming off Africa. Forecasters also looked at something called the "multi decadal signal," or weather patterns that tend to last several decades. Since 1995, the Atlantic basin has been in a pattern of high activity.
Meteorologists use all of these patterns, tools and data to predict the storm season, which runs through Nov. 30.
The National Hurricane Center released its seasonal hurricane forecast May 19, while another prominent group of forecasters from the University of Colorado has already predicted that 2011 will have 16 named storms, nine hurricanes and five major hurricanes.
The hurricanes could inflict more misery on places like Louisiana and Alabama that have been plagued by flooding and deadly twisters this spring.
Ultimately, experts say, people should take note of the seasonal forecasts but not rely on them. Having an emergency kit and an evacuation plan is important even if the season isn't expected to be active.
Said Kottlowski: "You prepare for the worst, regardless of what the forecast is."
Marshall Gaines, 75, recently moved from Maine to a barrier island on the Gulf of Mexico near St. Petersburg. He visited city hall and was given him a brochure on what to do to prepare for a hurricane - none of which he has done yet.
He said that his top hurricane-season priorities are stockpiling water and batteries, obtaining renter's insurance and determining which shelters or hotels take pets in case he and his two dogs must evacuate.
"I'm not too worried, I guess," he said. "I'm planning on getting the basics. Certainly, hurricanes are a possibility."
Tamara Lush can be reached at http://twitter.com/TamaraLush