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 Oceanographic and Atmospheric Administration’s Climate Prediction Center 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.
The seasonal average is 11 named storms, including six hurricanes, two them major.

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 Climate Prediction 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.