ST. PETERSBURG, Fla. -- One major factor in whether the Atlantic Ocean will experience an active 2011 hurricane season is fluctuating thousands of miles away in the Pacific.
The La Nina weather phenomenon, which is linked to above average hurricane seasons in the Atlantic, appears to be weakening and it is unclear as the season begins June 1 whether it will exacerbate or have no effect on this year's storms.
La Nina helped make the 2010 hurricane season the third-most active hurricane season on record, said meteorologist Jeff Masters, who writes a popular weather blog. There were 19 named storms last year and of those, 12 became hurricanes. Meteorologists say it also helped cause this past winter's barrage of blizzards in the northern United States, heavy summer flooding in Australia and recent tornadoes in the Southern U.S.
La Nina is expected to be gone by June or July, but the federal Climate Prediction Center says that it could continue to affect weather for months.
La Nina is an unusual cooling of the Pacific Ocean near the equator. During years that La Nina is in effect, there's a decrease in the vertical wind shear thousands of feet over the Caribbean Sea and tropical Atlantic -- that gives hurricanes a chance to form and strengthen before they are blown apart. It's the opposite weather pattern of El Nino, which warms Pacific waters near the equator and increases Atlantic basin wind shear -- that can blow storms apart as they form.
To be sure, there were other important factors that caused last year's storms to form and strengthen: record warm Atlantic waters, barometric pressure in the Caribbean and favorable winds coming off Africa. Forecasters also looked at something called the "multi decadal signal," which refers to weather patterns that tend to last several decades at a time -- since 1995, the Atlantic basin has been in a pattern of high activity.
"Last year there was La Nina, on top of already high conditions," said Gerry Bell, the head forecaster on the National Hurricane Center team that makes seasonal predictions.
Meteorologists use all of these patterns, tools and data sets to help predict the upcoming storm season, which runs until Nov. 30. They also are aided by two models -- the GFS and European -- when making forecasts.
"Those models aren't going to care what happened in the past, they're only going to care about what's going on now," Masters said.
But here's the not-so-good news: so-called "neutral" years -- or years in which there is neither a La Nina or an El Nino -- are often more difficult for meteorologists to forecast.
"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."
One of the most notable "neutral" seasons was in 2005, when Hurricanes Katrina, Rita and Wilma made landfall along the Gulf Coast.
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 whether the hurricane season is expected to be active or not.
Said Kottlowski: "You prepare for the worst, regardless of what the forecast is."