It's Official: There's A La Niña

TAMPA Florida can brace for half the normal rainfall through February and temperatures up to 5 degrees higher as a La Niña settles into the Pacific Ocean.

Reduced rainfall and higher temperatures are not pleasant prospects for West Central Florida and the Panhandle, two areas of the state already facing drought conditions.

On Tuesday, the Climate Prediction Center said cooling water in the Pacific Ocean has become a La Niña that should last through early next year.

Water from South America west to the international date line has been cooling for months, creating La Niña conditions. Temperatures range from about 3 degrees below normal near South America's coast to about 1 degree below normal near the date line.

A La Niña typically reduces rainfall in Florida's driest months, which run from fall through the spring.

"It means a very warm and dry winter. That's not good for parts of Florida. It's highly likely drought conditions will continue or get worse," said David Zierden, state climatologist.

A La Niña can cut rainfall in half from November through February, Zierden said.

Around Tampa Bay, the normal 8.7 inches of rain that come in those four months would be reduced to 4.4.

With rainfall already below normal over nearly 80 percent of the state, any reduction before the even drier spring begins could be trouble.

"It's going to set up some problems in the traditional dry season this spring," Zierden said.

However, it is impossible to say absolutely what effect the La Niña will have in the state.

"There's a small probability it will go against the grain, but only a small one," Zierden said.

Last winter, an El Niño, warmer than normal water in the Pacific, was expected to bring more rain to the state. Instead, the storms and rain passed north of Florida.

The La Niña dries and warms Florida by altering the rivers of air moving west to east across the country called the jet streams.

The southern branch of the streams comes across the United States near the Pacific Northwest and brings storms to that region of the country.

That change helps pin the northern branch called the polar jet stream farther north, trapping cold fronts trying to migrate south and preventing most from reaching Florida.

Once the summer afternoon thunderstorms diminish, most of Florida's rain in the fall and winter comes from cold fronts. During a La Niña, fewer reach the state.

"We just get the tail end of the fronts that are typically drier," Zierden said.

A La Niña also can create conditions more favorable for hurricanes by reducing winds blowing from the west that disrupt storms as they try to form. The expected formation of the La Niña is one reason forecasters predict 2007 will be an active hurricane season.

During a La Niña, hurricane activity in the eastern Pacific drops while increasing in the Atlantic. Though most did not last long, eight tropical storms formed in September, only two fewer than the number of storms that form during an entire average season.

The government declares a La Niña when water from South America to the date line is about 1 degree or more below normal for five consecutive months.

The climate center predicts this La Niña will be weak or moderate, though Zierden said the current La Nina has been gaining strength and should be at least a moderate event or possibly a strong La Niña.

Either would have the same effect on Florida.