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Expect Dry And Drier

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By NEIL JOHNSON, The Tampa Tribune

Published: November 29, 2007

TAMPA - The La Nina in the Pacific Ocean should last through early next year and grow stronger by the spring, forecasters say.

That could make drought conditions in the Tampa Bay area and much of Florida worse because stronger La Ninas tend to reduce rainfall more than ones classified as weak.

During a moderate or strong La Nina, when water across a large stretch of the equatorial Pacific cools about 1 to 3 degrees below normal, most of Florida receives 3 to 4 inches less rain from December through February.

For the Tampa Bay area, those three months produce an average of about 7.3 inches of rain. If the forecast is accurate, that could be cut by more than half.

"That will aggravate a situation that's already pretty bad," said John Feldt, hydrologist in charge at the Southeast River Forecast Center.

The National Climate Prediction Center's latest La Nina forecast issued on Monday said the La Nina grew in strength during October to the threshold of a moderate La Nina and is expected to continue becoming more intense.

The difference for Florida between a weak La Nina and moderate or strong is an inch or more of rain.

A weak La Nina typically reduces rainfall 1 to 2 inches from December through February.

The possibility of reduced rainfall during Florida's dry winter and spring was one reason the Southwest Florida Water Management District decided on Monday to keep rules in place limiting lawn watering to once a week.

The management district imposed the restrictions in January and they were set to expire Friday. Instead the rules will remain in place through June and the district board said outdoor watering could be banned if conditions grow worse.

Across the 16 counties the water district covers, rainfall for the past 12 months is nearly 10 inches below normal. That includes the Green Swamp, which forms the headwaters of the Hillsborough River, Tampa's main water supply.

Though rainfall at Tampa International Airport where the National Weather Service records the area's rain is only about 1.6 inches below normal for the year, rainfall in Sarasota is 18 inches below normal.

The climate center considers West Central Florida, including the Tampa Bay area, to be in a moderate drought. Long-range forecasts predict the drought in this region will grow stronger during the next three months.

Feldt said that except for Tropical Storm Barry on June 2, Florida received little rain from tropical systems this year.

A La Nina reduces rainfall in Florida by changing the jet stream to prevent fronts from crossing the state. After summer rains end in early October, approaching cold fronts produce the state's rainfall.

"The fronts you've been having have been dry. It tends to feed on itself, and right now it's not looking good," Feldt said.

Florida's last drought lasted from 1998 until the summer of 2001, also propelled by a pair of back-to-back La Ninas. The first formed in June 1998 and lasted until July 2000. After warming to slightly above the threshold for a La Nina, water again cooled in October 2000 and lasted until March 2001.

Forecasters do not expect the current La Nina to be as strong or persistent.

But losing half the winter's normal rainfall would leave the state facing even hotter and drier spring months.

"You may have to look for summer for any significant rain," Feldt said.

Reporter Neil Johnson can be reached at (813) 259-7731 or njohnson@tampatrib.com.

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