TAMPA - The El Nino weather pattern that helped stymie hurricane development last year is fading and could be replaced by conditions that favor hurricane formation.

Weather scientists said Tuesday that a vast stretch of warm water in the Pacific Ocean that forms El Nino is cooling rapidly and may switch quickly to cooler water, called La Nina.

Although the National Climate Prediction Center cannot say positively a La Nina will form, preliminary forecasts indicate Pacific water will cool below normal temperatures.

"Our models have become pretty reliable," said retired Navy Vice Adm. Conrad C. Lautenbacher, administrator of the National Oceanic and Atmospheric Administration.

El Nino causes a shift in the jet stream blowing from the west miles above Earth, and that shift disrupts hurricanes as they develop. Its opposite, La Nina, weakens that jet stream, creating conditions that make it easier for storms to form.

There is a "positive correlation" between a La Nina in the Pacific and an active Atlantic hurricane season, Lautenbacher said.

Forecasters will know more about whether the La Nina forms by the time NOAA makes its hurricane season predictions in May, a month before hurricane season begins.

"It's a factor that will play into our forecasting," Lautenbacher said.

An El Nino formed quickly last summer and was a major factor in limiting the number of tropical storms and hurricanes to 10.

That made last year the quietest hurricane season since 2002, another El Nino summer, when there were 12 tropical storms and hurricanes.

While El Nino and La Nina play a part in the number of storms, they do not affect their tracks, which are guided more by areas of high and low pressure.

In addition to its effects on hurricanes, El Nino usually brings more rain to Florida during the winter and spring.

But as a drought buster, this El Nino was a failure. Most of the storms and rain passed north of the Florida Peninsula.

"No large portion of the state had enough rain," state meteorologist Ben Nelson said.

Though it didn't bring enough rain to relieve the drought, the El Nino did help spawn tornadoes that struck on Christmas and Feb. 2. Strong tornadoes are another effect of El Nino during the winter.

"We didn't get the rain, but we got the severe weather," Nelson said.

If La Nina forms in the spring, it could mean even less rainfall for Florida during the normally dry months.

"We're dominated by high pressure, which is hot, dry air," Nelson said.

The last time conditions switched quickly from an El Nino to a La Nina was in 1998, when Florida went from record rainfall to a drought that lasted three years.

Under normal conditions, March is a relatively wet month. La Nina could lead into a hot, dry spring and make drought conditions worse.

"Next month will be very critical," Nelson said.

WEATHER MAKERS

El Nino

Water temperature in the tropical Pacific Ocean rises several degrees above normal and causes a shift in the jet stream. That brings rainy winter storms to Florida but helps block severe cold fronts coming from Canada. During hurricane season, the jet stream makes it difficult for storms to develop.

La Nina

Water in the tropical Pacific cools several degrees below normal, weakening the jet stream and allowing warm dry air to settle over Florida, reducing rainfall. In hurricane season, the weakened jet stream makes conditions favorable for storms to form.

Reporter Neil Johnson can be reached at (352) 544-5214 or njohnson@tampatrib.com.