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Add La Nina to storm worry

By CASEY CORA
Published February 28, 2007

La Nina appears to be on its way, unwelcome news for the 2007 hurricane season.

Forecasters with the National Oceanographic and Atmospheric Administration's Climate Prediction Center announced Tuesday that a weak El Nino is over and conditions are favorable for a potentially volatile La Nina.

Officials said the formation of La Nina isn't concrete, but satellite and buoy observations indicate conditions are in place for it to form.

"Generally speaking, La Nina tends to be more favorable for hurricane development," said Russell Henes, a forecaster with the National Weather Service.

During La Nina, more storms form in the deep tropics from systems that move off Africa, according to the prediction center. The systems are more likely to become major hurricanes that threaten the United States.

La Nina is the opposite of El Nino, a warm current of water that appears every three to seven years in the eastern Pacific Ocean.

In El Nino, trade winds weaken, allowing warm surface water in the Pacific to slough back to the east. This tends to produce strong upper-level winds that can shear the tops off Atlantic Basin hurricanes.

"Any shear tends to pull the storm apart," Henes said.

That's why NOAA's forecast is particularly bad news. La Nina, which typically comes every three to five years and lasts anywhere from nine to 12 months, inhibits the formation of those storm-disrupting winds.

La Nina tends to change in late spring, which makes it difficult this early in the year to assess its impact on the hurricane season, which begins June 1.

But if conditions persist, it could be a long season. La Nina plagued the 1999 season, which produced 12 named storms, including eight hurricanes and five Category 4 hurricanes, a record at the time. It also helped make 1995 one of the most active on record.

Fast Facts:

What is La Nina?

Trade winds from the east push warm surface waters to the western Pacific, which affects tropical rainfall and wind patterns. This scenario tends to inhibit the formation of strong upper-level winds that can shear the tops off hurricanes developing in the Atlantic Basin and make them less threatening.

[Last modified February 28, 2007, 00:50:21]