The growing strength of the current La Nina event will cause above-average activity during the final two months of the 2007 Atlantic hurricane season and could extend the season as well, according to weather forecasters at Colorado State University (CSU).

La Nina events -- the cooling of ocean surface temperatures off the western coast of South America -- have been found to disrupt normal weather patterns in the United States.

"Typically, the end of the Atlantic basin hurricane season is governed by rising values of vertical wind shear," said CSU forecaster William Gray. "We expect La Nina conditions through this fall. La Nina conditions tend to reduce levels of vertical wind shear in the tropical Atlantic, and therefore the end of the Atlantic basin hurricane season will likely be extended this year."

CSU's Department of Atmospheric Science forecast team said a total of four named storms will develop during October and November, a slight downgrade from an earlier CSU forecast. The team previously predicted that five named storms, including two hurricanes -- one of them a major hurricane (Category 3 or greater, with sustained winds of at least 111 mph) -- would form during the Atlantic hurricane season's final two months. CSU forecasters are still calling for two hurricanes, one of them a major hurricane, during October and November.

The Atlantic hurricane season runs from June 1 to Nov. 30. A total of 13 named storms, four hurricanes and two major hurricanes developed in the Atlantic through the end of September. Dean, a long-lived Category 5 hurricane, slammed into Mexico's Yucatan Peninsula Aug. 21. Another Category 5 hurricane, Felix, struck portions of Central America Sept. 4, the same day that Henriette, a Category 1 Pacific hurricane, hit Baja California, Mexico. Humberto, a Category 1 hurricane, made landfall Sept. 13 on the Texas coast southeast of Houston. Another Category 1 hurricane, Lorenzo, made landfall on central Mexico's east coast Sept. 28.

"August had somewhat above-average activity, about 130% of average, while September had about average activity, about 92% of average," said Phil Klotzbach, lead author of the CSU hurricane forecast. "We expect October-November to be very active."

Eight named storms formed during September, tying a record for most named storm formations during the month, but most of the storms were short-lived and not particularly intense. Sea surface temperature and vertical wind shear values were near their long-term averages during the month, while sea level pressure values remained somewhat below average. September is usually the most active month of the hurricane season.

If CSU's October-November forecast comes to pass, there will be a total of 17 named storms, six hurricanes and three intense hurricanes during the entire 2007 Atlantic hurricane season (CSU forecasters said the National Hurricane Center will likely upgrade Karen, a tropical storm that remained at sea from Sept. 24 to Sept. 29, to hurricane status during post-season analysis, bringing the total number of Atlantic hurricanes to seven). WSI Corp. recently said it expects a total of 14 named storms, six hurricanes and four intense hurricanes this year (see NGI, Oct. 1). WeatherBug, which owns 8,000 weather-monitoring stations in the United States, has also maintained its forecast for an above-average storm season in the Atlantic Basin.

The CSU forecast team said it will issue its forecast verification on Nov. 27 and an early 2008 season forecast in early December.

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