(Recasts, adds details, quotes) By Jim Loney MIAMI, Oct 2 (Reuters) - The La Nina weather phenomenon in the eastern Pacific will likely extend the Atlantic hurricane season this year, with four more storms forming and two becoming hurricanes, a noted forecasting team said on Tuesday. The Colorado State University hurricane research team boosted its season forecast from 15 to 17 storms, of which six would be hurricanes. Thirteen storms have already formed this year and four of those have become hurricane. The CSU team, founded by forecasting pioneer Bill Gray and now led by researcher Phil Klotzbach, also said it expected Tropical Storm Karen to be raised to hurricane status in a post-season analysis, increasing the hurricane total to seven. The official six-month Atlantic hurricane season runs from June 1 to Nov. 30, but historically the busiest part ends in mid-October. The CSU forecasters said typically the end of the season is marked by rising wind shear, a difference in wind speeds at different altitudes that can tear apart hurricanes. But the formation of the cool-water La Nina phenomenon in the eastern Pacific could change that pattern this year. "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," the report said. While the season so far has seen more tropical storms than normal, surprisingly strong wind shear has prevented many from strengthening and made them short-lived. A month ago, the Colorado State team predicted the season would produce 15 tropical storms, of which seven would become hurricanes and four would be major hurricanes with winds over 110 mph (177 kph). Its preseason prediction, issued at the end of May, was for 17 tropical storms with nine hurricanes and five major hurricanes. The latest forecast called for one more "major" hurricane, Category 3 or higher on the five-stage Saffir-Simpson scale of hurricane intensity with sustained winds topping 110 mph (177 kph). The season has already seen two major hurricanes, Dean and Felix, both of which reached Category 5, the top rank on the five-stage Saffir-Simpson scale of hurricane intensity. Dean skirted Jamaica and then hammered Mexico in August and Felix struck Central America in early September, killing scores of people. It was the first time two Category 5 hurricanes have hit land in the Atlantic basin in a single season since record-keeping began in 1851. The long-term average for Atlantic hurricane seasons is about 10 storms, of which six become hurricanes. There were 28 named storms in 2005, a record-setting year, and 15 strengthened into hurricanes, including Katrina, which killed 1,500 on the U.S. Gulf Coast and swamped New Orleans.
