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ShareScienceDaily (Apr. 3, 2007) — The U.S. Atlantic basin will likely experience a very active hurricane season, the Colorado State University forecast team announced today, increasing its earlier prediction for the 2007 hurricane season.
The team's forecast now anticipates 17 named storms forming in the Atlantic basin between June 1 and Nov. 30. Nine of the 17 storms are predicted to become hurricanes, and of those nine, five are expected to develop into intense or major hurricanes (Saffir/Simpson category 3-4-5) with sustained winds of 111 mph or greater.
No hurricanes made landfall along the U.S. coastline in 2006. The 2006 season witnessed a total of 10 named storms, 5 hurricanes and two major hurricanes. The 2005 season, considered unusual by the Colorado State forecast team, witnessed 27 named storms, 15 hurricanes and seven intense hurricanes. Long-term averages are 9.6 named storms, 5.9 hurricanes and 2.3 intense hurricanes per year.
"We are calling for a very active hurricane season this year, but not as active as the 2004 and 2005 seasons," said Phil Klotzbach of the Colorado State hurricane forecast team. "Based on our latest forecast, the probability of a major hurricane making landfall along the U.S. coastline is 74 percent compared with the last-century average of 52 percent.
"In December and January, we had a weak to moderate El Nino event in the tropical Pacific Ocean. When you have El Nino conditions during the hurricane season, it increases vertical wind shear across the tropical Atlantic and typically results in a weaker tropical cyclone season," Klotzbach said. "However, we've seen El Nino conditions dissipate quite rapidly late this winter, so we do not think that's going to be an inhibiting factor this year. Also, we have warm Atlantic sea surface temperatures this year which we've seen just about every year since 1995."
The hurricane forecast team predicts tropical cyclone activity in 2007 will be 185 percent of the average season. By comparison, 2005 witnessed tropical cyclone activity that was about 275 percent of the average season.
The hurricane forecast team reiterated its probabilities for a major hurricane making landfall on U.S. soil:
- A 74 percent chance that at least one major hurricane will make landfall on the U.S. coastline in 2007 (the long-term average probability is 52 percent).
- A 50 percent chance that a major hurricane will make landfall on the U.S. East Coast, including the Florida Peninsula (the long-term average is 31 percent)
- A 49 percent chance that a major hurricane will make landfall on the Gulf Coast from the Florida Panhandle west to Brownsville (the long-term average is 30 percent).
The team also predicted above-average major hurricane landfall risk in the Caribbean.
"We were quite fortunate last year in that we had no hurricane landfalls," Klotzbach said. "The 2006 season was only the 12th year since 1945 that the United States witnessed no hurricane landfalls. Since then, we have had only two consecutive-year periods where there were no hurricane landfalls - 1981-1982 and 2000-2001."
The Colorado State hurricane forecast team has cautioned against reading too much into the hurricane seasons of 2004 and 2005 when Florida and the Gulf Coast were ravaged by four landfalling hurricanes each year. Hurricanes Charley, Frances, Ivan and Jeanne caused devastating damage in 2004 followed by Dennis, Katrina, Rita and Wilma in 2005.
"The activity of these two years was unusual, but within the natural bounds of hurricane variation," said William Gray, who began forecasting hurricane seasons at Colorado State 24 years ago. "Following the two very active seasons of 2004 and 2005, 2006 experienced slightly below-average activity with no landfalling hurricanes.
"We've had an upturn of major storms since 1995," Gray said. "We think this upturn of major storms will continue for another 15 or 20 years."
Probabilities of tropical storm-force, hurricane-force and intense hurricane-force winds occurring at specific locations along the U.S. East and Gulf Coasts within a variety of time periods are listed on the forecast team's Landfall Probability Web site. The site provides U.S. landfall probabilities for 11 regions, 55 sub-regions and 205 individual counties along the U.S. coastline from Brownsville, Texas, to Eastport, Maine. The Web site, available to the public at http://www.e-transit.org/hurricane, is the first publicly accessible Internet tool that adjusts landfall probabilities for regions, sub-regions and counties based on the current climate and its projected effects on the upcoming hurricane season. Klotzbach and Gray update the site regularly with assistance from the GeoGraphics Laboratory at Bridgewater State College in Massachusetts.
The hurricane team's forecasts are based on the premise that global oceanic and atmospheric conditions - such as El Niño, sea surface temperatures and sea level pressures - that preceded active or inactive hurricane seasons in the past provide meaningful information about similar trends in future seasons.
For 2007, Gray and the hurricane forecast team expect continued warm tropical and north Atlantic sea-surface temperatures, prevalent in most years since 1995, as well as neutral or weak La Nina conditions - a recipe for greatly enhanced Atlantic basin hurricane activity. These factors are similar to conditions that occurred during the 1952, 1964, 1966, 1995 and 2003 seasons. The average of these five seasons had well above-average activity, and Klotzbach and Gray predict the 2007 season will have activity in line with the average of these five years.
Gray does not attribute changes in recent and projected Atlantic hurricane activity to human-induced global warming.
"Although global surface temperatures have increased over the last century and over the last 30 years, there is no reliable data available to indicate increased hurricane frequency or intensity in any of the globe's seven tropical cyclone basins, except for the Atlantic over the past 12 years," Gray said. "Meteorologists who study tropical cyclones have no valid physical theory as to why hurricane frequency or intensity would necessarily be altered significantly by small amounts of global mean temperature change."
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