Portable Planet

Colorado State University hurricane forecasters today maintained an earlier forecast that called for a very active 2007 season with a 74 percent chance of a major hurricane making landfall on the U.S. coastline.

The hurricane forecast team is calling for 17 named storms forming in the Atlantic basin between June 1 and Nov. 30. Nine of the storms are expected to become hurricanes with five becoming intense hurricanes (Saffir/Simpson category 3-4-5) with sustained winds of 111 mph or greater.

The entire report is available on the Web here.

No hurricanes made landfall along the U.S. coastline in 2006. The 2006 season witnessed a total of 10 named storms, five hurricanes and two major hurricanes. Long-term averages are 9.6 named storms, 5.9 hurricanes and 2.3 intense hurricanes per year.

“We expect an above-average hurricane season with ENSO conditions on the cool side, which will help increase the likelihood of major storm activity in the Atlantic,” said Phil Klotzbach of the Colorado State hurricane forecast team and the lead author of the forecast. “El Nino conditions during the summer and fall - similar to those that developed in 2006 - tend to decrease Atlantic hurricane activity by increasing vertical wind shear across the area where Atlantic tropical cyclones develop.”

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.

The 2006 season was only the 12th year since 1945 that the United States experienced no hurricane landfalls. Since then, there have been only two consecutive-year periods where there were no hurricane landfalls: 1981-1982 and 2000-2001.

“There were a lot of challenges in 2006 that we didn’t expect such as a late-developing El Nino, which causes increased vertical wind shear and results in less tropical cyclone activity,” said William Gray, who began forecasting hurricane seasons at Colorado State 24 years ago.

For 2007, Klotzbach and Gray 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, 1954, 1964, 1966, 1995 and 2003 seasons. The average of these six seasons had well above-average activity, and Klotzbach and Gray predict the 2007 season will have activity in line with the average of these six years.

“We are in a new era for storms that is part of a natural cycle,” Gray said. “We’ve had an upturn of major storms in the Atlantic since 1995. This active cycle is expected to continue for another decade or two at which time we should enter a quieter Atlantic major hurricane period like we experienced during the quarter century periods of 1970-1994 and 1901-1925. These changes in storm activity are not caused by human-induced global warming but by natural forces.”

The Colorado State hurricane forecast team has said the hurricane seasons of 2004 and 2005 were anomalies: 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.

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 Nino, 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.

The team will issue seasonal updates of its 2007 Atlantic basin hurricane activity forecast on Aug. 3, Sept. 4 and Oct. 2. The August, September and October forecasts will include separate forecasts for each of those months.

Source: Colorado State University