Note to Editors: The complete hurricane forecast and a detailed description of forecast factors are available on the Web at www.colostate.edu or at http://hurricane.atmos.colostate.edu.
Heading into the fifth month of what has already been one of the most active hurricane seasons on record, William Gray and Philip Klotzbach of the Colorado State University hurricane forecast team are calling for above-average storm activity to continue through October. The team today issued a monthly prediction for October and an updated seasonal forecast. Gray and Klotzbach also continue to call for an above-average likelihood of landfalling hurricanes.
"Unfortunately, the very active season we have seen to this point is not yet over. We project that October will continue the trend of above-average activity that we have witnessed in the preceding four months of the hurricane season," said Gray. "This year is already the most destructive hurricane season on record. We expect that by the time the 2005 hurricane season is over, we will witness seasonal tropical cyclone activity at or very near record levels."
As detailed in a report issued this morning, the team’s October-only forecast calls for three named storms, two hurricanes and one major hurricane – nearly double the long-term average tropical storm activity for October.
Today’s October-only forecast follows the team’s successful August-only and September-only forecasts. In August, Gray and Klotzbach called for five named storms, three hurricanes and one major hurricane; the month recorded five named storms, two hurricanes and one major hurricane. The team’s September-only forecast predicted five named storms, four hurricanes and two major hurricanes; September witnessed five named storms, five hurricanes and two major hurricanes.
"In addition to our seasonal forecasts, we have recently created new schemes to forecast August-only, September-only and October-only Atlantic basin hurricane activity to be released at the beginning of each respective month," said Klotzbach. "These forecasts have developed and become quite successful."
As of the end of September, 2005 has already witnessed 17 named storms, nine hurricanes and five major hurricanes. The long-term (1950-2000) average for an entire season is 9.6 named storms, 5.9 hurricanes and 2.3 intense hurricanes per year. This year, three major hurricanes have made United States landfall (Dennis, Katrina and Rita) and Ophelia struck the North Carolina coastline (although the eye passed just offshore) as a Category 1 storm. Hurricane Katrina became the most destructive storm on record after coming ashore in southeast Louisiana as a Category 4 storm and devastating parts of the Louisiana, Mississippi and Alabama coastline.
The team cites several factors that are causing the very active Atlantic season this year, including: warmer than average Atlantic sea surface temperatures; lower than normal Atlantic sea level pressures; lower than average vertical wind shear; and moister lower and middle atmosphere conditions.
"In addition," according to Klotzbach, "as was the case during the 2004 hurricane season, there has tended to be an anomalous ridge along the East Coast of the United States during the 2005 hurricane season that has steered storms westward across the United States coastline. From 1995-2003, there tended to be an anomalous trough along the East Coast which allowed most storms to recurve before making United States landfall."
Another significant focus of the Colorado State team’s research involves efforts to develop forecasts of the probability of hurricane landfall along the U.S. coastline. The team has recently developed a methodology for calculating the probability of hurricane landfall along the entire U.S. coastline specifically for the month of October.
According to today’s report, for the month of October, there is a 49 percent chance of a named storm hitting somewhere along the U.S. coastline (long-period average is 29 percent). The probability of a hurricane making landfall in October is 21 percent (long-term average is 15 percent), and the likelihood of an intense hurricane crossing the coastline is 15 percent (long-term is 6 percent).
The storm seasons spanning 1995-2004 comprised the most active 10 consecutive hurricane years on record, and the 2005 season has continued this active trend. The forecasters believe that the United States is in a new, multi-decadal era for increased storm activity.
"Our research indicates that the United States has entered an era of increased major hurricane activity which has been reflected in high activity during eight of the last 10 years," Klotzbach said. "We expect this active tropical cyclone era to likely span the next two or three decades."
The Colorado State forecast team does not attribute changes in recent or projected Atlantic basin hurricane activity to human-induced global warming. These changes in hurricane activity are viewed as resulting from long-period natural climate alterations that historical and paleo-climate records show to have occurred many times in the past.
"Many individuals have queried whether the unprecedented landfall of four destructive hurricanes in a seven-week period during August-September 2004 and the landfall of three more major hurricanes during July-September 2005 are related in any way to human-induced climate changes. There is little evidence that this is the case," said Gray. "If global warming were the cause of the increase in United States hurricane landfalls in 2004 and 2005 and the overall increase in Atlantic basin major hurricane activity of the last past eleven years (1995-2005), we would expect to see an increase in tropical cyclone activity in the other storm basins as well, such as in the West Pacific, East Pacific and Indian Ocean basins. This has not occurred. It is unusual but not unprecedented that two major hurricanes come ashore near Galveston and New Orleans, respectively. In 1915, a Category 4 hurricane passed just west of Galveston and another Category 4 hurricane tracked over New Orleans."
When tropical cyclones worldwide are summed, there has actually been a slight decrease since 1995. In addition, Gray added, it has been well-documented that the measured global warming during the 25-year period of 1970-1994 was accompanied by a downturn in Atlantic basin major hurricane activity over what was experienced during the 1930s through the 1960s.
Gray and Klotzbach continuously work to improve their forecast methodologies based on a variety of climate-related global and regional predictors. For a detailed description of the many detailed forecast factors, visit the Web at http://hurricane.atmos.colostate.edu.
The team will be issuing their forecast verification for the 2005 Atlantic basin hurricane season on Nov. 19. The first forecast for the 2006 hurricane season will be issued on Dec. 6. All forecasts are available on the team’s Web site at: http://hurricane.atmos.colostate.edu/Forecasts.