Note to Editors: October and updated seasonal forecast total charts, as well as a detailed description of forecast factors, press releases and access to downloadable broadcast-quality audio clips and video clips, are available on the Web at www.colostate.edu.
Following two months of relentless Atlantic basin tropical storm activity that witnessed four hurricanes – three of which were intense hurricanes – and one tropical storm making landfall in Florida, William Gray and Philip Klotzbach of the Colorado State University hurricane forecast team expect a much quieter finish to the storm season. The team today issued a monthly prediction for October and an updated seasonal forecast.
The five named storms that have dramatically impacted Florida in August and September are unparalleled in terms of historical records going back 130 years, according to Gray.
"It is not the number of hurricanes in the Atlantic basin that has been so unusual, but rather the rare combination of high hurricane activity and very favorable surrounding hurricane steering conditions that drove so many storms from the deep tropics across Florida in such a short time period," said Gray. "This year has been a once-in-a-lifetime kind of year. Although Floridians should always be prepared for landfalling hurricanes, they should not expect what we have experienced this year to become the norm for future years."
In a 2004 storm season that has already seen 220 percent of the activity of the average season and witnessed 12 named storms, seven hurricanes and six intense hurricanes, the team’s October-only forecast released today calls for only three additional named storms, two hurricanes and no intense hurricanes, slightly above the long-term average for October. Gray and Klotzbach anticipate little activity in November.
Gray and his team also provided the probability of landfalling tropical cyclones during October. The probability of at least one named storm making landfall along the U.S. coastline this month is 33 percent (29 percent is the long-term average). The probability of at least one hurricane making landfall along the U.S. coastline is 17 percent (15 percent is the average). The probability of at least one major hurricane (Saffir/Simpson category 3-4-5) crossing the U.S. coastline in October is 8 percent (6 percent is the average).
The team additionally released its updated seasonal forecast today that calls for a total of 15 named storms, nine hurricanes and six intense hurricanes, which would be approximately 240 percent of the average storm season. This total is slightly adjusted from the team’s early September forecast of 16 named storms, eight hurricanes and five intense hurricanes. The long-term average is 9.6 named storms, 5.9 hurricanes and 2.3 intense hurricanes per year.
"Although we forecast an above-average 2004 forecast, there is no way we or anyone else could have foreseen the amount of Atlantic basin hurricane activity that occurred in August and September and the huge impact this would have on southeastern United States residents," said Gray. "We have studied more than 100 years of storm data, and this year did not behave like any other year we have studied."
According to the report issued today, the 2004 season has been distinguished by persistent tropical cyclone activity with at least one named storm in existence every day from August 25 to the present. Additionally, September witnessed two very long-lived intense hurricanes: Frances with 7.25 intense hurricane days and Ivan with 10 intense hurricane days. Ivan’s 10 intense hurricane days are the most for any single tropical cyclone since 1900. September additionally had more intense hurricane days than any September since 1950.
According to the forecast team, four factors all came together in the tropical central Atlantic that made the past two months so active with storm activity and landfalling hurricanes:
– very warm sea surface temperatures,
– strong low-level convergence,
– high low-level horizontal wind shear, and
– low vertical wind shear.
"It was very unusual to have these four required conditions come together so perfectly at one time, especially in a year where we also measured high sea level pressures in the tropical Atlantic and warm sea surface temperatures in the tropical Pacific – generally both strong indicators of reduced storm activity in the Atlantic basin," Klotzbach said.
Today’s report also highlights how this year’s hurricanes had long westward tracks that were not typical of the tracks of most hurricanes throughout the past decade. Throughout the active storm seasons of 1995-2003, the wind patterns created a trough over the eastern North America coast and the western Atlantic that in effect deflected westward moving hurricanes to the north before crossing the U.S. coastline. In contrast, this year’s wind patterns created a ridge over the eastern North American continent and western Atlantic that did not curve hurricanes to the north until the storms reached the longitude of the southeast U.S.; namely over Florida.
As a result of these conditions, only three of the 32 major hurricanes in the Atlantic basin crossed the U.S. shoreline from 1995-2003. In 2004, three of the six major hurricanes hit the United States.
"Florida has obviously been unlucky in 2004, but extremely fortunate overall in recent years with only three major storms making landfall from 1995-2003 and only one intense storm hitting the highly populated Florida Peninsula when Andrew hit in 1992," Gray said. "For many years, we have been discussing that it was inevitable that climatology would eventually reassert itself. There was no way, however, of knowing that the law of averages would try to catch up to its deficit so rapidly in one year."
The team also points out that this season, which has been devastating to many with insured damage estimates currently at about $22 billion and total losses easily doubling or tripling that number, could have been much worse if the storms had hit the highly populated areas of three major Florida coastal populations: Tarpon Springs to Sarasota, West Palm Beach to South Miami and Daytona Beach to Melbourne (and inland to Orlando).
Economic loss many times greater would have occurred if the center of any one of the landfalling hurricanes had come into even one of these concentrated population areas in Florida.
The Colorado State forecast team does not attribute changes in recent and projected Atlantic hurricane activity to human-induced global warming.
Gray and his team 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://typhoon.atmos.colostate.edu.
Gray and his forecast team will issue a seasonal wrap-up and verification report in late November.
In addition to Gray and Klotzbach, other team members include William Thorson, Barbara Brumit, Amie Hedstrom and others.