Season Review

'Once-in-a-lifetime' Season Explained
CSU's fprecast team says many factors combined to steer major hurricanes toward the U.S. coast

by Howard Shingle

It is not the number of hurricanes in the Atlantic basin that was so unusual last year, but rather the rare combination of high major hurricane formation and very favorable hurricane steering conditions that drove so many storms from the deep tropics across the Caribbean and into Florida in such a concentrated period of six weeks,” said William Gray, who heads the Colorado State University (CSU) hurricane forecast team.

In a report summarizing tropical cyclone activity in the Atlantic basin during the 2004 hurricane season Gray and Philip Klotzbach noted that the team believes several major factors all came together and played a role in making the 2004 hurricane season very active with so many intense landfalling hurricanes.

First, Atlantic sea-surface temperatures were anomalously warm throughout most of the basin—a factor shown to enhance tropical cyclone formation.

Additionally, strong low-level convergence, high low-level horizontal wind shear and low vertical wind shear combined perfectly to form so many major hurricanes. Associated with these favorable formation conditions were middle latitude wind patterns in the western Atlantic that became arranged in a way to drive these major hurricanes from the deep tropics across the southeast corner of the United States.

“It was unusual to have these conditions come together so perfectly at one time, especially in a year where we also measured high Atlantic basin sea-level pressures and warm sea-surface temperatures in the tropical Pacific—generally indicators of reduced storm activity in the Atlantic basin,” Klotzbach said.

EIGHT CYCLONES

The report highlights how last season’s hurricanes had long westward tracks that were not typical of the tracks of most major hurricanes throughout the past decade. Throughout the active storm seasons of 1995 to 2003, the wind patterns tended to create a trough over the eastern coast of North America. These trough conditions deflected westward moving hurricanes to the north before they were able to reach the U.S. coastline.

In contrast, last year’s wind patterns created a ridge over the eastern North American continent and western Atlantic. This prevented recurvature to the north until the storms reached the longitudes of the southeast United States, namely Florida.

As a result, from 1995 to 2003, only three of the 32 major Atlantic basin hurricanes hit the United States; in 2004, four of the six major hurricanes made U.S. landfall.

It was “a once-in-a-lifetime kind of year,” said Gray. “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.”

Florida was severely affected by four hurricanes in 2004, three of which were major storms (Category 3, 4, 5). Hurricanes Frances (Category 2), Jeanne (Category 3), Ivan (Category 3) and Charley (Category 4) caused devastating damage in Florida. Although not making landfall, Hurricane Alex caused damage along the Outer Banks of North Carolina. In addition, tropical storms Bonnie, Gaston, Hermine, Ivan and Matthew made landfall along the United States coastline. Gray pointed out that the last time eight different tropical cyclones impacted the United States coastline was 1916.

Gray’s team estimates that the four hurricane landfalls have caused over $50 billion in total damage, more than that of Hurricane Andrew in 1992.

ON TARGET, AGAIN

For the sixth consecutive year, the CSU forecast team was on track with their seasonal hurricane forecast. The team’s forecast numbers were on target with the number of named storms and hurricanes that occurred, but fell short in predicting the large amount of intense hurricanes that formed and concentrated in and around Florida in August and September.

The team made its long-range seasonal forecast, which called for an active hurricane season, on December 5, 2003, and then issued seasonal updates on April 2, May 28, August 6, September 3 and October 1. In December 2003, the team predicted that 13 named storms, seven hurricanes and three intense hurricanes would form in the Atlantic basin in 2004. In its May 28 update, issued just before the official start of hurricane season, the team slightly adjusted its prediction to 14 named storms, eight hurricanes and three intense hurricanes. When the 2004 hurricane season ended, the Atlantic basin had witnessed 14 named storms, eight hurricanes and six intense hurricanes.

“Overall, we consider 2004 a successful forecast year with regard to most of our forecast categories,” Gray said. "However, although we called for an above-average season, there is no way that we or anyone else could have foreseen the number of Atlantic basin major hurricanes that developed during August and September or the large impact this activity would have on Caribbean and southeastern U.S. residents. This year had characteristics unlike any other year we have studied.”

MAKING PROGRESS

William Gray and his Colorado State team have provided seasonal Atlantic basin hurricane forecasts for the past 21 years. In addition to Gray and Klotzbach, other team members include William Thorson, Barbara Brumit, Amie Hedstrom and others.

Until Gray began developing his forecast model in the early 1980s, there were no objective methods for predicting whether forthcoming hurricane seasons were likely to be active, inactive or near average.

“As the last six years indicate, we are making progress in better understanding and consequently improving seasonal prediction skill,” Gray said. “With more research, this understanding will likely continue to improve, and we hope these forecasts will continue to be of assistance to coastal populations, emergency managers, insurance providers and others concerned about Atlantic basin hurricane activity.”

Gray and his team are already working on their first seasonal forecast of the 2005 Atlantic basin hurricane season (see page 6).