season review

Beyond Expectations
The 2005 season goes down as the most destructive on record.

The 2005 hurricane season ended up being the most active and destructive season on record, say William Gray and Philip Klotzbach of the Colorado State University (CSU) forecast team, Fort Collins, CO.
For the seventh consecutive year, the CSU forecast team was on track with its seasonal hurricane forecast as highlighted in a summary report of 2005 Atlantic tropical cyclone activity, which compares the team’s seasonal and monthly forecasts to what actually occurred.

The CSU team made its long-range seasonal forecast, which called for an above-average hurricane season, on December 3, 2004 (see HP, Show Issue 2005, page 20), and then issued five updates throughout the season. On May 31, just before the official start of hurricane season, the team called for 15 named storms, eight hurricanes and four intense hurricanes. As the 2005 hurricane season came to an end, the Atlantic basin has witnessed 23 named storms, 13 hurricanes and seven intense hurricanes.

“Overall, we consider 2005 a successful forecast year with regard to most of our forecast categories, and we are pleased with our seasonal forecasts,” Gray said. “By the start of the hurricane season, we were predicting a very active season. However, we did not anticipate that this season would break many Atlantic basin records.”

FOLLOWING THE TROUGH
The team believes several major factors all came together to play a role in making the 2005 hurricane season the most active on record. 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 in such a way to provide very favorable conditions for major hurricane development. Associated with these favorable conditions were middle latitude wind patterns in the Western Atlantic that became arranged in a way to drive these major hurricanes from the tropics across the Gulf and southeast coastlines of the United States.

The CSU report highlights how this year’s hurricanes, similar to last year’s, 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 East Coast of the United States. These trough conditions deflected westward moving hurricanes to the north before they were able to reach the U.S. coastline. In contrast, wind patterns in the past two years 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. As a result, from 1995 to 2003, only three of the 32 major Atlantic basin hurricanes hit the United States; in the last two years alone, eight major hurricanes made U.S. landfall.

KATRINA CAUSES MOST DAMAGE
The United States was affected by four major hurricanes this year: Hurricanes Dennis (Category 3), Rita (Category 3), Wilma (Category 3) and Katrina (Category 4)—making the 2005 season the most destructive season on record with over $65 billion dollars in insured damage (and probably more than $150 billion dollars in total damage).

Most of this damage was caused by Hurricane Katrina, which devastated Louisiana and Mississippi. In addition to these four major hurricanes, tropical storms Arlene, Cindy and Tammy made landfall in the southeast United States, and Hurricane Ophelia, although not technically making U.S. landfall, brushed by the coast of North Carolina with hurricane-force winds affecting the Outer Banks and causing substantial damage.

The seven different tropical cyclones that made landfall in the United States in 2005 follow the very active 2004 season when eight different tropical cyclones made United States landfall.

NEW RESEARCH, FORECASTS
There are often monthly periods within active and inactive hurricane seasons that do not conform to the overall season. To this end, a relatively new aspect of the CSU team’s climate research is the development of tropical storm activity predictions for the individual months of August, September and October, traditionally the three most active months in a hurricane season. August-only monthly forecasts have now been made for the past six seasons, and September-only forecasts have been made for the last four seasons. This is the third year the team has issued an October-only forecast.

The team’s monthly forecasts for August-only and September-only activity were quite successful, especially when evaluated against the Net Tropical Cyclone (NTC) activity metric. The October-only forecast also called for a very active month; however, the team did not anticipate that this would be one of the most active Octobers on record.

Another novel initiative in Colorado State’s research involves efforts to develop forecasts of the seasonal probability of hurricane landfall along the U.S. coastline. Although individual hurricane landfall events cannot be pinpointed weeks ahead of time, the net seasonal probability of landfall can be forecast with statistical skill. The team’s landfall probabilities for the 2005 hurricane season were well above their long-term averages, and the season was notable for having four intense landfalling hurricanes along the U.S. coastline. This is the second straight year that the team’s landfall probabilities have been well above average, and frequent landfalls have occurred in both seasons.