Hurricane Season Review: Colorado State’s Gray and Klotzbach Again on Target with Seasonal and Monthly Forecasts

Note to Editors: Forecast verification totals, the complete 2005 season verification report, as well as related research, detailed descriptions of forecast factors, previous forecasts and explanations of individual storms, are available on the Web at or

William Gray and Philip Klotzbach of the Colorado State University forecast team were on target again this year as they predicted a very active hurricane season for the Atlantic Basin. The 2005 hurricane season ended up being the most active and destructive season on record.

For the seventh consecutive year, the forecast team was on track with their seasonal hurricane forecast as highlighted in a summary report of 2005 Atlantic tropical cyclone activity released today. The report, available in its entirety on the Web at, summarizes tropical cyclone activity in the Atlantic basin during the 2005 hurricane season and compares the team’s seasonal and monthly forecasts to what actually occurred.

The Colorado State team made their long-range seasonal forecast, which called for an above-average hurricane season, on Dec. 3, 2004, and then issued seasonal updates on April 1, May 31, Aug. 5, Sept. 2 and Oct. 3. On May 31, just before the official start of hurricane season, the team called for 15 named storms, eight hurricanes and four intense hurricanes. By August, the team updated their forecast to 20 named storms, 10 hurricanes and six intense hurricanes. As the 2005 hurricane season comes 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."

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 formation 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 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 1995-2003, 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 U.S. As a result, from 1995-2003, 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.

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 over $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 United States 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 follows the very active 2004 season when eight different tropical cyclones made United States landfall.

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 Colorado State 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 that 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.

"Overall, we consider our seasonal and monthly forecasts for the 2005 hurricane season to be one of the most skillful that we have issued," Klotzbach said.

Gray and Klotzbach’s August-only forecast called for five named storms, three hurricanes and one intense hurricane; the month witnessed five named storms, two hurricanes and one intense hurricane. The September-only forecast predicted five named storms, four hurricanes and two intense hurricanes; it witnessed five named storms, five hurricanes and two intense hurricanes. The October-only forecast called for three named storms, two hurricanes and one intense hurricane; it witnessed six named storms, four hurricanes and two intense hurricanes.

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 United States 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.

Landfall probabilities include specific forecasts of the probability of landfalling tropical storms and hurricanes of category 1-2 and 3-4-5 intensity for 11 regions and 55 subregions along the U.S. Gulf and East Coast. These subregions are further subdivided into 205 coastal and near-coastal counties. The climatological and current-year probabilities are available online via the Landfalling Hurricane Probability

Web page at

As highlighted in the report released today, unique characteristics of the 2005 hurricane season include the following:

– 23 named storms formed during the 2005 season, the most named storms in a single season, breaking the old record of 21 in 1933.

– 13 hurricanes formed during the 2005 season, the most hurricanes in a single season, breaking the old record of 12 hurricanes in 1969.

– 7 intense or major (Category 3-4-5) hurricanes formed during the 2005 season, which ties the single-season record for intense hurricanes set in 1950.

– 103.25 named storm days recorded during the 2005 season, the second most named storm days in a single season, trailing only the 1995 season that had 102.5 named storm days.

– The season had Net Tropical Cyclone activity of 249 percent of average, which breaks the record of 230 set in 1950.

– Three Category 5 hurricanes formed during the 2005 season (Katrina, Rita and Wilma), the most Category 5 hurricanes recorded in a single season, breaking the old record of two Category 5 hurricanes set in 1960 and 1961.

– Seven named storms made United States landfall during 2005 (Arlene, Cindy, Dennis, Katrina, Rita, Tammy and Wilma). This puts the 2005 season in a tie for second place for landfalling storms behind the 1916 and 2004 seasons, when eight named storms made landfall.

– The 2005 season was the most damaging in history for United States landfalling storms, largely due to Hurricane Katrina. Insured damage estimates for the 2005 season are nearly $70 billion dollars, which shatters the old record set in 1992 and 2004, which, when adjusted for inflation, were each about $25 billion dollars in insured damage.

Additional breakdown of records set for the individual months of the 2005 season follow.

– June

o Two named storms formed (Arlene and Bret). Only 1957, 1959, 1968 and 1986 have had two or more named storms form during the month of June.

– July

o Five named storms formed (Cindy, Dennis, Emily, Franklin and Gert). This is the most on record for July.

o Two major hurricanes formed (Dennis and Emily), the most on record.

o 25.25 named storm days occurred, the most on record.

o 10.75 hurricane days occurred, the most on record.

o 5.75 intense hurricane days occurred, the most on record.

o 64% Net Tropical Cyclone (NTC) activity was recorded, which more than doubles the old record of 26% set in 1996.

– August

o Five named storms formed (Harvey, Irene, Jose, Katrina and Lee). Only 1990, 1995 and 2004 have had more than five named storms form during the month of August.

– September

o Five hurricanes formed (Maria, Nate, Ophelia, Philippe and Rita). This ties 1955, 1969, 1981, 1998 and 2000 for the most hurricanes to form during the month of September.

– October

o Six named storms formed (Stan, Tammy, Vince, Wilma, Alpha and Beta). This ties 1950 for the most named storm formations during the month of October.

o Four hurricanes formed (Stan, Vince, Wilma and Beta). Only 1950 had more hurricanes develop during the month of October.

o Two intense hurricanes formed (Wilma and Beta). This ties 1950, 1961, 1964 and 1995 for the most intense hurricanes to form during the month of October.

o Five intense hurricane days occurred. Only 1954 and 1961 recorded more intense hurricane days.

o 66% Net Tropical Cyclone (NTC) activity was recorded. This breaks the old record of 63% set in 1950.

– November

o As of November 17, no tropical cyclone activity was recorded during the month. Since 1950, 33 of 56 years have had no named storm development during November. Very few seasons have witnessed tropical cyclone development after November 17.

William Gray and his Colorado State team have provided seasonal Atlantic basin hurricane forecasts for 22 years. 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 seven 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 2006 Atlantic basin hurricane season. This forecast report will be issued on Dec. 6, 2005, and will be available on the Web at