Hurricane Season Review: El Nino and Tropical Atlantic Conditions Complicate 2006 Colorado State Forecast

Note to Editors: The complete 2006 season verification report is available at Forecast verification totals, the complete 2006 season verification report, as well as related research, detailed descriptions of forecast factors, previous forecasts and explanations of individual storms, will be available no later than 9:30 a.m. MST at

A late-developing El Nino and increased dryness in the tropical Atlantic led to a slightly below-average hurricane season that complicated hurricane forecasts by William Gray and Philip Klotzbach of the Colorado State University forecast team.

Gray and Klotzbach had anticipated that 2006 would be well above-average in their early December, early April and early June forecasts.

In the seven consecutive years prior to 2006, the forecast team correctly predicted above- or below-average activity with its seasonal hurricane forecasts from their early June forecast.

"A variety of factors interact with each other to cause year-to-year and month-to-month hurricane variability," said Phil Klotzbach, lead author on the forecasts. "It is impossible to understand how all these processes interact with each other to 100 percent certainty. Continued research should help us better understand these complicated atmospheric/oceanic interactions."

The newest report, available in its entirety on the Web at, summarizes tropical cyclone activity in the Atlantic basin during the 2006 hurricane season and compares the team’s seasonal and monthly forecasts to what actually occurred.

"The 2006 Atlantic basin hurricane season was much less active than the 2004 and 2005 seasons, but 2006 was also atypical in that there were no landfalling hurricanes along the U.S. coastline this year," said Gray, who has led the forecasting team at Colorado State for 23 years. "This is the first year that there have been no landfalling hurricanes along the U.S. coastline since 2001, and this is only the 11th year since 1945 that there have been no U.S. landfalling hurricanes."

The Colorado State team made its long-range seasonal forecast, which called for an above-average hurricane season, on Dec. 6, 2005, and then issued seasonal updates on April 4, May 31, Aug. 3, Sept. 1 and Oct. 3.

On May 31, just before the official start of hurricane season, the team called for 17 named storms, nine hurricanes and five intense hurricanes. By August, the team updated and lowered their forecast to reflect the downward trend caused largely by the late arrival of El Nino.

The 2006 hurricane season contrasted sharply with 2005. The 2005 season witnessed 27 named storms, 15 hurricanes and seven intense hurricanes – activity that Colorado State forecasters have called an anomaly.

"The record number of tropical cyclones in 2005 (27 named storms, 15 hurricanes and 7 major hurricanes), should not be taken as an indication of something beyond natural processes," Gray said.

"There have been several other years with comparable hurricane activity to 2005," Gray said, pointing to 1933, which experienced 21 named storms in a year when there was no satellite or aircraft data.

The 2006 hurricane season had the following special characteristics:

-Another early-starting season. Alberto formed on June 11. The climatological average date for the first named storm formation in the Atlantic, based on 1944-2005 data, is July 10.

-Nine named storms formed during the 2006 season. This is the fewest named storms to form in the Atlantic since 1997, when only seven named storms formed.

-Five hurricanes formed during the 2006 season. This is the fewest hurricanes to form in the Atlantic since 2002, when four hurricanes formed.

-Two major hurricanes formed during the 2006 season; 1997 was the most recent year to have fewer than two major hurricanes form (one named Erika).

-50 named storm days occurred in 2006. This is the lowest value of named storm days since 1997, when only 28.75 named storm days occurred.

-20 hurricane days occurred in 2006. This is the lowest value of hurricane days since 2002, when 10.75 hurricane days were observed.

-Three intense hurricane days occurred in 2006. This ties 2002 for the lowest value of intense hurricane days observed since 1997, when only 2.25 intense hurricane days occurred.

-No Category 4 or 5 hurricanes formed in the Atlantic basin this year.  This is the first year with no Category 4-5 hurricanes in the Atlantic since 1997.

-Three named storms made United States landfall in 2006.  This is the fewest number of named storms to make landfall in the United States since 2001 when three named storms (Allison, Barry and Gabrielle) made landfall.

-From Alberto-Helene, each tropical cyclone lasted as long or longer than the cyclone preceding it.  For example, Alberto and Beryl lasted 2.75 named storm days, Chris and Debby lasted 3.25 named storm days, Ernesto lasted 6 named storm days, etc.

-Both Gordon and Helene accumulated 7.5 hurricane days.  These two storms accrued as many hurricane days as Wilma, which was the longest-lived hurricane of the 2005 season.

A more detailed analysis of the season:

June and July

-June and July experienced average amounts of tropical cyclone activity with two named storms forming – Alberto and Beryl.  Unlike 2005, when two major hurricanes – Dennis and Emily – developed and intensified in the tropical Atlantic and Caribbean, no hurricane activity occurred in the deep tropics during June and July 2006.


-Three named storms (Chris, Debby and Ernesto) formed during August, but only Ernesto briefly reached hurricane status. This is the fewest hurricanes to form in August since 2002, when no hurricanes formed.


-Four named storms (Florence, Gordon, Helene and Isaac) formed during September, and all four became hurricanes. Gordon and Helene became major hurricanes.

-18.25 hurricane days occurred in September 2006. This is more than were observed in September 2005 (16.75 hurricane days).


-No named storms formed in October.  This is the first time that no named storms have formed in October since 2002.  Prior to 2006, only eleven years since 1950 witnessed no named storm formations in October.

-Only two named storm days were observed in October (from Isaac which formed in late September).  This is the fewest named storm days in October since 1994, when zero named storm days were observed.

Gray and his Colorado State team have provided seasonal Atlantic basin hurricane forecasts for 23 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.

In 2004, the Colorado State team helped create the Landfalling Hurricane Probability Web page – at – that includes 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.

"We continue to modify our seasonal predictions based on the information we obtain every year," 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 2007 Atlantic basin hurricane season. This forecast report will be issued on Dec. 8, 2006 and will be available on the Web at