Note to Reporters: The full verification report and a chart showing the predictions vs. observed storms are available with the news release at http://www.news.colostate.edu/ and at http://hurricane.atmos.colostate.edu/. The report includes an extensive discussion of the climate features that caused the 2011 Atlantic basin hurricane season to have activity at levels that were observed.
The Colorado State University hurricane forecast team accurately predicted above-average hurricane activity for the Atlantic basin for 2011 even though the storms weren’t as severe as expected, according to its seasonal verification report issued today.
“This season was notable for having many weak tropical cyclones but only slightly above-average intense tropical cyclone activity,” said Phil Klotzbach, lead author of the forecast. “We slightly underpredicted named storms and named storm days while we overpredicted more intense hurricane activity for the entire Atlantic basin and particularly for the Caribbean.”
Following several weak, short-lived tropical cyclones that began the hurricane season, Hurricane Irene became the first official hurricane of the season on August 22. Irene caused billions of dollars worth of damage in the Lesser Antilles, Puerto Rico, the Bahamas and the US East Coast, especially in North Carolina, Vermont, New Jersey and upstate New York.
The report summarizes tropical cyclone activity in the Atlantic basin during the 2011 hurricane season and compares the team’s seasonal and two-week forecasts to what actually occurred. A combination of warm tropical Atlantic sea surface temperatures and the onset of La Nina likely contributed to the somewhat above-average named storm activity that occurred this year. Stronger vertical shear, drier mid levels and cooler subtropical Atlantic sea surface temperatures than expected likely combined to make this season less active for intense hurricane activity than predicted.
The Colorado State team of Phil Klotzbach and William Gray made its long-range seasonal forecast, which called for an above-average hurricane season, in early December 2010 and issued seasonal updates on April 6, June 1 and August 3, all of which called for an active season.
Just as hurricane season began in June, the team called for 16 named storms, 9 hurricanes and five major (Category 3-4-5) hurricanes. Observed were 19 named storms, 7 hurricanes and three major hurricanes.
The team bases its annual forecasts on 60 years of previous data that includes factors such as Atlantic sea surface temperatures and sea level pressures, levels of vertical wind shear (the change in wind direction with height), El Nino (an anomalous warming of waters in the central and eastern tropical Pacific) and other factors.
“The global atmosphere and oceans in combination have stored memory buried within them that can provide clues as to how active the upcoming Atlantic basin hurricane season is likely to be,” said Gray, who has been forecasting for 28 years.
Hurricane statistics for 2011 contained in the report include:
• Nineteen named storms occurred during 2011. Only 2005 (28 storms) and 1933 (21 storms) have had more named storms than 2011.
• 90.5 named storm days occurred in 2011. This is the 6th most named storm days to occur in a single season since 1944.
• No Category 5 hurricanes developed in 2011. This is the fourth consecutive year with no Category 5 hurricanes. The last time that four or more years occurred in a row with no Category 5 hurricanes was 1999-2002.
• No major hurricanes made U.S. landfall in 2011. The last major hurricane to make U.S. landfall was Wilma (2005), so the nation has now gone six years without a major hurricane landfall. Since 1878, the United States has never had a six-year period without a major hurricane landfall.
• No named storms formed between September 24 and October 24. The last time that no named storms formed between these dates was 2002, when the last storm of the season (Lili) formed on September 21.
• Irene became the first hurricane of the season, following eight tropical storms. This is the longest string of tropical storms to start a season without a hurricane on record, breaking the old record of six in 2002.
• Hurricane Irene became the first hurricane to make U.S. landfall since Hurricane Ike (2008). Irene was the first system to make landfall at hurricane strength in New Jersey since 1903.
• Hurricane Philippe took the longest time of any system in recorded history (since 1851) to reach hurricane strength. Philippe was a named storm for 11.75 days before reaching hurricane strength for the first time.
Although the Atlantic has seen a very large increase in major hurricanes during the 16-year period of 1995-2010 (average 3.8 per year) in comparison with the prior 25-year period of 1970-1994 (average 1.5 per year), few major hurricanes have made U.S. landfall (except for the two very damaging years of 2004-2005). Klotzbach and Gray attribute this upturn in Atlantic major hurricanes to natural multi-decadal variability in the strength of the Atlantic thermohaline circulation and a concomitant increase in tropical Atlantic sea surface temperatures.
The team will issue its first forecast for the 2011 hurricane season on Dec. 7, 2011.