Note to Reporters: The full verification report and a chart wrapping up the season 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 likely caused the 2009 Atlantic basin hurricane season to behave the way it did.
The Colorado State University hurricane forecast team accurately predicted below-average hurricane activity in its early June and early August forecasts for the Atlantic basin for 2009, according to its seasonal verification report issued today.
The report summarizes tropical cyclone activity in the Atlantic basin during the 2009 hurricane season and compares the team’s seasonal and 15-day forecasts to what actually occurred.
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 2008 and issued seasonal updates on April 9, June 2, and August 4, 2009.
Just as hurricane season began in June, the team called for 11 named storms, five hurricanes and two major (Cat 3-4-5) hurricanes. In August, the team lowered their prediction to 10 named storms, four hurricanes and two major hurricanes.
Observed were 9 named storms, three hurricanes and two major hurricanes.
“The skill of our early June and early August forecasts was reasonably good,” said Phil Klotzbach, lead author of the forecasts. “Our earlier predictions of early December 2008 and early April of this year over-estimated this year’s tropical cyclone activity because of our inability to judge the formation of the moderate El Nino event which began to develop late this spring.”
“Activity in 2009 was reduced considerably due largely to the moderate El Nino event that developed,” said William Gray, who has been issuing forecasts for 26 years. “This event generated significantly stronger-than-average vertical wind shear, especially in the Caribbean and Gulf of Mexico. “
Five named storms (Ana, Danny, Erika, Fred, and Henri) dissipated over the open ocean in the tropical and sub-tropical Atlantic this year. This is a fairly rare occurrence that typically happens only in years such as this year that are characterized by high levels of tropospheric vertical wind shear.
The team bases its annual forecasts on 60 years of previous data that includes such factors 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.
Only two weak tropical storms made landfall this year – Claudette in August and Ida in November. On average, the United States experiences approximately 3.6 named storms, 1.9 hurricanes, and 0.7 major hurricane landfalls per year.
The Atlantic has seen a very large increase in major hurricanes during the 15-year period of 1995-2009 (average 3.7 per year) in comparison with the prior 25-year period of 1970-1994 (average 1.5 per year). 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. These changes are not directly related to global sea surface temperature increases or atmospheric CO2 concentrations, Gray has said.
The 2009 hurricane season had the following special characteristics:
• A late-starting season. Ana did not form until August 15 – the latest ‘A’ storm of the season since Andrew formed in 1992 on August 17.
• Nine named storms occurred during 2009, which is the fewest named storms in a tropical cyclone season since 1997 when eight named storms formed.
• 27.25 named storm days occurred in 2009 – the fewest named storm days since 1991, when only 24.25 named storm days were recorded.
• Three hurricanes occurred in 2009 – the fewest hurricanes in a tropical cyclone since 1997 when there were also three hurricanes.
• 11.25 hurricane days occurred in 2009. This is the fewest hurricane days since 2002 when 10.75 hurricane days were reported.
• 2 major hurricanes formed during the 2009 hurricane season. The last time that fewer than two major hurricanes occurred in a season was in 1997 when only one major hurricane (Erika) formed.
• No Category 5 hurricanes developed in 2009. This is the second consecutive year with no Category 5 hurricanes. The last time that two or more years occurred in a row with no Category 5 hurricanes was 1999-2002.
• No named storms formed in June or July. The last time that no storm activity occurred in June or July was 2004 (Alex formed that year on August 1). This is the 18th year of the past 66 years with no storm formations in June or July.
The team will issue its first forecast for the 2010 hurricane season on Wednesday, December 9.
CSU ATLANTIC BASIN SEASONAL HURRICANE FORECASTS FOR 2009
Forecast Parameter and 1950-2000 Climatology
10 Dec 2008 Update
9 April 2009 Update
2 June 2009 Update
4 Aug 2009 Observed
Named Storms (NS) (9.6) 14 12 11 10 9
Named Storm Days (NSD) (49.1) 70 55 50 45 27.25
Hurricanes (H) (5.9) 7 6 5 4 3
Hurricane Days (HD) (24.5) 30 25 20 18 11.25
Major Hurricanes (MH) (2.3) 3 2 2 2 2
Major Hurricane Days (MHD) (5.0) 7 5 4 4 3.25
Accumulated Cyclone Energy (ACE) (96.2) 125 100 85 80 50
Net Tropical Cyclone Activity (NTC) (100%) 135 105 90 85 66