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 2012 Atlantic basin hurricane season to have activity at levels that were observed.
The 2012 hurricane season was one of the most unusual seasons on record with a significant number of weaker cyclones combined with a general lack of major hurricane activity, according to a new report from the Colorado State University hurricane forecast team.
“The 2012 hurricane season had more activity than predicted in our seasonal forecasts. It was notable for having a very large number of weak, high latitude tropical cyclones but only one major hurricane that lasted for a mere six hours,” said Phil Klotzbach, lead author of the forecast.
The report summarizes tropical cyclone activity in the Atlantic basin during the 2012 hurricane season and compares the team’s seasonal and two-week forecasts to what actually occurred. The anticipated El Nino discussed in the seasonal forecasts did not develop as predicted for a number of reasons, Klotzbach said, while anomalous sinking motion at mid-levels in the atmosphere was the primary reason why 2012 was not more active in the tropical Atlantic.
The Colorado State team of Klotzbach and William Gray made its long-range seasonal forecast, which called for a slightly below-average hurricane season, on April 4 and June 1. An update issued on August 3 called for average activity.
Just as hurricane season began in June, the team called for 13 named storms, five hurricanes and two major (Category 3-4-5) hurricanes. They updated that in August, calling for 14 named storms, six hurricanes and two major hurricanes.
Observed were 19 named storms, 10 hurricanes but only one major hurricane.
“Superstorm Sandy was a very atypical system that caused some of the most economic damage ever associated with a single storm in U.S. history,” Klotzbach said. “Its destruction was the result of a combination of a mid-latitude cyclone and tropical cyclone whose northwesterly track brought major flooding to the New York City and New Jersey coastal areas.”
“Although storms such as Sandy are extremely rare, these types of tropical cyclones are well within natural variability, and should not be attributed to increases in human-induced greenhouse gases,” Gray said. A more in-depth discussion of Sandy and its potential relationship to climate change will be available in a separate paper by Gray and Klotzbach to be released on http://hurricane.atmos.colostate.edu/ by the end of the week.
The team bases its annual forecasts on 60 years of historical data and 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.
Hurricane statistics for 2012 contained in the report include:
• Nineteen named storms occurred during 2012. Only 2005 (28) and 1933 (21) have had more named storms than 2012. This is most unusual for a season which only accrued 0.25 major hurricane days and had only one hurricane (Sandy) with a central pressure below 964 millibars.
• 99.50 named storm days occurred in 2012. This is the third most named storm days to occur in a single season since 1944. Only 1995 (121.25) and 2005 (131.50) had more named storm days than 2012.
• 10 hurricanes occurred in 2012. Only five other years have had more than 10 hurricanes occur in a single season since 1944.
• One major hurricane formed in 2012. This is the fewest major hurricanes to occur in the Atlantic basin since 1997.
• 0.25 major hurricane days occurred in 2012. This is the fewest major hurricane days to occur in the Atlantic basin since 1994.
• No Category 5 hurricanes developed in 2012. This is the fifth consecutive year with no Category 5 hurricanes. The last time that five or more years occurred in a row with no Category 5 hurricanes was 1993-1997.
• No tropical cyclones reached Category 4 or 5 hurricane strength in 2012. The last time that this occurred was in 2006.
• No major hurricanes made landfall on the United States in 2012. The last major hurricane to make U.S. landfall was Wilma (2005), so the U.S. has now gone seven years without a major hurricane landfall. Since 1878, the U.S. has never had a seven-year period without a major hurricane landfall.
• The maximum intensity reached by any tropical cyclone this year was 100 knots (Michael). This is the weakest maximum intensity achieved by the most intense tropical cyclone of a season since 1994 (Florence – 95 knots).
• Beryl became the strongest off-season tropical cyclone on record to make U.S. landfall, when it made landfall on May 28th at 60 knots near Jacksonville, Fla.
• Hurricane Nadine tied Hurricane Ginger (1971) for the most named storm days accrued (21.25) by a single storm since the dawn of aircraft reconnaissance in 1944.
• Post-tropical Cyclone Sandy (aka Superstorm Sandy) generated the lowest pressure ever recorded in the Northeast United States at landfall (943 mb), breaking the record set by the Great New England Hurricane or Long Island Express (1938).
Although the Atlantic has seen a very large increase in major hurricanes during the 18-year period of 1995-2012 (average 3.6 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.