Note to Reporters: The full verification report and a chart showing predictions vs. observed storms are available at http://www.news.colostate.edu/ and at http://hurricane.atmos.colostate.edu/. The report outlines factors that resulted in fewer hurricanes than expected in the Atlantic basin during the 2013 season.
The 2013 hurricane season was the quietest since 1995 and the first time in 19 years that no major storms formed in the Atlantic basin, according to an end-of-season report issued by Colorado State University forecasters.
CSU forecasters had predicted 2013 would be an above-average year for hurricanes.
“It was one of the largest busts for our research team in the 30 years we’ve been issuing this report,” said Phil Klotzbach, a CSU researcher and co-author of the report.
On April 10 and June 3, Klotzbach and his colleague, William Gray, predicted 18 named storms, nine hurricanes and four major hurricanes for the 2013 season. Their updated forecast in August still called for above-average activity.
Only 13 named storms developed during the 2013 season, a near-normal number. Two of those storms reached minimal hurricane strength, but neither achieved major hurricane status – a category 3-5 on the Saffir-Simpson wind scale.
It is the first time since 1994 that no major hurricanes have formed during the hurricane season, which runs between June and November.
Klotzbach said while this year’s forecast was off, the CSU team has correctly predicted an above- or below-average season with their August outlook about 75 percent of the time.
“These seasonal forecasts should be judged on their overall track record of success, not on a single or a few unsuccessful seasonal forecasts,” he said.
The CSU report summarizes all tropical cyclone activity in the Atlantic basin during the 2013 season and compares the team’s seasonal and two-week forecasts to what occurred.
Klotzbach said the quiet hurricane season was a result of several anomalous factors in the tropical Atlantic, including very dry mid-levels in the atmosphere, greater than anticipated sinking in the mid-levels, and an abrupt weakening of the Atlantic Multi-Decadal Oscillation (AMO) or Atlantic Thermohaline Circulation (THC) during spring and early summer.
The lower activity was not a result of an El Nino event, which can reduce the number of hurricanes that form.
Klotzbach and Gray base the annual forecasts on 60 years of historical data that include Atlantic sea surface temperatures, sea level pressures, levels of vertical wind shear (the change in wind direction and speed with height), El Nino (a significant warming of waters in the central and eastern tropical Pacific), and other factors.
Klotzbach said these factors generally work well and explain approximately 50 percent to 60 percent of the year-to-year hurricane variability over those 60 years of historical data. However, there remains 40 percent to 50 percent that is not explained.
This year’s report contains an extensive discussion about what failed with this year’s forecast and provides ideas for how to improve in the future.
Observations contained in the current report include:
• Two hurricanes formed in 2013. This is the fewest hurricanes since 1982, when two hurricanes also formed.
• No major hurricanes formed in 2013. The last year with no major hurricane formations was 1994.
• No major hurricanes made U.S. landfall in 2013. The last major hurricane to hit the United Sates was Wilma in 2005. The U.S. has now gone eight years without a major hurricane landfall – the longest stretch since reliable landfall records began around 1878. (Superstorm Sandy was not a major hurricane at landfall; its slow movement and massive size were responsible for the damage on the Eastern Seaboard.)
• Seasonal Accumulated Cyclone Energy (or ACE) was only 30 percent of the 1981-2010 median.
Although the number of major hurricanes in the Atlantic has increased substantially from 1995 to 2013 (3.5 per year compared to 1.5 per year from 1970 to 1994), few major hurricanes have made landfall in the U.S. during that time.
Klotzbach and Gray attribute the upturn as well as an increase in major hurricane activity from the late 1940s through the mid-1960s to natural multi-decadal variability in the strength of the AMO or THC. A concomitant increase in a number of favorable hurricane-enhancing parameters occurs in the tropical Atlantic during the positive phase of this oscillation.
The Tropical Meteorology Project has been issuing forecasts for 30 years. These predictions have served as a valuable information tool for insurance interests, emergency managers and coastal residents alike.
While these forecasts were largely developed with funding from various government agencies, attempts to obtain grants have been unsuccessful. In recent years, several insurance companies have provided funding for the forecast.
However, the CSU team has recently lost some of its financial support from industry. Consequently, new sources of revenue are required to keep the forecast going.
The Tropical Meteorology Project will suspend issuing its seasonal forecasts in April 2014 unless additional funding is obtained. The CSU forecast team is currently seeking partnerships with the private sector in order to continue these predictions.