The official end to hurricane season came Wednesday, and with it the annual how-we-did report from Colorado State University hurricane gurus Phil Klotzbach and William Gray.
“This season was notable for having many weak tropical cyclones but only slightly above-average intense tropical cyclone activity,” Phil Klotzbach, lead author of the forecast, said. “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.”
While the season may be officially over, the National Hurricane Center is watching a low-pressure system located about 500 miles north-northeast of the northern Leeward Islands. The agency is giving it a 20 percent chance of developing into something stronger in the next 48 hours. Since it is moving north at about 15 mph, it poses no threat to the Virgin Islands.
The team is calling their 2011 forecast an accurate prediction.
As hurricane season began in June, Klotzbach and Gray predicted 16 named storms, nine hurricanes and five major hurricanes. The numbers came in at 19 named storms, with seven of those reaching hurricane status. Three of the hurricanes reached major status with winds 111 mph or more.
Only 2005, with 28 storms, and 1933, with 21 storms, have had more named storms than 2011.
In their press release issued Wednesday, Gray and Klotzbach said no Category 5 hurricanes, which have winds over 155 mph, developed in 2011. This is the fourth consecutive year with no Category 5s. The last time that four or more years occurred in a row with no Category 5 hurricanes was 1999 to 2002.
The season began with 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 official hurricane of the season on Aug. 22. Irene caused billions of dollars worth of damage in the Lesser Antilles, Puerto Rico, the Bahamas, and the U.S. East Coast, especially in North Carolina, Vermont, New Jersey, and upstate New York. Irene brushed the Virgin Islands as a tropical storm, downing trees and causing flooding.
The Colorado State team termed it an above-average season caused by a combination of warm tropical Atlantic sea surface temperatures and the onset of La Nina. Stronger vertical shear, drier mid levels and cooler-than-expected subtropical Atlantic sea surface temperatures likely combined to make this season less active for intense hurricane activity than predicted, Klotzbach and Gray said.
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,” Gray said.
The team will issue its first forecast for the 2012 hurricane season on Dec. 7.