Wikipedia defines it as:
Accumulated cyclone energy (ACE) is a measure used by the National Oceanic and Atmospheric Administration (NOAA) to express the activity of individual tropical cyclones and entire tropical cyclone seasons, particularly the Atlantic hurricane seasons. It uses an approximation of the energy used by a tropical system over its lifetime and is calculated every six-hour period. The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season....By this measure the top ten hurricane seasons are:
Ryan N Maue at Florida State University maintains the ACE index on a daily basis. Here's the 2008 scorecard:
...For definitions of the terms "above", "near", and "below" normal, see the Climatology section above.
Season ACE TS HR MH Classification 2005 Atlantic hurricane season 248 28 15 7 Above normal (hyperactive) 1950 Atlantic hurricane season 243 13 11 8 Above normal (hyperactive) 1995 Atlantic hurricane season 228 19 11 5 Above normal (hyperactive) 2004 Atlantic hurricane season 225 14 9 6 Above normal (hyperactive) 1961 Atlantic hurricane season 205 11 8 7 Above normal (hyperactive) 1955 Atlantic hurricane season 199 12 9 6 Above normal (hyperactive) 1998 Atlantic hurricane season 182 14 10 3 Above normal (hyperactive) 1999 Atlantic hurricane season 177 12 8 5 Above normal (hyperactive) 2003 Atlantic hurricane season 175 16 7 3 Above normal (hyperactive) 1964 Atlantic hurricane season 170 12 6 6 Above normal (hyperactive)
Here's his ACE page, here's NOAA's backrounder. Here's Max Mayfield's (former director, National Hurricane Center) comment on ACE.
Here's the headline story, from NewsDaily:
Another forecaster predicted an active 2009 Atlantic hurricane season on Tuesday, six months ahead of the tropical cyclone period that begins on June 1.
WSI Corp. predicted 13 tropical storms in the 2009 season and said seven would develop into hurricanes.
The long-term average during the six-month season is for 10 or 11 tropical storms and six hurricanes.
WSI, based in Andover, Massachusetts, forecast that three of next year's hurricanes would be dangerous storms with a rank of Category 3 or above on the five-step Saffir-Simpson scale of hurricane intensity.
Such storms feature sustained winds of at least 111 miles per hour.