...DISCUSSION...
FINALLY A QUIET NIGHT AT WORK WITH LISA FAR OUT TO SEA AND
JEANNE NOW HPC'S RESPONSIBILITY. TO REFLECT ON HOW BUSY IT HAS
BEEN SO FAR... THIS IS THE MOST ACTIVE SEASON TO DATE UP UNTIL
SEPT 27 EVER SEEN IN TERMS OF NOAA'S ACCUMULATED CYCLONE ENERGY
(ACE) INDEX. IN FACT...IF THE SEASON ENDED TODAY...2004 WOULD BE
THE 5TH MOST ACTIVE ACE SINCE RECORDS BEGAN IN 1851...WITH 1950-
1893-1995-1926 BEING THE TOP 4. ONTO THE CURRENT WEATHER...
SOURCE: CPC
Measuring total seasonal activity: The Accumulated Cyclone Energy (ACE) index
The phrase "total seasonal activity" refers to the collective intensity and duration of Atlantic tropical storms and hurricanes occurring during a given season. The measure of total seasonal activity used by NOAA is called the Accumulated Cyclone Energy (ACE) index. The ACE index is a wind energy index, defined as the sum of the squares of the estimated 6-hourly maximum sustained wind speed (knots) for all named systems while they are at least tropical storm strength.
Two other measures of total seasonal activity, developed by Dr. William Gray at the Colorado State University, are the Hurricane Destruction Potential (HDP) index and the Net Tropical Cyclone (NTC) index. These indices are correlated at approximately 0.95 with the ACE index. NOAA uses the ACE index instead of the HDP index when making and verifying their seasonal outlooks because the ACE index includes the contribution from systems while at tropical storm strength, whereas the HDP index does not. The ACE index is used instead of the NTC index because it allows one to easily quantify activity occurring in different parts of the Atlantic Basin, and because it does not suffer from the re-sampling issues inherent in the mathematical formulation of the NTC index.
NOAA uses the ACE index, in combination with the numbers of named storms, hurricanes, and major hurricanes, to categorize North Atlantic hurricane seasons as being above normal, near normal, or below normal.
NOAA definitions of above-normal, near -normal, and below-normal Atlantic hurricane seasons
The 1951-2000 mean value of the ACE index is 93.2, and the median value is 87.5
Above-normal season: An ACE index value well above 103 x 104 kt2 (corresponding to 117% of the median ACE value or 110% of the mean), or an ACE value slightly above 103 x 104 kt2 combined with at least two of the following three parameters being above the long-term average: number of tropical storms, hurricanes, and major hurricanes.
Near-normal season: An ACE index value in the range 66-103 x 104 kt2 (corresponding to 75%-117% of the median or 71%-110% of the mean), or an ACE index value slightly above 103 x 104 kt2 but with less than two of the following three parameters being above the long-term average: numbers of tropical storms, hurricanes, and major hurricanes.
Below-normal season: An ACE index value below 66 x 104 kt2, corresponding to below 75% of the median or 71% of the mean.
