Before and during each hurricane season, several forecasts of hurricane activity are issued by national meteorological services, scientific agencies, and noted hurricane experts. The forecasts include weekly and monthly changes in significant factors that help determine the number of tropical storms, hurricanes, and major hurricanes within a particular year. According to NOAA, the average Atlantic hurricane season between 1950 and 2005 contained 10.3 tropical storms, 6.2 hurricanes, 2.7 major hurricanes, and an accumulated cyclone energy (ACE) index of 66–103 units. Broadly speaking, ACE is a measure of the power of a tropical or subtropical storm multiplied by the length of time it existed. Therefore, a storm with a longer duration will have high values of ACE. It is only calculated for full advisories on specific tropical and subtropical systems reaching or exceeding wind speeds of . Accordingly, tropical depressions are not included in the count. After the storm has dissipated, typically after the end of the season, the NHC re-examines the data and produces a final report on each storm. These revisions can lead to a revised ACE total either upward or downward compared to the original value. The National Oceanic and Atmospheric Administration (NOAA) typically categorizes a season as either above-average, average, or below-average based on the cumulative ACE Index, but the number of tropical storms, hurricanes, and major hurricanes within a hurricane season are occasionally considered as well.
