I have been asked how many storms and hurricanes are forecast by Phil Klotzbach and Bill Gray for the 15- day period from September 5 to September 19. I should have made it clear that the Accumulated Cyclone Energy (ACE) index is the metric that Phil and Bill are forecasting during this 15-day period.
The ACE index is a wind energy index, and is defined by NOAA as the sum of the squares of the maximum sustained surface wind speed (in knots) measured every six hours for all named storms while they are at least tropical storm strength. The ACE index accounts for the intensity and duration of storms and hurricanes. This is a much more robust measure of tropical cyclone activity than simply the number of storms or hurricanes (which could last short periods of time). See my blog from August 13, 2007 for comments on the ACE index.
Note that the 15-day forecast from Phil and Bill says nothing about landfalls. Some may say they don’t care how many tropical cyclones like Fred (currently over the far eastern Atlantic) there are or how strong they are as long as they stay away from land. Others will find that a forecast of reduced amounts of activity during the peak of the hurricane season, if correct, is of high scientific interest. Time will tell how useful these 15-day forecasts will be.