September typically has more tropical cyclone activity than any other month of the hurricane season. This year, the Atlantic Basin has seen three tropical storms, two of which were Category 1 hurricanes, during September.
However, the number of tropical storms and hurricanes don't tell the whole story. I prefer to look at the Accumulated Cyclone Energy (ACE) index to determine the overall activity. The ACE index accounts for the number, intensity, and duration of storms. Technically, it is the sum of the squares of the maximum wind speed at six-hour intervals for all tropical storms and hurricanes. It is a much more robust parameter than just looking at the number of storms and hurricanes.
Dr. Ryan N. Maue's webpage helps put this hurricane season in perspective. The average yearly ACE is 104 based on 1981-2010 climatology. If this were an average year, the ACE should be 79 by today. However, the actual ACE calculated for this season so far is only 23.
That means that the ACE to date for this season is only 30 percent of what would be expected for an average year. We obviously can still see some additional activity in October and November, but the seasonal forecasters who called for above average activity this year appear headed toward a busted forecast.