If one were to look at strictly the raw data, the Atlantic's 11 tropical storms this year compares rather close to the 150 year average of 12 storms. However, the number of short duration and lower intensity storms has combined to produce one of the least intense hurricane seasons overall for the Atlantic in 30 years.
Accumulated Cyclone Energy (ACE) is a metric used by meteorologists at NOAA and elsewhere to express wind energy in a storm over its life based on the maximum winds in each six hour advisory that is issued on a tropical system. The higher the maximum winds, the increased "points" the storm earns in the ACE department. Storms that have longer duration and higher winds can rack up a large number of points.
This year, we've had neither. Our strongest storms in terms of winds (to date) are Humberto and Ingrid, both topping out at 85 miles per hour. Humberto was the longest duration system overall. Given we have not had a major hurricane (111 miles per hour or stronger) to date, it is setting up as the first year since 1994 without one...and the overall ACE for 2013 is currently on track for the lowest we have seen since 1983.
Our current ACE tally of 28 is tied for third lowest since 1950 -- with only 1983 (17) and 1977 (25) ranking lower and 1972 tied with this year. Those years had significantly fewer storms (only ten storms combined between 1983 and 1977). Our average ACE tally during the recent tropical active phase (1995 to 2012) is 139...we're running at about 20 percent of the average seasonal total with six weeks of "official" season to go. Given we're about to exit out of the historically active part of tropical season (storms are possible into November but frequency of storms wanes quickly), the odds of getting our ACE tally to increase much, if at all, is going to decrease in the coming weeks.
What's been to "blame" for the inactive season? A couple of factors as far as the Atlantic is concerned. The combination of dry air episodes in the Atlantic (Saharan dust getting pushed west across the tropical Atlantic) and occasional upper wind shear events (unfavorable winds aloft that prevent tropical development) basically sucked the life out of the few Cape Verde storms that would try to fire up. Chantal, Dorian, and Erin were three of the storms that would get beat up in the Central Atlantic thanks to dry air and/or wind shear, sometimes in tandem.
Until recent weeks in the Northwest Pacific and Indian Oceans, the tropics were relatively quiet globally overall this year. Even in the Western Pacific, the recent activity has only helped push their seasonal ACE totals to within 70 percent of the seasonal average. It isn't just us.