OCTOBER 2nd, 2017 - If you thought the last month was an active one for hurricanes, you'd be right. September 2017 goes down as the most active month on record in the North Atlantic basin.
Overall, the month of September was extremely active with four named storms, all of which became hurricanes, and three of those four became major hurricanes. On top of that, both Hurricane Irma and Hurricane Maria reached Category 5 intensity.
Hurricane Irma: September 6th, 2017
Hurricane Maria: September 18th, 2017
The month of September generated a very high amount of ACE (Accumulated Cyclone Energy). ACE measures the combined strength and duration of tropical storms and hurricanes. Overall, this September was 3.5 times MORE ACTIVE than an average September (1981-2010). The normal ACE for an ENTIRE season is 93. Just for the month of September, the Atlantic generated 175 ACE.
So why have the tropics been so active, especially in the Atlantic? Well, there are two reasons for that: the lack of El Nino in the Eastern Pacific and abnormally warm ocean temperatures.
El Nino acts to generate warmer waters in the Pacific Ocean, which will usually lead to more wind shear in the Atlantic Ocean. Wind shear acts to rip storms apart before they can strengthen into the monsters we've seen so far this season. This year has been rather neutral with a La Nina perhaps developing. That has kept wind shear to a minimum in the Atlantic and an active season persists.
Additionally, above normal water temperatures allow storms to strengthen quicker. Warm water acts as an engine for hurricanes, the warmer the water, the stronger a storm can become. Temperatures typically average in the lower 80°F in the central Atlantic, but this season, those numbers have been higher, in the mid to upper 80s at times.
As of the first week of October the Atlantic is now storm free for the first time since August 23rd, so it's been over 5 weeks since we've been this quiet. While the Atlantic basin is quiet now, the 2017 Atlantic Hurricane Season is far from over, with the season officially ending on November 30th. That leaves us with nearly 2 months of what could be a continued active hurricane season.