Why the 2017 Atlantic hurricane season ranks among the top 7 most intense ever recorded

One of the most destructive Atlantic hurricane seasons in recent history has concluded.

From Harvey's historic flooding of southeastern Texas to the Irma-battered Florida Keys and Maria's widespread devastation in Puerto Rico, the fallout from a series of major hurricanes was dramatic and will be felt for years.

This tropical season, which ended Nov. 30, is the busiest in the Atlantic since 2012 as 17 storms were named. While it was not a record-setting season in terms of the number of storms, it will likely be remembered as the one of the most intense in United States history.

A hurricane season’s intensity is quantified by Accumulated Cyclone Energy, or ACE, which measures the combined strength and duration of tropical storms and hurricanes.

An above-average season is about 111 units, while a below average season is less than 66. According to Colorado State University, the current ACE value for the Atlantic is 226, making it well above normal.

As of Dec. 1, this season’s ACE value ranks as the seventh most active on record to date, trailing only 1893, 1926, 1933 and 2005, according to the National Hurricane Center.

There were several factors for the above-normal activity including a decrease in wind shear and the presence of above-normal sea surface temperatures, according to AccuWeather Hurricane Expert Dan Kottlowski.

A key development was the transition from a neutral El Niño-Southern Oscillation (ENSO) pattern at the start of the season to a weak La Niña, Kottlowski explained. This meant that strong vertical wind shear weakened considerably by the latter part of August and September in primary tropical storm breeding areas such as the Caribbean Sea and southwestern Atlantic Ocean.

Vertical wind shear, or the change of wind speed and direction with altitude, can limit the development of tropical systems, according to Kottlowski. When wind shear is not present a tropical cyclone's center will be vertically aligned, which keeps it intact and allows it to strengthen. But when upper-level winds come over top of a system, they can tilt the system in one direction and make it harder for the system to intensify further.

This is similar to a spinning top that, when completely upright, can spin continuously without problem. However, when it becomes angled or tilted, it can unravel and come to a halt, Kottlowski explained.

“We are still experiencing warmer-than-normal sea surface temperatures across the Atlantic Basin, so a combination of lower shear and warm water certainly helped things to develop,” Kottlowski said.

Warm water can act as fuel for developing systems by adding moisture to the atmosphere and thus allowing more shower and thunderstorm development around a storm's center.

Of the 17 named storms, 10 became hurricanes. Six hurricanes developed into major hurricanes while six named storms made landfall in the mainland U.S. this year.

This season brought the rare circumstance when two Category 4 hurricanes, Harvey and Irma, made landfall in the U.S. within a month's time. Harvey made its first Texas landfall on Aug. 25, while Irma made landfall in the Florida Keys on Sept. 10. This also marked the first time that two Category 4 storms made landfall in the mainland U.S. in the same Atlantic hurricane season since at least the beginning of the satellite era, which goes back to the early 1960s.

Another factor Kottlowski pointed out related to the tropical waves moving off the coast of Africa. While about 58 tropical waves have been observed this year, a number which is around normal, Kottlowski said the frequency of the waves increased from mid-August and through September, when conditions are optimal for tropical development.

“If you have more tropical waves, you’re going to end up with more tropical storms that develop,” he said.

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