2017 Hurricane Season, One Of The Most Destructive On Record, Comes To An End

November 30 marked the last day of the 2017 Hurricane Season, one that is sure to be remembered in the U.S. Virgin Islands and a host of other Caribbean islands, as well some parts of the U.S. mainland, as one of the worst seasons in history.

There were 17 named storms, 10 of which became hurricanes. The storms caused $369 billion in damage, and killed 441 people, according to the Associated Press (AP). The 2017 season is one of only six to include multiple Category 5 hurricanes, and only the second season to include two Cat 5 storms making landfall at that intensity, AP said.

Hurricane Harvey, which struck South Texas on August 25, did not cause the traditional wind damage, but rather massive flooding with more than 50 inches of rain. The flooding destroyed an estimated 200,000 homes in Houston and Texas’s smaller, coastal areas.

Following Harvey, Hurricane Irma brought her wrath first to the Caribbean, walloping St. Martin, Barbuda, Anguilla, St. Thomas and St. John, the British Virgin Islands, and Turks and Caicos before slamming almost the entire state of Florida on Sept. 11.

There were other strong systems, including Hurricane Jose, which initially threatened St. Thomas but spun farther north into open waters.

Then, as if the earlier battering of the Caribbean was not enough, Hurricane Maria formed, ruining the island of Dominica, ravaging St. Croix, and pummeling Puerto Rico before moving to U.S.’s east coast as a tropical storm. Hurricane Maria was the cause of multiple deaths in Dominica and Puerto Rico.

The Colorado State University (CSU) Tropical Meteorology Project forecast team said it under-predicted the 2017 hurricane season.

“The 2017 hurricane season was extremely active. Overall, our predicted numbers from our early July and August issue dates for named storm and hurricane formations were relatively close to what was observed, but our early season predictions and our predictions for integrated metrics such as Accumulated Cyclone Energy (ACE) were far too low,” said Phil Klotzbach, lead author of the forecast.

CSU said the tropical Atlantic was also much warmer than normal during the peak of the hurricane season. Very warm sea surface temperatures and reduced levels of vertical wind shear led to anomalously hurricane-enhancing conditions, especially during September when the Atlantic generated three long-lived, intense hurricanes: Irma, Jose and Maria. While September generated record levels of Atlantic ACE, all other months of the Atlantic hurricane season had activity at close to normal levels, CSU said.

CSU’s initial forecast for the 2017 season was issued on April 6 and predicted slightly below-average hurricane activity, due to anticipated shear-enhancing El Niño conditions.  As the odds of El Niño diminished and the tropical Atlantic anomalously warmed, CSU increased its forecasts, calling for a near-average season on June 1, with an above-average season predicted on July 5 and August 4.

In the first forecast issued on April 6, the team called for 11 named storms, four hurricanes and two major hurricanes. The CSU team increased its forecast and on June 1 called for 14 named storms, six hurricanes and two major hurricanes.

The July 5 and Aug. 4 updates continued to increase levels of activity predicted in 2017, calling for 15 named storms, eight hurricanes and three major hurricanes, and 16 named storms, eight hurricanes and three major hurricanes, respectively.

Observed activity was 17 named storms, 10 hurricanes and six major hurricanes. Slightly below-median ACE of 85 was predicted on April 6, increasing to 100 on June 1 and 135 on July 5 and August 4.  Observed ACE was a whopping 226. The 1981-2010 median Atlantic ACE was 92.

The team bases its annual forecasts on 60 years of historical data and includes factors such as Atlantic sea surface temperatures and sea level pressures, levels of vertical wind shear (the change in wind direction and speed with height), El Niño (an anomalous warming of waters in the central and eastern tropical Pacific) and other factors. While these forecast factors generally work well and explain approximately 50-60 percent of the year to year hurricane variability in these 60 years of historical data, there remains 40-50 percent of this variability which is not explained.