November 30 marks the official end of the Atlantic and East Pacific hurricane seasons. After seven months of activity, beginning with Tropical Storm Arlene in April and ending with Tropical Storm Rina in November, it is finally over. While the Eastern Pacific had an active, but fairly uneventful season, the Atlantic recorded its fifth most active season in terms of named storms and seventh most active season in terms of Accumulated Cyclone Energy (ACE). The season will generally be most remembered for the destructive Hurricanes Harvey, Irma and Maria, which accounted for most of the damage in what is almost certainly the costliest Atlantic hurricane season on record.

2017 Atlantic hurricane season storms

Storm Name	Classification	Formed	Dissipated	Maximum 1-minute sustained winds (mph)	Minimum pressure (mbar)
Arlene	Tropical Storm	April 19	April 21	50	990
Bret*	Tropical Storm	June 19	June 20	45	1007
Cindy*	Tropical Storm	June 20	June 23	60	992
Four	Tropical Depression	July 5	July 7	30	1009
Don*	Tropical Storm	July 17	July 19	50	1007
Emily*	Tropical Storm	July 31	August 2	45	1005
Franklin*	Category 1 Hurricane	August 7	August 10	85	981
Gert*	Category 2 Hurricane	August 13	August 17	105	967
Harvey*	Category 4 Hurricane	August 17	September 1	130	938
Ten**	Potential Tropical Cyclone	August 27	August 29	45	1003
Irma*	Category 5 Hurricane	August 30	September 12	185	914
Jose*	Category 4 Hurricane	September 5	September 22	155	938
Katia	Category 2 Hurricane	September 5	September 9	105	972
Lee*	Category 3 Hurricane	September 15	September 30	115	962
Maria*	Category 5 Hurricane	September 16	September 30	175	908
Nate*	Category 1 Hurricane	October 4	October 9	90	981
Ophelia*	Category 3 Hurricane	October 9	October 16	115	960
Philippe*	Tropical Storm	October 28	October 29	60	997
Rina*	Tropical Storm	November 6	November 9	60	995

* These storms have not yet had post-season analysis completed by the National Hurricane Center (NHC) and their dates and intensities are not final. 

** Potential Tropical Cyclone Ten was not a tropical cyclone, but had advisories issued by the NHC. 

The 2017 Atlantic hurricane season began very early, with the formation of Tropical Storm Arlene in mid-April. Arlene was the first Atlantic tropical storm to form in April since Ana in 2003, and the strongest April Atlantic tropical cyclone on record in terms of minimum central pressure. No tropical cyclones formed in May for the first time since 2014. The season officially began on June 1, and two storms formed in the month of June: Tropical Storm Bret, which became the earliest named storm in the Atlantic tropics east of the Lesser Antilles, and Tropical Storm Cindy, which made landfall in Western Louisiana. Three tropical cyclones formed in July: Tropical Depression Four, which did not affect land and remained over the tropical Atlantic, Tropical Storm Don, which formed east of the Windward Islands and dissipated prior to entering the Caribbean, and Tropical Storm Emily, which rapidly developed and made landfall near Anna Maria Island, Florida. While the early-season storms were generally weak, this was not a trend for the peak season months of August through October.

August featured four tropical cyclones. Early in the month, Franklin made landfall in the Yucatan Peninsula as a tropical storm, and strengthened into the first hurricane of the season over the Bay of Campeche, making landfall in Veracruz, Mexico with minimal impact. Gert formed over the western Atlantic in mid-August, strengthening into a Category 2 hurricane over the Gulf Stream without making landfall. After Gert dissipated, Tropical Storm Harvey formed east of the Lesser Antilles, passing through the Islands as a tropical storm before opening up into a tropical wave. Harvey then regenerated over the Southwestern Gulf of Mexico three days later, and rapidly intensified into a Category 4 hurricane and made landfall at this intensity near Rockport, Texas; Harvey stalled over Texas, causing extreme rainfall in the Houston area. While Harvey was inland over Texas, Potential Tropical Cyclone Ten was designated off the coast of the Southeastern United States; it did not become a tropical cyclone. Harvey finally dissipated September 1.

In late August, Tropical Storm Irma formed over the Eastern Tropical Atlantic; strengthening into a major hurricane just one day after formation. Irma fluctuated in intensity before reaching Category 5 status by September 5 when it approached the Leeward Islands. As Irma reached Category 5 status, Tropical Storm Jose formed over the Eastern Atlantic, and a new Tropical Depression formed over the Bay of Campeche which would eventually become Hurricane Katia. September 6 through 8 featured a rare event: the Atlantic had three simultaneous hurricanes active for the first time since 2010: Irma (Category 5 at peak), Jose (Category 4 at peak) and Katia (Category 2 at peak). Katia dissipated September 9 after making landfall in Veracruz, Mexico, while Irma continued moving west-northwestward and made landfalls in the Florida Keys and near Marco Island September 10 as a major hurricane. Irma and Katia dissipated shortly thereafter, and two new storms formed over the tropical Atlantic in mid-September: Lee and Maria. Lee initially struggled to strengthen, while Maria rapidly intensified into a Category 5 hurricane and made landfall in Dominica at this intensity September 18. Two days later, Maria made landfall in eastern Puerto Rico as a high-end Category 4 hurricane. Maria then turned to the northwest out to sea and did not make landfall in the continental United States. Lee dissipated around this time, regenerating four days later over the central subtropical Atlantic. Lee eventually attained Category 3 intensity far from any land areas in late September. Lee and Maria both transitioned into post-tropical cyclones at the end of September over the far Northern Atlantic, and activity briefly paused.

This lull in activity did not last long, as Tropical Depression Sixteen formed over the Southwestern Caribbean in early October. The depression eventually strengthened into Hurricane Nate over the Gulf of Mexico and made landfalls in Louisiana and Mississippi as a minimal hurricane. Nate’s rapid movement prevented it from rapidly intensifying into a major hurricane. As Nate dissipated, a new tropical depression formed over the eastern subtropical Atlantic, which eventually became Hurricane Ophelia. Ophelia became the easternmost major hurricane in the Atlantic basin on record, attaining this intensity just south of the Azores. Ophelia then made landfall in Ireland as a hurricane-force post-tropical cyclone in mid-October, causing moderate damage.  Tropical Storm Philippe formed in the northwestern Caribbean in late October and made landfall in southwestern Florida as a tropical storm; minimal damage was reported. The final named storm of the season, Tropical Storm Rina, formed over the central subtropical Atlantic in early November and did not affect land. However, Rina’s remnants partially contributed to the formation of “Medicane” Numa over the Mediterranean Sea later in the month.

In total, the 2017 Atlantic hurricane season featured 18 tropical depressions (the most since 2012), 17 tropical storms (the most since 2012), 10 hurricanes (the most since 2012) and 6 major hurricanes (the most since 2005). It also featured an estimated Accumulated Cyclone Energy (ACE) of 226 units according to Colorado State University, the most since 2005 and the fourth-highest ACE since 1950. The season was much more active than predicted by most seasonal forecasts.

Why was this Atlantic hurricane season so active?

Many people may wonder what caused such high activity this Atlantic hurricane season. In short, the lack of an El Niño event over the equatorial Pacific, in addition to significantly warmer-than-normal sea surface temperatures over the tropical Atlantic, resulted in the Atlantic recording its most active season since the record-breaking 2005 season in terms of ACE. In addition, the African Sahel region was wetter than normal, which resulted in stronger than normal tropical waves moving through the Atlantic basin. Prior to this season, there was uncertainty as to whether we were still in the high-activity era of Atlantic hurricane activity, which began in 1995. The hyperactive 2017 season (and even the above average  2016 season) appears to have answered that question: we are almost certainly indeed still in it. Barring the sudden development of El Niño or offseason tropical Atlantic cooling, I am currently leaning towards that the idea that the 2018 season could also feature above-average activity, though likely less active than this year.

It should be noted that while the Atlantic saw extremely high activity, the other Northern Hemisphere basins were not nearly as active, and all featured below-average by the ACE index. The East Pacific recorded its least active season since 2012 in terms of named storms, and the Western Pacific featured only two super typhoons – the fewest since 2010. As a whole, Northern Hemisphere ACE for 2017 was slightly below normal despite the hyperactive Atlantic season.

I hope everyone has a great offseason! The 2018 Atlantic hurricane season will begin on June 1, and the 2018 Pacific hurricane season will begin on May 15. However, it is not out of the question that we could see Sean before the end of the year or Alberto prior to next June. Approximately 3 percent of Atlantic hurricane seasons – including six of the last ten – have featured off-season activity, most recently Tropical Storm Arlene last April.  If an off-season disturbance with a significant chance of development forms, I will write a blog post about it.