The 2017 Atlantic hurricane season was a hyperactive and catastrophic hurricane season that, with a damage total of at least $282.28 billion (USD), was the costliest tropical cyclone season on record. With over 3,300 estimated deaths, 2017 was the deadliest season since 2005 and also featured both the highest total accumulated cyclone energy (ACE) and the highest number of major hurricanes since 2005. About 99.7% of the season's damage was due to three of the season's major hurricanes – Harvey, Irma, and Maria. Another notable hurricane, Nate, was the worst natural disaster in Costa Rican history; Harvey, Irma, Maria, and Nate had their names retired due to their high damage costs and loss of life. Featuring 17 named storms, 10 hurricanes, and 6 major hurricanes, the 2017 season ranks alongside 1936 as the fifth-most active season since reliable records began in 1851.
This season is also one of only six years on record to feature multiple Category 5 hurricanes, and only the second after 2007 to feature two hurricanes making landfall at that intensity. All ten of the season's hurricanes occurred in a row, the greatest number of consecutive hurricanes in the satellite era, and tied for the greatest number of consecutive hurricanes ever observed in the Atlantic basin since reliable records began in 1851. Additionally, this season is the only season on record in which three hurricanes each had an ACE of over 40: Irma, Jose, and Maria.
The season officially began on June 1 and ended on November 30. These dates historically describe the period of year when most tropical cyclones form in the Atlantic basin. However, as shown by Tropical Storm Arlene in April, the formation of tropical cyclones is possible at other times of the year. In mid-June, Tropical Storm Bret struck the island of Trinidad, which is rarely struck by tropical cyclones, due to its low latitude. In late August, Hurricane Harvey became the first major hurricane to make landfall in the United States since Wilma in 2005 and the first Category 4 hurricane since Charley in 2004, while also tying the record for the costliest tropical cyclone on record, as well as the most rainfall dropped by a tropical cyclone in the United States. In early September, Hurricane Irma became the first Category 5 hurricane to impact the northern Leeward Islands on record, later making landfall in the Florida Keys as a Category 4 hurricane. In terms of maximum sustained winds, Irma is the strongest hurricane ever recorded in the Atlantic Ocean outside of the Gulf of Mexico and Caribbean Sea. In late September, Hurricane Maria became the first Category 5 hurricane to strike the island of Dominica on record. It later made landfall in Puerto Rico as a high-end Category 4 hurricane with catastrophic effect. Most of the deaths from this season occurred from Maria, and caused a humanitarian crisis in Puerto Rico. In early October, Hurricane Nate became the fastest-moving tropical cyclone in the Gulf of Mexico while also becoming the fourth hurricane of the year to make landfall in the contiguous United States. Slightly over a week later, Hurricane Ophelia became the easternmost major hurricane in the Atlantic basin on record, and later impacted most of Northern Europe as an extratropical cyclone. The season concluded with Tropical Storm Rina, which became a post-tropical cyclone on November 9, although the season did not officially end until November 30.
Initial predictions for the season anticipated that an El Niño would develop, lowering tropical cyclone activity. However, the predicted El Niño failed to develop, with cool-neutral conditions developing instead, later progressing to a La Niña – the second one in a row. This led forecasters to raise their predicted totals, with some later anticipating that the season could be the most active since 2010.
Beginning in 2017, the National Hurricane Center (NHC) had the option to issue advisories, and thus allow watches and warnings to be issued, on disturbances that are not yet tropical cyclones but have a high chance to become one, and are expected to bring tropical storm or hurricane conditions to landmasses within 48 hours. Such systems are termed "Potential Tropical Cyclones". The first storm to receive this designation was Potential Tropical Cyclone Two, which later developed into Tropical Storm Bret, east-southeast of the Windward Islands on June 18. In addition, the numbering that a potential tropical cyclone receives would be retained for the rest of the hurricane season, meaning that the next tropical system would be designated with the following number, even though potential tropical cyclones do not qualify as tropical cyclones. This was first demonstrated with Potential Tropical Cyclone Ten, which failed to develop into a tropical cyclone.