The 2017 Atlantic hurricane season was a devastating and extremely active Atlantic hurricane season that became the costliest tropical cyclone season on record worldwide, with a damage total of at least $294.92 billion (USD). It was the second consecutive above–average season dating back to 2016. The season featured 17 named storms, 10 hurricanes, and 6 major hurricanes. Most of the season's damage was due to hurricanes Harvey, Irma, and Maria. Another notable hurricane, Nate, was the worst natural disaster in Costa Rican history. These four storm names were retired following the season due to the number of deaths and amount of damage they caused. Collectively, the tropical cyclones were responsible for at least 3,364 deaths. The season also had one of the highest accumulated cyclone energy (ACE) on record, with an index of 225 units. A record three hurricanes generated an ACE of over 40: Irma, Jose, and Maria. This season featured two Category 5 hurricanes, and the only season other than 2007 with two hurricanes making landfall at that intensity. The season's ten hurricanes occurred one after the other, the greatest number of consecutive hurricanes in the satellite era, and tied for the highest number of consecutive hurricanes ever observed in the Atlantic basin.
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 and are adopted by convention. However, as shown by Tropical Storm Arlene in April, the formation of tropical cyclones was possible at other times of the year. In late August, Hurricane Harvey struck Texas and became the first major hurricane to make landfall in the United States since Wilma in 2005, ending a record 12-year span in which no hurricanes made landfall at the intensity of a major hurricane throughout the United States and was the strongest since Charley in 2004. With damages estimated at $125 billion, the storm sequentially tied Hurricane Katrina for the costliest tropical cyclone on record and broke the record for most rainfall dropped by a tropical cyclone in the United States, with extreme flooding in the Houston area. 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 sustained winds, Irma, at the time, became the strongest hurricane ever recorded in the Atlantic basin outside of the Gulf of Mexico and Caribbean, with maximum sustained winds of 180 mph (285 km/h); it was later surpassed in 2019 by Hurricane Dorian. In mid September, Hurricane Maria became the first Category 5 hurricane in history to strike the island of Dominica. 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. In early October, Hurricane Nate became the fastest-moving tropical cyclone in the Gulf of Mexico on record and the third hurricane to strike the contiguous United States in 2017. 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 extratropical on November 9.
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 in late May, with some later anticipating that the season could be the most active since 2010.
Prior to the start of this season, the National Hurricane Center (NHC) changed its policy to permit issuance of advisories on disturbances that were not yet tropical cyclones but had a high chance to become one, and were expected to bring tropical storm or hurricane conditions to landmasses within 48 hours. As a result of this change, early watches and warnings could be issued by local authorities. Such systems would be 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. Additionally, the number assigned to a potential tropical cyclone would remain with that disturbance, meaning that the next identified tropical system would be designated with the following number, even if the potential tropical cyclone did not develop into one. The first such system was Potential Tropical Cyclone Ten in August.
Seasonal forecasts
Ahead of and during the season, several national meteorological services and scientific agencies forecast how many named storms, hurricanes, and major hurricanes will form during a season, and/or how many tropical cyclones will affect a particular country. These agencies include the Tropical Storm Risk (TSR) Consortium of the University College London, the National Oceanic and Atmospheric Administration (NOAA) and Colorado State University (CSU). The forecasts include weekly and monthly changes in significant factors that help determine the number of tropical storms, hurricanes, and major hurricanes within a particular year. Some of these forecasts also take into consideration what happened in previous seasons and the dissipation of the 2014–16 El Niño event. On average, an Atlantic hurricane season between 1981 and 2010 contained twelve tropical storms, six hurricanes, and two major hurricanes, with an accumulated cyclone energy (ACE) index of between 66 and 103 units. ACE is, broadly speaking, a measure of the power of a hurricane multiplied by the length of time it existed; therefore, long-lived storms and particularly strong systems result in high levels of ACE. The measure is calculated at full advisories for cyclones at tropical storm strength—storms with winds in excess of 39 mph (63 km/h).
Pre-season outlooks
The first forecast for the year was issued by TSR on December 13, 2016. They anticipated that the 2017 season would be a near-average season, with a prediction of 14 named storms, 6 hurricanes, and 3 major hurricanes. They also predicted an ACE index of around 101 units. On December 14, CSU released a qualitative discussion detailing five possible scenarios for the 2017 season, taking into account the state of the Atlantic multidecadal oscillation and the possibility of El Niño developing during the season. TSR lowered their forecast numbers on April 5, 2017, to 11 named storms, 4 hurricanes, and 2 major hurricanes, based on recent trends favoring the development of El Niño. The next day, CSU released their prediction, also predicting a total of 11 named storms, 4 hurricanes, and 2 major hurricanes. On April 17, The Weather Channel (TWC) released their forecasts, calling for 2017 to be a near-average season, with a total of 12 named storms, 6 hurricanes, and 2 major hurricanes. The next day, on April 18, North Carolina State University released their prediction, also predicting a near-average season, with a total of 11–15 named storms, 4–6 hurricanes, and 1–3 major hurricanes. On May 20, TWC issued an updated forecast, raising their numbers to 14 named storms, 7 hurricanes, and 3 major hurricanes to account for Tropical Storm Arlene as well as the decreasing chance of El Niño forming during the season. On May 25, NOAA released their prediction, citing a 70% chance of an above average season due to "a weak or nonexistent El Niño", calling for 11–17 named storms, 5–9 hurricanes, and 2–4 major hurricanes. On May 26, TSR updated its prediction to around the same numbers as its December 2016 prediction, with only a minor change in the expected ACE index amount to 98 units.
Mid-season outlooks
CSU updated their forecast on June 1 to include 14 named storms, 6 hurricanes, and 2 major hurricanes, to include Tropical Storm Arlene. It was based on the current status of the North Atlantic oscillation, which was showing signs of leaning towards a negative phase, favoring a warmer tropical Atlantic; and the chances of El Niño forming were significantly lower. However, they stressed on the uncertainty that the El Niño–Southern Oscillation could be in a warm-neutral phase or weak El Niño conditions by the peak of the season.
On the same day, the United Kingdom Met Office (UKMO) released its forecast of a very slightly above-average season. It predicted 13 named storms, with a 70% chance that the number would be in the range between 10 and 16, and 8 hurricanes, with a 70% chance that the number would be in the range between 6 and 10. It also predicted an ACE index of 145, with a 70% chance that the index would be between 92 and 198.
On July 4, TSR released their fourth forecast for the season, increasing their predicted numbers to 17 named storms, 7 hurricanes, and 3 major hurricanes, due to the fact that El Niño conditions would no longer develop by the peak of the season and the warming of sea-surface temperatures across the basin. Additionally, they predicted a revised ACE index of 116 units. During August 9, NOAA released their final outlook for the season, raising their predictions to 14–19 named storms, though retaining 5–9 hurricanes and 2–5 major hurricanes. They also stated that the season had the potential to be extremely active, possibly the most active since 2010.
Seasonal summary
Background
The Atlantic hurricane season officially began on June 1, 2017. Among the busiest on record, the season produced eighteen tropical depressions, all of which but one of which further intensified into tropical storms. Ten hurricanes occurred in succession, the greatest number of consecutive hurricanes since the start of the satellite era in 1966; six of these further strengthened into major hurricanes. The season had the most named storms and hurricanes, along with most major hurricanes, since 2005. It also produced the first major hurricanes to strike the continental U.S. since 2005.
Unlike the pattern of previous years that acted to steer many tropical cyclones harmlessly into the open Atlantic, 2017 featured a pattern conducive for landfalls; in fact, the season culminated into 23 separate landfalls by Atlantic named storms. At least 3,364 fatalities were recorded and damage totaled $294.92 billion, cementing the 2017 season as the costliest in recorded history and the deadliest season since 2005.
The extremely active season came to fruition through a multitude of different factors. Pre-season projections noted the potential for a weak to moderate El Niño event to evolve through the summer and fall on the basis of statistical model guidance. Instead, equatorial Pacific Ocean temperatures began cooling throughout the summer, reaching La Niña threshold in November and curtailing the negative effects on Atlantic hurricane activity originally expected. In addition, tropical Atlantic Ocean temperatures—previously below average in months prior to the start of the season—underwent rapid warming by late May, providing lower sea level pressures, weaker trade winds, increased mid-level moisture, and all-around a more conducive environment for above-average activity.
Early/pre-season activity
The season's first tropical cyclone, Arlene, developed on April 19, becoming only the second April tropical storm on record. Above-average activity continued throughout June and July with the formations of tropical storms Bret, Cindy, Don, and Emily, along with Tropical Depression Four. These were, however, mostly weak and short-lived storms.
August yielded four storms—Franklin, Gert, Harvey, and Irma—all of which intensified into hurricanes. Harvey attained Category 4 status prior to reaching the Texas coastline, ending the record streak of 4,323 days without a major hurricane landfall in the United States; with damage estimates up to $125 billion, Harvey is tied with Hurricane Katrina as the costliest natural disaster on record in the United States. Hurricane Irma reached peak winds of 180 mph (285 km/h), making it the strongest Atlantic hurricane outside the Gulf of Mexico and Caribbean Sea until surpassed by Hurricane Dorian in 2019. Irma ravaged the northern Leeward Islands and produced a trail of destruction through the Greater Antilles and Southeast United States. Harvey and Irma struck the continental United States as Category 4 hurricanes in the span of two weeks; this marks the first time the country has suffered two landfalls of such intensity during the same hurricane season.
Peak to late-season activity
September featured copious activity, with four hurricanes forming: Jose, Katia, Lee, and Maria, the final of which became the tenth-most-intense Atlantic hurricane on record. As Irma also persisted into September and reached Category 5 intensity, it and Maria marked the first recorded instance of two Category 5 hurricanes occurring in the same month. The season became only the sixth at the time to feature at least two Category 5 hurricanes, after 1932, 1933, 1961, 2005, and 2007. With Maria becoming the first Category 5 hurricane on record to strike Dominica, 2017 also became the only season other than 2007 to have at least two cyclones make landfall at Category 5 intensity. Maria caused a major humanitarian crisis in Puerto Rico, resulting in nearly $91 billion in damage and a death toll that exceeded 3,000. October featured hurricanes Nate and Ophelia, as well as Tropical Storm Philippe; Nate was the worst natural disaster in Costa Rican history, as well as the fastest-moving cyclone in the Gulf of Mexico. Ophelia became the easternmost major hurricane on record in the Atlantic. Activity concluded with the formation of Tropical Storm Rina in early November, though the season did not officially end until November 30.
The seasonal activity was reflected with an accumulated cyclone energy index value of 223 units, the seventh-highest value on record in the Atlantic. Despite an above average number of storms to begin 2017, many were weak and short-lived, resulting in the lowest ACE value for a season's first five named storms on record. However, Hurricane Irma produced the third-highest ACE value on record, 64.9 units. The season ultimately featured three storms that produced an ACE value above 40 units, the first occurrence on record. September 2017 featured more ACE than any month in recorded history in the Atlantic (surpassing September 2004), and September 8 alone produced more ACE than any other day on record. Overall, September's ACE value represented activity about three-and-a-half times more active than the 1981–2010 average for the month.
Systems
Tropical Storm Arlene
A potent extratropical cyclone formed well east of Bermuda on April 16. The cyclone moved southeast, becoming disconnected from the surrounding environment and gradually losing its frontal characteristics. Deep convection formed in bands north and east of the center by 00:00 UTC, and gradually on April 19, leading to the formation of a subtropical depression. Despite an unfavorable environment, with ocean temperatures near 68 °F (20 °C) and moderate wind shear, convection coalesced near the center and allowed the subtropical depression to become fully tropical by 00:00 UTC on April 20. It intensified into Tropical Storm Arlene six hours later. After attaining peak winds of 50 mph (85 km/h) early on April 21, the storm began to revolve counterclockwise around a larger extratropical low. The storm tracked into the cold sector of the cyclone, causing Arlene to lose tropical characteristics around 12:00 UTC on April 21. The post-tropical cyclone moved south and east, before dissipating well west-southwest of the Azores on April 22.