2017 Atlantic hurricane season (SM - Updated)

The 2017 Atlantic hurricane season is an ongoing event in the annual formation of tropical cyclones in the Atlantic basin. The season officially began on June 1, and will end on November 30, 2017. 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 is possible at any time of the year.

For the third consecutive year activity began early, with the formation of Tropical Storm Arlene on April 19, nearly a month and a half before the official start of the season. It is only the second named storm on record to exist in the month in April, with the first being Ana in 2003, as well as the strongest overall for the month of April. In mid-June, Tropical Storm Bret struck the island of Trinidad, which rarely gets hit by tropical cyclones. Tropical Storm Cindy struck the state of Louisiana a few days later, becoming the first tropical cyclone to strike the state since Hurricane Isaac in 2012.

Beginning this year, the National Hurricane Center has 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". Advisories on these storms contain the same content, including track forecasts and cyclone watches and warnings, as advisories on active tropical cyclones.[1] This was first demonstrated on June 18 with the designation of Potential Tropical Cyclone Two—which later developed into Tropical Storm Bret—east-southeast of the Windward Islands.[2]

Seasonal forecasts
Ahead of and during the season, several national meteorological services and scientific agencies forecast how many named storms, hurricanes and major (Category 3 or higher on the Saffir–Simpson scale) 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.[4] 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.[3]

Pre-season outlooks
The first forecast for the year was issued by TSR on December 13, 2016.[4] 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.[4] 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.[15] 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.[5] The next day, CSU released their prediction, also predicting a total of 11 named storms, 4 hurricanes, and 2 major hurricanes.[6] On April 17, The Weather Company 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.[7] 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.[8] On May 20, The Weather Company 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.[9] 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.[10] 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.[11]

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.[12] 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.[12] 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.[13]