Since 1965, the Atlantic hurricane season has been defined to be between June 1 and November 30.

That could change soon if the recent trend of pre-season Atlantic tropical cyclone activity continues. The month of May has had 10 subtropical or tropical cyclone formations since 2007, 9 of which became named storms. None of these storms have reached hurricane strength, and there has not been a May hurricane recorded in the Atlantic basin since Alma in 1970. Since 2012, 7 named storms (6 of which were fully tropical) developed in the period between May 15 and May 31. The Atlantic has had a May named storm develop in five of the last six hurricane seasons, and four of them made landfall in the United States. In fact, 2012 and 2020 both had two named storms form in the second half of May. The document attributes at least 20 deaths to these May systems.

It should be noted, however, that from 1982 to 2006, not one May named storm formed in the Atlantic basin, so it is possible that this recent pre-season activity could be a statistical anomaly that could end soon. Another example is the anomalous December activity in the Atlantic basin between 1998 and 2007; during this time six of 10 Atlantic hurricane seasons ended after the November 30 official end date. Since then, only one unnamed subtropical storm in 2013 has developed or persisted into the month of December.

The World Meteorological Organization recently published a document that proposed plans to start routine Tropical Weather Outlooks for the Atlantic basin on May 15 in 2021. Prior to this year, routine issuance of the product did not begin until 2:00 a.m. EDT on June 1, right in line with the official start of the season. It should be made clear that the Atlantic hurricane season start date will remain June 1 at this time, but routine Tropical Weather Outlooks will not be much additional work for the National Hurricane Center (NHC). This would not require a significant increase in workload for the NHC, as the Eastern Pacific hurricane season already begins on May 15. The document also proposed that the NHC develop quantitative criteria to determine whether to change the official boundaries of hurricane season, and present the findings at a future World Meteorological Organization meeting. The World Meteorological Organization plans to meet virtually from March 15-17, 2021, to discuss the past hurricane season and potentially retire names from the 2019 and 2020 seasons.

In a May 2020 interview with USA Today, National Hurricane Center spokesman Dennis Feltgen said [The hurricane center] “is weighing the potential advantages and disadvantages of changing the official start date of the Atlantic hurricane season based on the possibility that the recent uptick in late May storms will continue.”

If the Atlantic hurricane season boundaries are changed, this would not be the first time. In the early 1940s, when seasonal bounds were first proposed, the season was considered to begin on June 16 and end on October 31. In the late 1940s, the season’s official start date was moved up one day to June 15, and the official end date was pushed back to November 15. In 1965, the season start date was moved up to June 1, and the season end date was pushed back to November 30, which remains the official Atlantic hurricane season boundaries to this date.

I would not expect an official announcement from the National Hurricane Center about routine Atlantic Tropical Weather Outlook issuance beginning on May 15 until after the World Meteorological Organization meeting.