The Atlantic hurricane season has begun, as of June 1st. This season occurs for six months, ending on November 30th. It’s this time period when the environment is most conducive to produce tropical cyclones. There are several factors that affect the development of hurricane season: wind shear, sea surface temperatures, and dry air. During the middle of hurricane season, these three factors typically reach their most favorable status. Wind shear is low, ocean temperatures are warm, and the air is moist, thus making for a perfect environment for tropical cyclones to form and possibly rapidly develop.
In the below chart, courtesy of Michael Lowry who is an atmospheric scientist, the most active time for tropical cyclone activity is the first half of September. This coincides with the lowest of wind shear and the warmest of sea surface temperatures.
— Michael Lowry (@MichaelRLowry) May 8, 2018
Despite an early start of this year’s hurricane season with the formation of Subtropical Storm Alberto during the Memorial Day Weekend, the first named storm in the Atlantic forms on July 9th, on average. The first hurricane develops about a month later, typically on August 10th. It really takes a perfect environment for a major hurricane (category three or higher) to be produced. Therefore, the first major hurricane normally forms on September 4th, which is just before the climatological peak of the Atlantic hurricane season.
It is interesting based on recent times when comparing it to climatology. Alberto, the first named storm of the Atlantic hurricane season of 2018, was declared on May 25th. Out of the last seven years (including 2018), five of them have featured a named storm before June 1st, the start of hurricane season, and over a month before the typical first named storm. Furthermore, four of the last seven years have had tropical storm landfall or influence on the continental U.S. before June. Despite these early-season storms, there is no correlation to how active the season ends up becoming. Some seasons have been relatively quiet while others have been active.
In some years, it’s these early-season storms that can be the most impactful. This is because they form close to home, and in many cases they have directly impacted the United States. In June, the most dense of tropical cyclone origins is located over the Gulf of Mexico or off the Southeast U.S. coast. As the season progresses, the origins tend to become more widespread and eventually reach the MDR, or Main Development Region. The MDR is the area where typically the most intense tropical cyclones form, and is found between 20° and 80° W and 10° and 20° N, or from the central Caribbean eastward to the western African coast. By September, the most active time for hurricane season, at least on average, much of the western Atlantic Ocean is in play for tropical cyclone origins. Storms this time of the year tend to form over the western Caribbean Sea and track into the Gulf of Mexico, over the MDR and track toward the Southeast U.S or track out to sea, or over the Gulf of Mexico and track into the United States.
Based on this year’s forecast, the Atlantic hurricane season can expect near-average activity. It only takes one storm to define a season, however. The best odds for an intense, landfalling hurricane will be in September at the climatological peak of the season.