Hurricane season is the annual period when tropical cyclones most frequently form and strengthen across specific ocean basins. Understanding when this season begins and ends is a crucial first step for coastal communities and anyone whose travel or business could be affected by these powerful weather systems. The timing is determined by the annual alignment of specific atmospheric and oceanic conditions necessary for storm development. This period marks the time when preparation and vigilance should be at their highest.
Official Start and End Dates
The official calendar dates for hurricane season are established by the National Hurricane Center (NHC), a division of the National Oceanic and Atmospheric Administration (NOAA). The length of the season is set to encompass the vast majority of historical tropical cyclone activity in each region. These dates serve as a standardized framework for government agencies and the public to focus their preparedness efforts.
The Atlantic Basin, which includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico, has an official season that runs from June 1st through November 30th. This six-month window accounts for over 97% of all tropical storm and hurricane occurrences in this region. The Eastern Pacific Basin, extending westward from the coast of Mexico and Central America, begins its season earlier, on May 15th, but also concludes on November 30th.
These official dates reflect a long-term average of when the environment becomes consistently favorable for storm formation. The peak activity for the Atlantic season occurs between mid-August and mid-October, with September 10th historically marking the most active day. The highest risk of major storms is concentrated within this two-month peak period.
Meteorological Drivers Behind the Timing
The start of the season in late spring is directly linked to the changing meteorological conditions over the oceans. Tropical cyclones require a specific set of ingredients to form, and these ingredients only reliably come together once the Northern Hemisphere warms sufficiently. The most fundamental requirement is warm sea surface temperatures (SSTs), which must generally be 80 degrees Fahrenheit or higher to provide the necessary heat and moisture for the storm’s engine.
As the year progresses, solar radiation heats the surface waters of the tropical Atlantic and Pacific, making the environment supportive of deep thunderstorm convection. Another requirement is low vertical wind shear, which is the change in wind speed or direction with height in the atmosphere. High wind shear tears apart the structure of developing storms, but it typically decreases in the summer months, allowing storms to organize vertically.
The combination of warm SSTs, low wind shear, and high atmospheric moisture levels typically aligns consistently around late May and early June, marking the beginning of the favorable period. The season ends in November as the oceans cool and wind shear increases due to stronger mid-latitude weather systems moving across the continent.
Navigating Storms Before the Official Season
Despite the official start date of June 1st for the Atlantic, named storms have been increasingly common in May in recent years. Named tropical systems have formed before the official start in about half of the past 10 to 15 seasons. This trend suggests that the historical average used to set the dates may be shifting due to modern atmospheric and oceanic conditions.
This consistent pre-season activity has led to discussions within the NHC about potentially moving the official Atlantic start date forward to May 15th, matching the Eastern Pacific season. While the official start date has not been formally changed, the NHC has already adjusted its operational schedule to reflect the earlier threat. The agency now routinely begins issuing its Tropical Weather Outlooks, which monitor areas of potential development, on May 15th instead of waiting for June 1st.
The existence of pre-season storms is a reminder that the ocean’s ability to produce a tropical cyclone is not strictly limited to the arbitrary six-month window. These early storms demonstrate that the necessary meteorological ingredients can align earlier than the historical average.
How Seasonal Forecasts Are Determined
Before the season officially begins, agencies like NOAA and academic groups issue seasonal outlooks, which are predictions for the overall activity level of the upcoming season. These forecasts estimate the expected number of named storms, hurricanes, and major hurricanes, but do not predict specific storm tracks or landfalls. These outlooks are released in late May, just before the June 1st start, and are updated throughout the summer.
The forecast models rely heavily on large-scale climate patterns that influence the environment across the Atlantic basin. The El Niño-Southern Oscillation (ENSO), with its warm El Niño and cool La Niña phases, is a primary driver. El Niño typically increases vertical wind shear across the Atlantic, which suppresses storm formation, while La Niña reduces shear, leading to more active seasons.
Another important factor is the Atlantic Multi-decadal Oscillation (AMO), a long-term fluctuation in North Atlantic sea surface temperatures that can last for decades. When the tropical North Atlantic is warmer than usual, it provides an additional boost of heat energy to fuel stronger and more numerous storms.