June 1 is the official start of the Atlantic hurricane season, but as this decade has demonstrated a half-dozen times, the tropics can come to life early.

The June 1 to Nov. 30 dates for hurricane season were chosen to include about 97 percent of Atlantic tropical cyclones. Three percent of the Atlantic's activity has historically occurred before or after those dates.

(MORE: Hurricane Central)

Since 1950, 20 Atlantic Basin storms have developed before the start of June, according to NOAA's historical hurricane database. That's an average of roughly one storm developing before June 1 every three to four years.

Since 2012, preseason storm activity has been an outlier from the norm.

Four of the past six years have featured named storms before June 1 in the Atlantic, including 2012, 2015, 2016 and 2017. Two of those years – 2012 and 2016 – featured the genesis of two named storms before June 1.

May, with 13, has historically had the overwhelming majority of the preseason storms since 1950.

Clustered near the Southeast coast in May the last few years were tropical storms Bonnie (2016), Ana (2015), Beryl (2012) and Alberto (2012).

Bonnie, Ana and Beryl all brought direct impacts to the Southeast coast.

Beryl was the strongest tropical cyclone to make a U.S. landfall of those three, narrowly avoiding hurricane status by a mere 5 mph of maximum sustained winds. The storm swamped the 2012 Memorial Day holiday weekend in North Florida and South Georgia.

Arlene in 2017 was an April tropical storm in the North Atlantic, becoming just the second one to occur in that month since satellite observations began in the mid-1960s.

Alex in 2016 was an extreme outlier in the historical record after it became a hurricane near the Azores in January. The last hurricane that formed in the Atlantic during January was in 1938, according to NOAA's historical hurricane database.

(MORE: 2018 Hurricane Season Outlook)