Andrea Kicks Off Hurricane Season Early for the Fifth Consecutive Year

Hurricane season technically runs from June through November, but there's nothing magical about those dates.

At least one named storm has developed before June each hurricane season since 2015, some of which had impacts in the U.S. and elsewhere in the Atlantic Basin.

The latest one, Andrea, will not impact the United States and is likely to fizzle soon as it becomes absorbed by a cold front.

Tropical Storm Alberto made a Memorial Day landfall along the Florida Panhandle last year, remained intact and took a strange May track into Lower Michigan before losing its tropical characteristics.

(MORE: Alberto Was a Rare Midwest Tropical Cyclone)

In 2017, Arlene came even earlier than Alberto and became only the second April Atlantic tropical storm of record.

Tropical Storm Bonnie soaked the coast of the Carolinas in late-May 2016, but the weirdest part of the 2016 season was eastern Atlantic Hurricane Alex, only the second known January Atlantic hurricane. Alex eventually made landfall in the Azores as a tropical storm.

Tropical Storm Ana made the second-earliest U.S. landfall of at least a tropical storm strength on record on Mother's Day weekend in 2015 along the coast of the Carolinas.

This early start has also happened in 2012 (Alberto, then Beryl in May), 2008 (Arthur), 2007 (another Subtropical Storm Andrea) and 2003 (another Ana, this time in April). Beryl nearly became a hurricane before coming ashore on Memorial Day weekend near Jacksonville Beach, Florida.

About half of the 17 years from 2003 through 2019 had at least one pre-June 1 named storm, a total of 11 named storms during that time. The majority of these developed, meandered or made landfall along the coast from North Carolina to northeastern Florida.

What Hurricane Season Means

The Atlantic hurricane season is the most condensed of any basin in the Northern Hemisphere. The dates were selected to account for the large majority of Atlantic Basin tropical storms and hurricanes, but doesn't capture all of them.

Of the 288 Atlantic systems to become at least either tropical or subtropical storms from 2000 through 2018, roughly 3.5% of those formed before June 1 and another 1.7% formed in December, according to the National Hurricane Center.

There are statistical outliers with many phenomena, including storms before and after hurricane season.

They've Been Happening For Decades

The current decade has had the most such storms, and there has been a steady increase since the 1980s.

However, the 1950s had six such storms, the 1930s had four and there was another four pre-season storm streak from 1887 through 1890.

It's possible there were other such storms in the era before satellites – before the 1960s – that were missed by ship observations or reports from areas impacted.

Pre-Season Hurricanes, Too

A few of these storms have even become hurricanes before June 1.

Before January 2016's oddball Alex, NOAA's database lists six other Atlantic Basin hurricanes that formed before June 1.

Three of those hurricanes - Able (1951) and unnamed hurricanes in May 1908 and 1889 - made close passes to the East Coast. Alma in 1970 weakened to a tropical depression before gliding across the Southeast.

A bizarre March 1908 hurricane slammed into the Leeward Islands from the north.

While not officially listed in the historical database, a 2013 study unearthed a once-forgotten Civil War-era U.S. hurricane landfall in late May 1863.

Named Amanda in honor of a Union ship driven ashore, this was the only May U.S. hurricane landfall of record, coming ashore in the Florida Panhandle and killing at least 110.

How Early Development Usually Happens

Ocean heat content tends to be too marginal to support the development of a tropical storm or hurricane this early in the season.

Wind shear, the change in wind speed and/or direction with height, also tends to be strong before June and can rip apart tropical systems before they can organize.

But low pressure systems with cold and warm fronts can stall over the ocean sometimes in late spring.

If the ocean water is just warm enough, and winds at jet-stream level aren't too strong, thundershowers can build and persist around the center of this stalled low, slowly warming the column of air near it enough to form a subtropical storm, a mix of a tropical storm and one you'd typically see over land areas with warm and cold fronts.

If thunderstorms persist and cluster long enough, a subtropical storm can evolve into a tropical storm, as happened with Alberto in 2018, or even a hurricane, as occurred with Alex in January 2016.