At a Glance

The Atlantic hurricane season officially kicks off June 1.That doesn't mean tropical storms or hurricanes can't form before then.At least one named storm has formed before hurricane season each of the past five years.

The Atlantic hurricane season kicked off early for the the fifth-consecutive year Monday, when formed south of Bermuda.

Hurricane season technically runs from June through November, but there's 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.

Tracks of the six named storms that have formed before the official June 1 start of the hurricane season since 2015. (Note: Black tracks indicate those parts of the storm's lifetime during which it was either a tropical wave or remnant.)

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

(MORE: Alberto Was a Rare Midwest Tropical Cyclone)

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

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

Tropical Storm Ana made the 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.

Tracks of the 11 Atlantic Basin named storms that have formed before the official June 1 start of the hurricane season since 2000. (Note: Black tracks indicate those parts of the storm's lifetime during which it was either a tropical wave or remnant.)

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.

Distribution of when Atlantic systems first reached tropical storm or subtropical storm strength by month from 2000 through 2018.

(Data: NOAA/NHC)

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

They've Been Happening For Decades

According to NOAA's database, 35 storms have formed in the Atlantic Basin before June 1 from 1887 through 2018, a long-term average of one such early storm every three to four years.

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.

Breakdown of pre-June 1 Atlantic storms by decade. The era where full satellite surveillance of the Atlantic Basin was in place, since the mid-1960s, is highlighted by the arrow.

(Data: NOAA Historical Hurricane Tracks, NHC)

Pre-Season Hurricanes, Too

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

Before , NOAA's database lists .

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.

Track history of Hurricane Amanda from May 24-31, 1863, the only known May hurricane landfall in U.S. history.

(NOAA HRD)

A bizarre slammed into the Leeward Islands from the north.

While not officially listed in the historical database, a 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.

A typical setup for a subtropical storm.

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 , 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 , or even a hurricane, as occurred with .