<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/alberto-satellite_0.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/alberto-satellite_0.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/alberto-satellite_0.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Visible satellite of Subtropical Storm Alberto in the Gulf of Mexico on May 27, 2018, the day before it made landfall in the Florida Panhandle as a tropical storm. (NOAA/NASA)

At a Glance The Atlantic hurricane season runs from June through November.

That doesn't mean tropical storms, even hurricanes, can't form in other months.

Each of the last five years, we've had at least one named storm before June 1.

This is more common than you might think, according to records dating to the 1800s.

The 2020 Atlantic hurricane season is one month away, but the past five seasons have each gotten off to an early start.

Officially, the Atlantic hurricane season runs from June 1 through Nov. 30. That time frame was selected to encompass 97% of all Atlantic tropical storms and hurricanes , according to NOAA's Hurricane Research Division.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/hur-season-graph-jan-highlight.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/hur-season-graph-jan-highlight.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/hur-season-graph-jan-highlight.jpg?v=ap&w=980&h=551 800w" > Tropical storms, hurricanes and major hurricanes in the Atlantic Basin, by month, from 1851 to 2013. The June 1 to Nov. 30 hurricane season is highlighted in the warmer colors. A few named storms are shown in other times of year in the teal-colored sections of the graph at left and right. (NOAA/AOML/HRD)

But what about the other 3%?

You can see in the NOAA graph above that a small number of tropical storms, even hurricanes, have occurred primarily in May and December, but also in every other month outside of hurricane season.

Since 2015, at least one named storm has developed before June 1 each hurricane season, some of which had impacts in the United States and elsewhere in the Atlantic Basin.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/Preseason-ATL-tracks-since-2015.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/Preseason-ATL-tracks-since-2015.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/Preseason-ATL-tracks-since-2015.jpg?v=ap&w=980&h=551 800w" > Tracks of all Atlantic named storms that have formed before June 1 in each hurricane season from 2015 through 2019. The black segments of tracks denote when each system was either a remnant low-pressure center or an area of low pressure before becoming a depression or storm. (Data: NOAA)

Last May, Subtropical Storm Andrea formed southwest of Bermuda the week before Memorial Day, but only lasted about 24 hours.

In 2018, Tropical Storm Alberto made a Memorial Day landfall along the Florida Panhandle, remained intact and took a strange track into Lower Michigan before losing its tropical characteristics.

Tropical Storm Arlene developed even earlier than Alberto and, in 2017, became only the second April Atlantic tropical storm of record.

Perhaps 2016 was the strangest early start to an Atlantic season in recent memory.

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

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/alex-hurricane-14jan16-meteosat.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/alex-hurricane-14jan16-meteosat.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/alex-hurricane-14jan16-meteosat.jpg?v=ap&w=980&h=551 800w" > A Meteosat-10 infrared satellite image shows Hurricane Alex (circled) in the eastern Atlantic Ocean on Jan. 14, 2016, at 10:12 a.m. EST. (NOAA/NCEI/GIBBS)

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

This early start 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 near Jacksonville Beach, Florida, on Memorial Day weekend 2012.

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

They've Been Happening for Decades

According to NOAA's database, 38 storms formed in the Atlantic Basin before June 1 from 1851 through 2019, a long-term average of one such early storm every four to five years.

The 2010s had the most such storms, and there has been a steady increase since the 1990s.

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

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

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/pre-june1-atl-storms-since-1851.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/pre-june1-atl-storms-since-1851.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/pre-june1-atl-storms-since-1851.jpg?v=ap&w=980&h=551 800w" > 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)

Does a pre-June 1 start mean an active hurricane season is ahead?

The short answer is probably not.

Of the 32 years with an early start to the hurricane season since 1851, only 14 had more named storms than an average season.

Other Preseason Hurricanes

Before January 2016's oddball Alex, NOAA's database lists seven 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 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 on record, coming ashore in the Florida Panhandle and killing at least 110.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/hur-amanda-track-may1863-HRD.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/hur-amanda-track-may1863-HRD.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/hur-amanda-track-may1863-HRD.jpg?v=ap&w=980&h=551 800w" > Track history of Hurricane Amanda from May 24-31, 1863, the only known May hurricane landfall in U.S. history. (NOAA HRD)

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

How Early-Season 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 sometimes stall over the ocean in late spring.

If the ocean water is 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 nearby column of air 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.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/subtropical_low.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/subtropical_low.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/subtropical_low.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > A typical setup for a subtropical storm.

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

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.