How Rare are Named Storms in May?

Tropical Storm Alberto (May 20, 2012)

Although hurricane season doesn’t begin in the Atlantic Ocean until June 1, named storms sometimes form before the season starts.  It happened again this week when the National Hurricane Center named “Ana” off the southeast coast of the United States.  Tropical storms and hurricanes are rare in June so named systems in May are rarer still.  The last time a tropical storm was named before the season began was in 2012.

Ana was initially a “subtropical” storm, meaning it has characteristics of both a tropical storm and an extratropical or non-tropical storm.  A tropical storm has the potential to grow into a hurricane if the sustained winds around the center increase to at least 74 mph, while a subtropical storm does not.  If a subtropical storm becomes a hurricane, that means it’s changed enough to become a fully tropical system.  A tropical storm or hurricane is a warm core system that derives its energy from warm ocean water as opposed to other sources such as temperature gradients the way non-tropical or mid-latitude storms do.  The National Hurricane Center has since determined that Ana has transitioned from a "subtropical" to a tropical storm.

Early or late in the hurricane season, it’s common for a cold front to become stationary over the Gulf of Mexico or off the southeast coast of the United States.  Both are favored areas for tropical development close to June 1 since the water warms more quickly there.  If the front lingers long enough then an area of low pressure sometimes forms.  Such was the case with Tropical Storm Ana this week.  Ana is not expected to become an intense system but will continue to bring gusty winds and rainfall to the southeast United States from Georgia to the Carolinas and southern Virginia.  The second named storm of 2015 will be "Bill."

On May 19, 2012, Tropical Storm Alberto formed in very similar location to Ana off the South Carolina coast.  It never became an intense system but did produce rough surf and gusty winds along the coast.  2012 was a unique season since a second named storm formed before the season began when Tropical Storm Beryl was named on May 26.  Beryl dissipated on May 30 and another four named storms – including the season’s first hurricane (Chris) – developed during the month of June. It’s quite rare for six named storms to form before the end of June.  Consequently, 2012 was the last busier than average hurricane season in the Atlantic Ocean with 19 named storms – almost double the seasonal average of 10 named storms.

The first April storm on record formed in 2003, when “Subtropical” Storm Ana was (ironically) named April 20 in the Central Atlantic Ocean.  My colleagues and I on the WUSA 9 Weather Team will continue to track the current version of Ana.