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11 May 15 18:30

After the recent forecast for a quiet Atlantic hurricane season in 2015, some might have been surprised to see Tropical Storm Ana form before the official June 1 hurricane season start.

Ana formed from a swirl of clouds off the coast of the Carolinas overnight May 7th. Ana was initially designated a subtropical storm, which has characteristics of both non-tropical weather systems and tropical weather systems. During the early morning hours of May 9th, the National Hurricane Center saw enough characteristics to classify Ana as a tropical storm; Ana also reached its peak intensity at this time, with maximum sustained winds of 60 mph. Tropical Storm Ana then went onto made landfall at 6 AM EDT on May 10th in South Carolina, near Myrtle Beach, with maximum sustained winds of 45 mph and a central pressure of 1002 mb. The main impacts from Ana were heavy surf, beach erosion and flooding rains along the immediate Carolina coast; there are no reports of any significant damage.

Naturally, two questions arise: First, is it normal to have tropical cyclones form in May? Second, does Ana's early formation contradict pre-season forecasts and imply an active hurricane season ahead? 

The answers to the questions are yes and no, respectively.  Mother Nature does not see calendars; a tropical cyclone can form any time of year when the conditions are favorable. Climatologically speaking, these conditions are present in the summer and autumn months in the Northern Hemisphere, hence the reason hurricane season is defined as June through November in the Atlantic. There are 39 pre-season storms in the official hurricane database, HURDAT, which archives hurricanes back to 1851. Since the 1960s, the beginning of the satellite era, a tropical cyclone has formed before June 1 approximately every other year. Just three years ago, we observed not one, but two tropical cyclones in May, Alberto and Beryl. Beryl went onto make landfall near Jacksonville, FL and was the strongest recorded US landfall of a tropical cyclone outside of hurricane season, with winds of 65 mph as it moved onshore. In 2003, Tropical Storm Ana (no relation to this storm) formed in late April, reaching a peak intensity of 60mph on April 22nd. In 1952, an unnamed tropical storm made landfall in Florida in early February, gaining the name, "The Groundhog Day storm." And while 2005 didn't get off to an early start, tropical storms formed well beyond the climatological end of hurricane season, with Tropical Storm Zeta forming in December, and Hurricane Epsilon reaching hurricane intensity in December. Tropical Storm Zeta even rung in 2006, finally dissipating on January7th.

Perhaps the most interesting case of a pre-season storm is Hurricane "Amanda," a recently rediscovered storm that made landfall in May 1863. With the country embroiled in the Civil War, and the Gulf Coast squarely in the Confederacy, accessible weather records from the time are sparse. Recent analysis published in the Bulletin of the American Meteorological Society found that a hurricane with wind speeds of 105 mph made landfall in the northwest Florida Panhandle on May 28th, killing over 100 people, destroying homes and farms and driving the Union ship, "Amanda," ashore. Hurricane "Amanda," is now the only known hurricane to make landfall in the US in the month of May, and if and when "Amanda," is included in the official HURDAT database, will become the strongest US landfalling tropical cyclone outside of hurricane season.

Although Ana formed before June 1, it isn't a harbinger of things to come. Looking back through the historical record, only 38% of seasons that contained storms that formed before June 1 had above average activity.  However, Ana does serve as an important reminder that hurricane season is almost upon us, and all coastal residents must be aware and prepared.


Category: Climate/natural disasters: Disaster risk, Floods/storms

Location: Myrtle Beach, SC, United States


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