As the traditional peak of the Atlantic hurricane season nears, forecasters have slightly increased the number of storms they expect to see this year. While the figures still suggest near-normal activity, they would mark the busiest season since 2012 if they pan out. The uptick in the forecast comes courtesy of shifts in the large-scale conditionsMore
Every year on Aug. 20, Bill Gray, the Colorado State University professor who pioneered seasonal hurricane forecasting, would ring a bell to mark the start of the most active part of the Atlantic hurricane season.
Gray died in April at the age of 86, but his protégé, Phil Klotzbach, remembered the tradition in a recent Twitter post.
With his bell ringing, Gray was signalling the tendency to get complacent during the first, relatively quiet months of the hurricane season. Even though the season begins on June 1, it usually doesn’t really heat up until mid-August, with the peak of activity running to mid-October.
Almost on cue, the tropical Atlantic has started to bubble into action. Although Tropical Storm Fiona faded earlier this week, Tropical Storm Gaston continues to churn in the North Atlantic.
Of most concern on Thursday is a tropical wave just north of the Dominican Republic which is heading toward the Bahamas. Conditions for development are expected to improve as the weekend nears, but it still has a long way to go to reach tropical storm status.
Beyond the Bahamas, the steering winds suggest that South Florida and other areas along the Gulf coast need to monitor the system, especially because the water temperatures in the Bahamas and the Gulf are extremely warm, between 86° and 90°F (30° and 32°C).
Air Force hurricane reconnaissance will be gathering data from the system today, and if it becomes strong enough to name, it will be called Hermine.
Late summer and early fall is normally the time when the ocean is the warmest and wind shear in the atmosphere is lightest. Warm water is the energy source for hurricanes. Wind shear is the changing of wind speed and direction vertically through the atmosphere, so when there is lot of shear, a hurricane has difficulty maintaining its core structure. In effect, high wind shear tears the circulation of a hurricane apart.
With last year’s El Niño weakening rapidly during the last few months, both Colorado State and the National Oceanic and Atmospheric Administration raised the number of tropical cyclones (the general term for tropical storms and hurricanes) they expect to form in their seasonal outlooks earlier this month.
Studies that examine the effect of climate change on tropical cyclones are mixed. According to the National Climate Assessment, the frequency of the strongest hurricanes in the Atlantic, the Category 4 and 5 storms, has increased since the 1980s.
But another measurement of tropical cyclone activity, the Accumulated Cyclone Energy (ACE) Index, has decreased in the Atlantic since a peak in 2005, when the basin had its busiest season on record. The ACE considers the number, strength, and duration of tropical cyclones in a season.
This paradoxical tendency is in line with projections from the most recent Intergovernmental Panel on Climate Change report, which indicates a likely increase in tropical cyclone wind speed and rainfall rates, while their global frequency may remain unchanged or even decrease.
The increase in rainfall rates and wind speed, coupled with sea level rise, suggests more damage from storm surge flooding in the decades ahead. The number of days with minor flooding is already dramatically on the rise, and with storm surge superimposed on the rising sea level, the flooding threat is multiplied.
The last major hurricane to hit the United States, one that was rated Category 3 or higher, was Wilma in 2005. But as Sandy reminded us in 2012, it does not take a major hurricane to do major damage.
The clock, however, is ticking.