The 2013 Atlantic hurricane season officially ended on Saturday. It was the least active year in 30 years. There were only 13 tropical storms, two hurricanes and no major hurricanes. The two hurricanes that formed were very weak. In an average year, there are 12 tropical storms; six or seven go on to become hurricanes and two of those reach major hurricane intensity.
A tropical storm is a rotating weather system with low pressure, strong winds and a storm center with heavy rain producing thunderstorms arranged in a spiral pattern. When the highest sustained winds in a tropical storm reach 74 mph, the storm is reclassified as a hurricane if it is over the Atlantic Ocean or a typhoon if over the Pacific Ocean. When the sustained winds reach 97 mph the storm is considered a major hurricane. Typhoons with winds above 150 mph are called super typhoons.
In late May and early June of 2013, the seasonal forecasts were predicting seven to nine hurricanes. Why was the forecast so far off? For one reason, seasonal forecasts are difficult to make. While the spring and early summer surface pressure and sea surface temperature suggested an above-average season, there were conditions that were not forecast and likely helped this hurricane season to be below average.
During the hurricane season, drier-than-normal air blew off the deserts of Africa into the tropical waters where hurricanes form. This dry air can weaken or inhibit the formation of hurricanes. In addition to this dry air, the average temperature structure was less conducive to thunderstorm development. These are likely two conditions that made this a below normal season. The coming year will bring forth research studies seeking to provide a complete explanation of this unexpected below-average season.