Increased likelihood of below-normal Atlantic hurricane season

“Tropical storms and hurricanes can and do strike the United States, even in below-normal seasons and during El Niño events,” said Gerry Bell, Ph.D., lead seasonal hurricane forecaster with NOAA’s Climate Prediction Center. “Regardless of our call for below-normal storm activity, people along the Atlantic and Gulf coasts should remain prepared and vigilant, especially now that the peak months of the hurricane season have started.”

Two tropical storms already have struck the United States this year. Ana made landfall in South Carolina in May, and Bill made landfall in Texas in June.

The 90 percent probability of a below-normal season is the highest confidence level given by NOAA since seasonal hurricane outlooks began in 1998.

The updated outlook also lowers the overall expected storm activity this season. The outlook now includes a 70 percent chance of 6-10 named storms (from 6-11 in the initial May Outlook), of which 1-4 will become hurricanes (from 3-6 in May), and 0-1 will become major hurricanes (from 0-2 in May). These ranges — which include the three named storms to-date (Ana, Bill, and Claudette) — are centered well below the seasonal averages of 12 named storms, 6 hurricanes and three major hurricanes.

Forecasters attribute the high likelihood of a below-normal season to three primary factors:

• El Niño has strengthened as predicted, and NOAA’s latest El Niño forecast calls for a significant El Niño to continue through the remainder of the hurricane season;
   
• Atmospheric conditions typically associated with a significant El Niño, such as strong vertical wind shear and enhanced sinking motion across the tropical Atlantic and Caribbean Sea, are now present. These conditions make it difficult for storms to develop, and they are predicted to continue through the remaining four months of the hurricane season; and
   
• Tropical Atlantic sea-surface temperatures are predicted to remain below average and much cooler than the rest of the global tropics.

 The Atlantic hurricane season runs from June 1 through November 30.

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