The National Oceanic and Atmospheric Administration has published its forecast for the upcoming hurricane season, which begins June 1 and ends Nov. 30. The agency predicts 13 to 20 named storms, seven to 10 hurricanes and three to six major hurricanes. Based on a median value of 13 named storms, the season’s overall severity is predicted to be near average.
“We expect this season to be above average in total (20 named storms), and above average in strength with a few storms being powerful hurricanes and several impacts,” said Gerry Bell, Ph.D., NOAA’s lead seasonal hurricane forecaster.
The forecast comes as research confirms that just 2 of last year’s 53 named storms were strengthened to a Category 3 or above with sustained winds of at least 111 mph. Hurricanes Irma and Maria alone caused an estimated $265 billion in damage.
NOAA defines a Category 3 as a storm with wind speeds greater than 105 mph, and a Category 4 storm with winds greater than 157 mph. A Category 5 hurricane (winds at least 185 mph) is categorized as a dangerous major hurricane.
The Atlantic hurricane season is different from other regions in the Americas, which include the Caribbean Sea and Pacific Ocean, due to the larger atmospheric features, deep water and higher-than-average ocean temperatures that are characteristic of the Atlantic basin. These areas are known as the higher atmosphere. The high winds in the high atmosphere serve as a key cause of hurricane formation. They tend to be persistent, often resulting in a clustering of storms that are continuously rotating, allowing them to cruise north and west as they rise in the atmosphere.
The models used in NOAA’s forecast are similar to those used in the hurricane-prediction models used by the National Hurricane Center, where forecasters use a similar mix of a low-end model, a high-end model and computer models with different ratings of strength.
The NOAA seasonal outlook uses an average storminess value of 15 named storms, eight hurricanes and four major hurricanes over a five-year period. For the past three seasons, the average for the hurricane season was 17 named storms, 10 hurricanes and six major hurricanes. For the average five-year period, the average is 14 named storms, eight hurricanes and four major hurricanes.
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But this past season saw the most storms on record, with season-end storm count close to the average value. The forecast aims to account for a range of possible outcomes and helps produce an estimate of near-normal-to-above-normal-season value for the Atlantic region. This is the first year that NOAA’s outlook covers both the North Atlantic and the subtropical Pacific.
“We are basing our outlook for the season on solid science about hurricane intensity, formation and impacts,” said Gerry Bell, Ph.D., lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center. “Forecasting hurricane activity is a constant challenge because there are so many forces that shape the atmosphere and ocean, which are constantly shifting. So we are trying to create our best view possible of what could happen in any particular season and make it timely and clear to people who need to make decisions.”
Climate change is amplifying these same forces and is expected to increase the intensity of storms in the Atlantic. It is also increasing the likelihood that tropical storms and hurricanes will create or strengthen a severe hurricane or two. NOAA has been studying the relationship between climate and hurricanes and where the potentially damaging effects will be felt. Hurricanes and tropical storms are storm systems that form in the Atlantic Ocean and the Gulf of Mexico. They occur with the presence of warm water, rising temperatures in the atmosphere and wind shear, a westerly or often counter-clockwise flow of wind from the seabed to the surface of the sea. Wind shear is a powerful wind that blows at speeds between 35 mph and 73 mph. Storms are usually preceded by warm water, high sea surface temperatures and easterly winds in the upper atmosphere.
In November, NOAA scientists were able to correlate the intensity and central pressure trends of the most intense storms to greenhouse gases. In the past 40 years, greenhouse gases, including carbon dioxide, methane and nitrous oxide, have increased about 23 percent. These gases can cause further warming and increase the magnitude of extreme weather events such as hurricanes and typhoons.
Those most at risk from hurricane season include people living in the U.S. mainland, coastal residents in Caribbean islands, and coastal residents in the East and Gulf coasts of the United States.