Forecasters with the National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center raised the likelihood for a below-normal season in last week’s update to the Atlantic Hurricane Season Outlook.
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| Image from NOAA |
The update predicts a 70% chance of a below-normal season, a 25% chance of a near-normal season and only a 5% chance of an above-normal season. The probabilities in the initial outlook issued on 22 May were 50%, 40% and 10%, respectively.
“We are more confident that a below-normal season will occur because atmospheric and oceanic conditions that suppress cyclone formation have developed and will persist through the season,” says Gerry Bell, Ph.D., lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center, a division of the National Weather Service. “Nonetheless, tropical storms and hurricanes can strike the U.S. during below-normal seasons, as we have already seen this year when Arthur made landfall in North Carolina as a category-2 hurricane. We urge everyone to remain prepared and be on alert throughout the season.”
The primary factors influencing the increased chance of a below-normal season are:
· Overall atmospheric conditions are not favorable for storm development. This includes strong vertical wind shear, a weaker West African monsoon, and the combination of increased atmospheric stability and sinking motion. These conditions mean fewer tropical systems are spawned off the African coast, and those that do form are less likely to become hurricanes. These conditions are stronger than originally predicted in May and are expected to last mid-August through October, the peak months of the hurricane season;
· Overall oceanic conditions are not favorable for storm development. This includes below-average temperatures across the Tropical Atlantic, which are exceptionally cool relative to the remainder of the global Tropics. This cooling is even stronger than models predicted in May and is expected to persist through the hurricane season; and
· El Niño is still likely to develop and to suppress storm development by increasing vertical wind shear, stability and sinking motion in the atmosphere.
The updated hurricane season outlook, which includes the activity to-date of hurricanes Arthur and Bertha, predicts a 70% chance of the following ranges: seven to 12 named storms (top winds of 39 mph or higher), including three to six hurricanes (top winds of 74 mph or higher), of which zero to two could become major hurricanes (Category 3, 4, 5; winds of at least 111 mph).
The Atlantic hurricane region comprises the North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea.
Read more:
EIA predicts lower risk from hurricanes
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