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While the Atlantic hurricane season does not officially end until Nov. 30, AccuWeather forecasters believe that the odds of any additional tropical storm formation in the near future are low. After a frenetic pace around the peak of hurricane season, there is now just one name left on the 2021 list of storm names: Wanda. Might that name go unused?
The 2021 Atlantic hurricane season got off to a record-fast start, with five storms forming by July 1, surpassing a record set just a year ago. The season continued at a fast pace, with development kicking off again in mid-August and continuing through mid-September. By the end of the period of rapid activity, eight storms had made landfall in the United States. But now, the tropics sit dormant.
“We’re going into a situation where there are less tropical features coming off Africa, which before October usually are the instigators in the development of tropical storms and hurricanes,” AccuWeather Hurricane Expert Dan Kottlowski said.
Tropical waves that emerge off the coast of Africa are usually the source of tropical storm development during the peak of hurricane season. According to Kottlowski, 85% of all tropical storm development can trace its origins to tropical waves.
Hurricane Ida, the strongest storm to make landfall in the United States this year, was born from a tropical wave that began to organize in the southern Caribbean Sea, just off the coast of South America. Ida rapidly intensified before making landfall in Louisiana. Even after losing wind intensity, the storm went on to devastate parts of the Northeast as a tropical rainstorm.
Hurricane Henri, which formed off the Northeast coast, had its unusual origin in a complex of thunderstorms that tracked across the United States nearly two weeks before its landfall in Rhode Island.
But, around October, the number of tropical waves usually decreases, with storms tending to originate in the Caribbean Sea.
Hurricane Nicholas, which formed this year in September, had its origin in a fashion more typical of October tropical development, forming in the Bay of Campeche before making landfall in Texas as a Category 1 hurricane.
However, wind shear over the Caribbean Sea is greatly depressing the chance of any further tropical development this year.
“When you have strong winds, what we call wind shear, those strong winds will weaken tropical systems, and that’s why it’s been so quiet since Sept. 15, because we’ve had a tremendous amount of wind shear in the Atlantic basin,” said AccuWeather Chief Broadcast Meteorologist Bernie Rayno.
Vertical wind shear, the most impactful when it comes to tropical systems, is the change in direction and speed of winds at increasing heights in the atmosphere.
When strong vertical wind shear is present, the top of a tropical storm or hurricane can be blown hundreds of miles downstream. In this case, the storm can become very lopsided or tilted and begin to unwind as dry air makes its way into the system or the flow of warm, moist air into the entire storm is disrupted. Strong wind shear can prevent a storm from forming in the first place.
“We are going into a La Niña,” Kottlowski said, referring to the climatological phenomenon in which sea surface temperatures in the equatorial Pacific Ocean drop below average and can have far-reaching effects on weather elsewhere in the world. “And typically when you go into a La Niña, that favors keeping the vertical wind shear to the north and that’s not happening just yet, the vertical wind shear has been deep into the tropics over the last several days, and that’s the reason we really have not seen real robust patterns to create the opportunity for tropical development,” he added. NOAA on Thursday announced that the La Niña phase had officially developed and that meteorologists expect it to remain into spring of next year.