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.
This water vapor satellite image shows Tropical Storm Beta over the Gulf of Mexico in September 2020. The orange arrows indicate winds in the upper level of the atmosphere, known as wind shear, which have displaced the moisture and storms within Beta to the east of the storm's center, preventing it from strengthening.
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.
Hurricane Henri fought wind shear throughout much of its lifetime, making the storm appear lopsided on Satellite. At the time this image was taken on Aug. 20, 2021, Henri was located about 400 miles (640 kilometers) southeast of Charleston, South Carolina, and was moving toward the northwest with sustained winds measured 65 miles per hour.
Wind shear can be visible to keen observers on satellite imagery. If an active tropical system looks lopsided, with the clouds on just one side of the system, that is likely due to high amounts of wind shear. In lieu of a tropical system, wind shear can be visible on water vapor satellite loops, which show the movement of water vapor in the upper atmosphere, and thus can indicate high winds present.
But, as Kottlowski mentioned, the weather pattern over the Atlantic basin is being influenced by a La Niña phase, which historically tends to shift vertical wind shear northward and out of the Caribbean.
La Niña, which translates to "little girl" in Spanish, is one phase of a three-pronged natural climate pattern that occurs across a large portion of the tropical Pacific Ocean known as the El Niño-Southern Oscillation (ENSO). ENSO's three phases are broken down into La Niña's cool phase, which amplifies the northern jet stream and weakens the southern jet stream, known as the subtropical jet stream, having downstream impacts in the United States.
If the La Niña helps to make conditions become more favorable, the Atlantic basin might be able to churn out a few more storms before the season ends, or even after the season officially ends on Nov. 30. Storms can develop well into November and have even formed as late as January,
"Don't let down your guard yet, because we still have plenty of warm water and, with us going into La Niña, that presents a unique opportunity for tropical development in the late season," Kottlowski said.
Last year, the strongest storm of the season formed in November. Hurricane Iota, which formed on Nov. 13 over the still-warm waters of the Caribbean, rapidly intensified and made landfall in Nicaragua as a Category 4 storm with winds of 155 mph. Two weeks earlier, Hurricane Eta made landfall in nearly the same spot, also as a Category 4 storm, before the storm made landfall twice in Florida as a tropical storm.
This year, the strongest storm of the season to date has been Category 4 Hurricane Sam, which put on a show of strength as it swirled over the Atlantic Ocean but did not impact land. The last storm of the season to date was Tropical Storm Victor, which spun up briefly off the coast of Africa before dissipating.
Unless conditions change soon, the 2021 Atlantic hurricane season looks to go out with a whimper instead of a bang.
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