"Current indications are this will be another above-normal season,” said AccuWeather hurricane expert and senior meteorologist Dan Kottlowski. “This can translate into high impacts on the United States.”
AccuWeather Global Weather Center – May 27, 2021 – One named tropical system down, potentially 19 more to go? According to AccuWeather's forecast for the upcoming 2021 Atlantic hurricane season, that could certainly be the case.
As AccuWeather’s team of tropical weather experts, led by veteran meteorologist Dan Kottlowski, predicted in late March, the Atlantic once again popped to life ahead of schedule with the formation of Ana, the first named system of 2021. It's the seventh straight year that a named system has formed prior to the official June 1 start date. Ana arrived as a subtropical system early on Saturday, May 22, about 200 miles northeast of Bermuda. It transitioned into a tropical storm on May 23 and dissipated by Monday, May 24.
With one storm already in the books before Memorial Day, forecasters are expected to have their hands full once again this summer and fall following the historic 2020 season.
The 2020 Atlantic hurricane season was like no other. Not only did it produce 30 named storms, the most on record, but the United States suffered 12 direct strikes, smashing the previous record of nine from 1916. Forecasters had to turn to the rarely used Greek alphabet for only the second time in history to name tropical systems.
Kottlowski's team predicts that the 2021 Atlantic hurricane season will result in 16-20 named storms, including seven to 10 hurricanes. Of the storms projected to reach hurricane strength, three to five are predicted to become major hurricanes (Category 3 or higher storms that have maximum sustained winds of 111 mph or greater).
New research published earlier this year from University of Miami researcher Brian McNoldy foundthat there is a new normal for named tropical systems in the Atlantic based on a 30-year average from 1991 to 2020.
AccuWeather’s forecast, when compared to that 30-year average, indicates that 2021 is expected to be an above-normal season for tropical activity in the Atlantic. A normal season is considered to have 14 storms, seven hurricanes and three major hurricanes. Last year, 14 hurricanes formed, and seven of those reached the major hurricane threshold, and, upon further review, no hurricanes last year reached Category 5 force.
A recent post-season analysis conducted by the National Hurricane Center found that Zeta was a major hurricane at the time it made landfall on Oct. 28, 2020. Previously the storm was rated as a Category 2 when it moved onshore in southeastern Louisiana. The 2020 season is now tied with the 2005 season for having the highest number of major hurricanes in a single season.
However, a similar post-season report released by the NHC revealed that Hurricane Iota did not reach Category 5 force as it closed in on Nicaragua last November. The storm was downgraded to Category 4, meaning that the 2020 season, despite its record activity, did not produce a Category 5 hurricane.
In terms of the number of storms that will directly impact the mainland U.S., Puerto Rico and the U.S. Virgin Islands in 2021, three to five are expected, according to Kottlowski's team. The annual average number of direct impacts is 3.5.
“Current indications are this will be another above-normal season,” said Kottlowski, who is in his 45th year of issuing forecasts for AccuWeather. “This can translate into high impacts on the United States.”
To come up with the forecast, Kottlowski and his team examined current weather patterns then studied long-range climate models to make a determination about what will happen during the heart of the season in August, September and early October. There are several key factors that will influence the upcoming season.
La Niña or no La Niña? That is the question
One of the determinants for how the season will shape up is the El Niño Southern Oscillation (ENSO). ENSO is defined simply as a short-term climate fluctuation that is determined by warming or cooling of the waters in the central and easternPacific Ocean. When the water in the equatorial Pacific is warmer than average, an El Niño is typically declared. When the reverse is true, it’s known as a La Niña.
Since AccuWeather's initial forecast was released in late March, the National Weather Service's Climate Prediction Center declared on May 13 that La Niña had ended and ENSO-neutral conditions had returned to the tropical Pacific. ENSO-neutral conditions mean that water temperatures in this part of the Pacific are closer to average.
"Long-range models are still showing ENSO easing back into a weak La Niña pattern sometime during September or October, Kottlowski said in the days leading up to June 1. "If ENSO goes into La Niña faster than expected (like during August or early September), the season could see more than 20 storms. If ENSO were to stay neutral through October, the number of storms could be less than 20."
This image, captured around 6 p.m. CDT Monday, Sept. 14, 2020, shows Hurricane Sally slowly approaching the Gulf Coast. (NOAA/GOES-East)
But what does the water temperature in part of the Pacific have to do with the Atlantic hurricane season?
During La Niña patterns, wind shear becomes less prevalent in the atmosphere over the Atlantic. Vertical wind shear is one of the biggest inhibitors of development for tropical systems. When there is less wind shear in the atmosphere, storms can develop with less obstruction. La Niña conditions were present during the height of 2020’s prolific season.
If La Niña returns and more than 21 tropical storms develop, there could be a new first for the Atlantic. Previously, if 22 named storms formed in one season, which only happened in two hyperactive years -- 2005 and 2020 -- Greek letters were used to name tropical cyclones. The World Meteorological Organization announced in March that a supplemental name list will now be used once the designated list is exhausted.
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