Outlook is for another busy season
By Skip Rigney
The preseason prognosticators have spoken. It looks like we can look forward to another exciting fall. As usual the center of attention will be in the Southeast.
Oh, did you think I was talking about college football?
No, you will have to consult with a different source to get the odds on whether the Crimson Tide of Alabama or the Tigers of Clemson, South Carolina are more likely to dominate the sports headlines in September.
Instead I am referring to the preseason outlooks for hurricane activity from June through November this year in the Atlantic Basin, which includes the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. On Thursday, April 8, Colorado State University’s (CSU) Tropical Weather and Climate Research team released their extended range forecast for the upcoming season.
Scientists at CSU pioneered seasonal hurricane prediction beginning in the 1970s. The current CSU team, led by Dr. Philip Klotzbach, predicts that, although not nearly as busy as the hyperactive 2020 season, the Atlantic Basin is likely to once again have more than the typical number of tropical storms and hurricanes.
The average number of named tropical cyclones, which comprises all tropical storms and hurricanes, was 14 per season in the Atlantic Basin based on statistics from 1991 through 2020. But, there is great variability. In 1983, there were only four. Last year there were thirty.
Klotzbach and his fellow CSU researchers predict that this summer and fall there will be about 17 named tropical cyclones in the Atlantic Basin. They cite two main reasons for expecting above-normal activity.
First are the atmospheric patterns associated with different phases of the multiyear changes in sea surface temperature (SST) in the eastern and central tropical Pacific Ocean. When SSTs in those parts of the Pacific are cooler than normal, the condition is known as La Nina.
The effects of La Nina’s cool surface waters extend high into the atmosphere resulting in weak winds above the Atlantic Basin, which in turn favors the development of tropical cyclones. That was the case during the La Nina that began late last summer.
La Nina is weakening. However, a flip to La Nina’s warm Pacific sibling, El Nino, whose associated upper wind pattern tears apart Atlantic Basin tropical storms, seems unlikely before the end of the 2021 hurricane season.
Meanwhile, SSTs in the tropical Atlantic for the past three months have been close to average. A little further north in the subtropical Atlantic they have been warmer than normal. This pattern is correlated with warmer-than-average SSTs during the hurricane season, which can provide the energy needed for above-normal tropical cyclone activity.
In addition to the scientists at CSU, three forecasting companies have also issued outlooks for the upcoming season. Weatherbell, Accuweather, and Stormgeo, also expect a higher-than-average number of storms.
But, don’t place any wagers. Despite fifty years of research, the spring outlooks of Atlantic Basin hurricane activity are not very accurate.
In fact, according to the statistics, relying on the long-term average is almost as good of a bet as the spring predictions.
Forecasters will issue another outlook in late May or June. Those early summer outlooks have proven to be considerably more accurate than the spring predictions.