Experts at Colorado State University announced the results of their first long-range forecast for the 2021 Atlantic hurricane season Thursday, calling for above-average activity. It comes on the heels of the most storm-packed season on record in 2020, during which an unprecedented 30 named storms formed.
The forecasters are calling for 17 named storms, compared with an average of 12.1, and eight hurricanes, compared with an average of 6.4.
The forecast is unwelcome news to hard-hit residents of the Gulf Coast, many of whom are still recovering from a flurry of high-end impacts in recent years, including that of Hurricane Laura last year and Michael, Harvey and others. The Bahamas, Puerto Rico, Cuba, the Virgin Islands and Central America have also suffered direct strikes from major hurricanes since 2016.
In their outlook, the Colorado State scientists are calling for four major hurricanes — up from a 30-year average of 2.7 — to swirl through the Atlantic this year; major hurricanes have winds in the Category 3 range or higher, corresponding to 111 mph or greater.
The forecast’s lead author, Philip Klotzbach, estimates a nearly 70-percent chance the Lower 48 will be struck by a major hurricane this season, up from a roughly 50/50 shot any given year.
The odds of the East Coast vs. the Gulf Coast being hit are roughly even — each about 45 percent. Their forecast says there’s a 58-percent shot that a major hurricane enters the Caribbean.
Anomalously warm sea surface temperatures, as well as the lingering effects of an easing La Niña, play roles in the forecast.
Last season, Klotzbach’s forecast was exactly the same as this year’s apropos to predicted activity, and it also called for four major hurricanes. The season wound up being about 21 percent more active than he had expected, with respect to how much energy was expended by named storms, and there were half a dozen major hurricanes.
While it’s impossible to reliably pin down the specifics of storms that may form in advance, Klotzbach’s forecasts have historically demonstrated considerable skill in identifying features that support above- or below-average seasons.
This year, he cites La Niña — a cooling of the waters in the eastern tropical Pacific Ocean — as a factor supporting a busier season. While the La Niña will probably dissipate into the summer, an El Niño pattern, during which the eastern Pacific is unusually warm, is unlikely. El Niños ordinarily spur sinking air over the Atlantic Ocean, which inhibits vertical thunderstorm development and the birth of tropical systems.
El Niños also enhance the amount of wind shear, or change of wind speed and/or direction with height, present over the Atlantic. Since El Niño conditions are not expected to coincide with the coming season, continued near- or above-average activity is likely.
Sea surface temperatures, both broadly across the Atlantic basin and in the southwest Pacific, have also started the year running a bit above average, something that’s historically been tied to more active seasons.
Things become a bit murkier when one looks to upper-level winds in the North Atlantic, which have trended stronger than typical. On the one hand, when high-altitude winds are strong, they act to tear apart fledgling systems and cut back on the odds of development. But they’re also connected to slower surface wind speeds, which reduces the amount of turbulence in the upper layer of the ocean. That in turn lets the waters heat up more, adding fuel to the seas to sustain stronger storms.
It’s an atmospheric tug-of-war that slightly hampers tropical cyclone activity, but, when combined with other mechanisms identified by Klotzbach, an above-average season is still favored
Putting it all together, Klotzbach and his team are calling for 17 named storms, five more than average, and eight hurricanes. They think nine days this season will feature one or more major hurricanes swirling through the Atlantic.
They’ve also identified the likelihood that storms in the Atlantic burn through about 150 ACE units of energy. ACE, or Accumulated Cyclone Energy, is a measure of how much energy a storm transforms into wind, and it’s a good indicator of a season’s activity. Stronger storms tally up more ACE, while weaker storms hardly net any. The average for a season is 106 units.
Despite the worrying forecast, it’s important to remember that it’s only April, and not a single storm has formed. Right now, it’s all about probabilities.
“The probability of landfall for any one location along the coast is very low and reflects the fact that, in any one season, most U.S. coastal areas will not feel the effects of a hurricane no matter how active the individual season is,” the report states.
That said, Klotzbach’s predictions underscore the importance of advanced preparation, and authorities urge being ready long before a storm strikes.
“Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much activity is predicted,” the report says.
The Colorado State forecast is similar to the outlook from AccuWeather, which is calling for about the same number of named storms, hurricanes and major hurricanes.