Echoing ominous outlooks issued earlier in the spring by other meteorologists, the National Oceanic and Atmospheric Administration said Thursday that odds favor above-average numbers of tropical storms and hurricanes in the basin, which includes the Caribbean and the Gulf of Mexico.
NOAA is calling for 13 to 20 named storms, those with winds of 39 mph or higher, with six to 10 of those gaining winds of at least 74 mph to qualify as hurricanes. In addition, it sees three to five “major” ones, with winds of 111 mph or better. The averages are 14 named storms, seven hurricanes, and three majors.
The season doesn’t start officially until June 1, but the National Hurricane Center says that in all likelihood a first named storm, Ana, will form near Bermuda on Friday.
An early start notwithstanding, the 2021 season is not expected to be quite as active as 2020′s, which produced a record number of named storms — including several that battered the Gulf Coast and two that were quite disruptive in the Philadelphia region.
But this one looms as another installment in a “high-activity era” that dates to 1995, said Matthew Rosencrans, long-range hurricane forecaster at the government’s Climate Prediction Center. In fact, the uptick led the government this year to bump up the annual averages from 12 named storms and six hurricanes.
A warming climate evidently has made storms juicier, increasing precipitation levels by about 3%, Rosencrans said, and rising seas have increased flooding threats. It also has been linked to hurricane intensities, he noted. However, he attributed the brisk storm traffic primarily to ponderous, cyclical changes in the Atlantic, he said.
“Climate change does not have a direct impact on the numbers of named storms,” he said. He attributed last year’s record number — 30 — to enhanced detection powers of satellite observations.
He said that sea surface temperatures in the hurricane breeding zone of the Atlantic have fallen slightly from last year’s, from 1 degree Fahrenheit, to 0.7, but they still would be favorable for storm development.
About those cycles
Rosencrans said the warmer waters would be symptomatic of a mouthful known as the Atlantic Multidecadal Oscillation, or AMO.
Historically, active and less active hurricane periods have occurred in 25- to 40-year cycles, depending on whether the AMO is in its positive — which favors active seasons — or negative phase. You can guess what phase it’s been in since 1995, an era that has included the deadly and devastating 2005 season.
In addition to the warm waters, other aspects of the positive phase include weakened trade winds and enhanced monsoon seasons in western Africa, which is near fertile hurricane spawning grounds.
“The data shows a clear oscillation going back to the 1800s,” Rosencrans said.
Another factor arguing for an active 2021 season is the state of the tropical Pacific, which won’t be interfering with Atlantic storms.
When sea surface temperatures out that way are above normal, they can interact with the overlying air in such a way as to generate west-to-east winds that can rip apart Atlantic storms. However, they are forecast to remain average, and might even enter a cool phase, which could increase the Atlantic storm numbers.
The NOAA forecast tracks quite closely with earlier outlooks.
At Colorado State University, a pioneer in long-range hurricane outlooks, forecaster Phil Klotzback is calling for 17 named storms, with eight of those becoming hurricanes, during the June 1-to-Nov. 30 season.
That’s in sync with the forecast by AccuWeather Inc. — 16 to 20 named storms, seven to 10 hurricanes, three to five majors. The WeatherBell Analytics’ forecast sees 16 to 22 tropical storms, nine to 13 hurricanes, and three to six Category 3s.
In short, should the 2021 Atlantic hurricane season turn out to be an unusually quiet one, some of the best minds in tropical meteorology will rediscover that the atmosphere still is guarding some of its secrets.