With Hurricane Florence flaring into a fierce Cat 4 hurricane Wednesday evening, and two waves hot on its heels, the deep tropics kicked into high gear right on schedule this week.
The first 10 days of September typically generate the highest number of storms for the season, when conditions become most favorable off the African coast where many storms are born, said National Hurricane Center specialist Jack Beven. Last year, Irma and Maria, brutal hurricanes blamed for nearly 3,000 deaths, formed in the fertile region.
On Wednesday, Florence spun into the season’s first major hurricane, with sustained winds reaching 130 mph at 5 p.m., less than a day after Tropical Storm Gordon rolled ashore in Mississippi.
A wave near the Cape Verde Islands off Africa is also expected to become a depression or tropical storm in the next two to five days, with another wave behind it.
“That’s not the only place it gets favorable, but it’s where you notice it the most,” Beven said.
Case in point: Gordon. The system came out of the Caribbean and quickly intensified, ahead of schedule, into a tropical storm Monday morning just west of Key Largo.
While the uptick is usual for the season, what has been slightly unusual so far, Beven said, is where storms have formed. Four out of the five, not counting Gordon, appeared at higher latitudes, outside the hurricane lane that funnels systems toward Florida and the U.S. coast, Beven said. That could be because an El Niño has been brewing in the Pacific. The weather pattern is linked to warming Pacific waters and generates upper atmospheric wind shear to help smother storms.
“Sometimes in years that are supposed to be El Niños or developing El Niños, we see that kind of distribution, but it’s not a perfect correlation between the two,” he said.
In August, the National Oceanic and Atmospheric Administration put the odds of an El Niño forming this month at 62 percent, said NOAA lead forecaster Gerry Bell. One hasn’t formed yet, so isn’t playing a part in the season, he said, but could develop in the coming months. NOAA plans to issue an update for the El Niño forecast Sept. 13, he said.
“If it’s developing during October, November, it could suppress the latter part of the hurricane season,” he said. “But we’re in the peak of the season right now, so people need to stay prepared.”
During the first half of the season, Saharan dust blowing off the African coast and cooler temperatures on the ocean surface kept storms from intensifying, he said. But in late August, the dust diminished and waters warmed.
As it heads west over the next day, Florence is expected to encounter strong shear that should begin to weaken it, hurricane forecasters said in a 5 p.m. advisory on Wednesday. But after two days, the shear should subside, allowing the storm to regain intensity. It’s also too soon to say if Florence will pose a threat to the U.S. coast — computer models are so far split and producing a higher than usual amount of uncertainty, Beven said.
But the two waves could mean trouble if history repeats itself. Last year, the Saharan dust settled around Aug. 20. On Aug. 30, Irma became a tropical storm about 420 miles west of the Cape Verde Islands. The next day it rapidly intensified into a Cat 2 storm before making landfall in the Keys Sept. 10 as a fierce Cat 4 hurricane.
Of the two systems lingering near Africa, forecasters gave the westernmost wave, which became better organized overnight, a 90 percent chance of forming over the next five days. The second system is expected to move off the coast as a wave over the next few days and has a 30 percent chance of forming over five days.
“Sometimes it’s like you open a window and you go from nothing to a week later and lots of tropical storms and hurricanes running around,” Beven said. “Then in the latter part of September, the area between Africa and the Lesser Antilles begins to shut down and by October the emphasis shifts back to the Caribbean. There’s a certain rhythm to this you see every year.”