PHILADELPHIA — What is normally rush hour is approaching for the Atlantic hurricane season, but so far the tropical storm traffic has been eerily unimpressive, despite all those ominous forecasts.
Rather than being hyperactive, so far this season only two hurricanes — storms with winds of at least 74 mph — have formed in the Atlantic, Caribbean Sea, and Gulf of Mexico. The average by now is three. Only five named storms, with winds of at least 39 mph, have developed. The normal is seven.
No storm worthy of a name formed between July 3 and Aug. 30, the first time that’s happened in 82 years, pointed out Colorado State University tropical storm specialist Philip Klotzbach.
What’s more, Hurricanes Earl and Danielle have come nowhere near the Atlantic or Gulf coasts.
“This has been quite the surprising hurricane season,” said Klotzbach, whose forecasts called for a more active than normal season, as did all the major outlooks.
So what’s happening?
Hurricane specialists point to the presence of ultra-dry Saharan air in the upper atmosphere, from wind-driven dust of the desert. That has put an arid damper on would-be storms in the Atlantic and Caribbean.
Would-be cyclones also have been battling “wind shear,” strong upper-level winds that shear off disturbances before they have a chance to mature into tropical storms.
One reason that forecasters foresaw a hyperactive season was the presence of cooler-than-normal sea-surface temperatures in the tropical Pacific, or La Niña. La Niña usually coincides with weaker shearing winds in the Atlantic Basin.
But part of what hasn’t been happening is mystifying, said Jason Dunion, a University of Miami meteorologist at NOAA’s Atlantic Oceanographic and Meteorological Lab.
Perhaps, he said, “something about the conditions in the Atlantic has tended to keep more of a lid on these tropical disturbances,” he said. “We’ll have to dig into this more during the offseason.”
As for why the Atlantic and Gulf coasts have been spared, pressure patterns over the North Atlantic have vectored them away from the United States, said Dunion.
West-to-east jet stream winds coming off North America have also been nudging them away, said Klotzbach.
But he said a major source of the good fortune is an obvious one: “The big reason we haven’t seen any impacts from storms yet has more to do with just the lack of overall storms.”
On average, Sept. 10 is the climatological peak of that Atlantic season, and on average four hurricanes and six named storms form during the rest of the season, which ends Nov. 30.
Dunion sees some indications that this season still very much has life. Saharan dust is a bit above normal over the Caribbean, but backing off over the Atlantic.
Sea-surface temperatures in the “main development region” of the subtropics are favorable for hurricanes, he said. Klotzbach added that shear has been lessening.
Dunion said longer-range models are showing changes in the North Atlantic circulation that could drive storms more toward the United States and Caribbean.
“We have plenty of time to see how those forecasts play out,” he said, “and we’ll also keep our eyes on the hurricane nursery over Africa.”
Late-season storms and their remnants have left their legacy in the Philadelphia region. Next month marks the 10th anniversary of the day that Sandy made landfall in New Jersey.
