Storm activity tends to peak in September, as water warms and wind shear fades
Experts anticipated an above-normal 2017 hurricane season in the Atlantic
Amid what feels like a never-ending barrage of storms, this year’s hurricane season still has weeks to go before emerging from its “most active and dangerous” period.
With Hurricanes Jose and Maria churning in the Atlantic – on the heels of Harvey, Irma and Katia – the season marked its statistical peak on September 10. Storm activity usually ramps up during the eight weeks that surround that date as key environmental factors align to fuel especially powerful storms, according to the National Oceanic and Atmospheric Administration, which considers the period a hurricane “season within the season.”
“There’s nothing magical about the September 10 date,” CNN meteorologist Brandon Miller said. “The peak of hurricane season really runs from the middle of August to the middle of October. But we really see the best of the conditions in early September, when the combination of ocean temperatures that have been baked by the long summer sun and the lack of wind shear that we commonly find in the late spring and early autumn leads to the greatest number of storms.”
The full hurricane season lasts from June 1 to November 30. And until a few weeks ago, this season had been quite ordinary.
Then Harvey came along.
Now, “potentially catastrophic” Maria is barreling toward Puerto Rico, while Jose could bring “dangerous surf and rip currents” this week to the US East Coast, and the remnants of the storm known as Lee threaten to redevelop. Meantime, residents of Florida and southeast Texas are digging in for a long recovery from Irma and Harvey, respectively.
Beyond these, more storms could form in the coming weeks, said Phil Klotzbach, research scientist at the Department of Atmospheric Science at Colorado State University.
“Hopefully, the rest of the storms will leave everyone alone. Pretty much everyone at this point is very done with hurricane season,” he said.
Hurricane activity tends to drop off by early October, but it’s possible to get a storm in November, Klotzbach said. Only 60 hurricanes have formed in November since 1851, compared with 205 October hurricanes, NOAA reports.
The last hurricane to make landfall in November was Otto last year.
Before Harvey and Irma, the United States hadn’t been hit by a major hurricane – a Category 3 or higher – since Hurricane Wilma in 2005.
“We went from hurricanes being a non-issue to hurricanes being a daily event,” Klotzbach said.
El Niño absence helps fuel storms
Warm sea surface temperatures and a no-show El Niño were expected contribute to an above-normal hurricane season this year in the Atlantic, NOAA said last month.
El Niño is a naturally occurring phenomenon characterized by warmer-than-normal water in the eastern Pacific equatorial region. While it occurs in the Pacific, El Niño has global impact, including increased wind shear across the tropical Atlantic, which creates hostile conditions for tropical storm development.
With El Niño failing to take shape, there was a 62% chance of a major hurricane making landfall in the United States this season, Colorado State forecasters had said. The average risk is 52%.
“The wind and air patterns in the area of the tropical Atlantic and Caribbean, where many storms develop, are very conducive to an above-normal season,” Gerry Bell, lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center, told CNN last month.
Climate change’s role hard to peg
The flurry of hurricanes also raises questions about whether climate change is playing a role.
But that’s hard to measure, Klotzbach said.
“If you look at storm activity over the last 50 to 60 years, there’s no trend in the Atlantic,” he said. “The Atlantic has these big broad ups and downs where you get a whole pile of storms. Trying to find climate change signals is pretty hard.”
Warmer water could be a key ingredient for a hurricane, but in these cases, the water may have warmed regardless of climate change, Klotzbach said, adding that the most direct ways climate change influences hurricane landfalls can come in the form of increased storm surge as a result of sea level rise.
Indeed, the consensus among scientists is that the effects of climate change, including rising sea levels and warmer oceans, made Harvey and Irma far more destructive than they would have been in previous decades, though conclusive data and analysis could take years to compile.
“The short version is, climate change makes these very bad storms worse,” said Sean Sublette, a meteorologist with Climate Central, a nonprofit group that studies climate change. “It’s not the proximate cause of the storm, but it makes these bad storms worse. And in the case of a really bad storm, climate change can make it totally disastrous or catastrophic.”
‘Extremely active’ season predicted
This hurricane season isn’t on pace to be the most active year for storms.
The 2005 Atlantic hurricane season – the year of Katrina – blew away records with 28 named storms, of which 15 were hurricanes.
Although 2017 may not set a new standard, NOAA had warned that this season had the “potential to be extremely active.” So far this year in the Atlantic, there have been six tropical storms and seven hurricanes, including Harvey, Irma, Jose, Katia and Maria.
NOAA in August predicted 14 to 19 named storms, five to nine hurricanes, and two to five major hurricanes for the season.
This season had more than the usual number of storms with 17 named storms, 10 hurricanes and six major hurricanes. In comparison, the average is 12 named storms, six hurricanes and two major hurricanes.
CNN meteorologist Taylor Ward and CNN’s Michelle Krupa contributed to this report.