Hurricane season is over at long last—a lengthy one for both the Atlantic and Pacific regions. The Atlantic saw two historic hurricanes make landfall in the United States, while the eastern Pacificsaw the greatest number of scale-topping storms on record. The stories of the two ocean basins are divergent lessons in how a hurricane season can land in the record books.
The Atlantic Ocean saw fifteen named storms this year, which is a few ticks above the average twelve. Eight of those storms went on to become hurricanes, and two of them—Florence and Michael—reached major hurricane status. Hurricane Florence made landfall in North Carolina and crawled its way inland with historic rains before the storm lifted away a few days later. Hurricane Michael reached the Florida Panhandle as one of the strongest storms ever recorded at landfall in the United States, packing winds just a hair below category five status.
The Pacific Ocean’s hurricane season was a much different story. This basin saw the most intense hurricane season ever recorded. The basin saw 22 named storms between the eastern and central parts of the Pacific, stretching from the west coast of Mexico to the Hawaiian Islands. The season saw a record-breaking ten hurricanes reach category four or five on the Saffir-Simpson Hurricane Wind Scale. Despite the high number of intense storms, fortunately, relatively few of them managed to make landfall. Hurricane Lane came extremely close to making landfall in Hawaii as a strong hurricane, and Hurricane Willa did make landfall near Mazatlan, Mexico, as a major hurricane.
The best way to measure the strength of a hurricane season isn’t to count the number of storms that form, but to take into account their strength and how long they last. That’s where Accumulated Cyclone Energy comes in. This value considers a storm’s maximum sustained winds and how many hours the event lasts. Stronger storms that last longer have higher ACE values than weak storms that quickly peter out.
The Pacific hurricane season saw a total ACE of 316.3, which blows away the basin’s previous record set in 1992. For perspective, the Atlantic Ocean has never seen a season’s total ACE exceed 259, which is a testament to the absolute strength and longevity of the storms that formed in the Pacific this year.
All the right ingredients came together to allow the Pacific to churn out one storm after another. The hyperactivity was likely influenced at least in part by a developing El Niño, the warm waters of which generate thunderstorms that seed the development of tropical cyclones and also provide those storms the fuel they need to strengthen.
The Atlantic hurricane season also finished with an above-average ACE thanks in large part to just three storms. It’s much harder for storms to find the perfect conditions to grow into monsters in the Atlantic Ocean because there are more opportunities for things to go wrong. Tropical cyclones require persistent thunderstorms, warm waters, tropical air, and low wind shear. There’s usually too much—or too little—of at least one of those factors in the Atlantic, which makes it difficult for storms to develop and then sustain themselves at full strength for very long.
Most of the storms we saw in the Atlantic this year were relatively weak and short-lived. A record number of the basin’s storms were subtropical at some point during their lives—a designation for a storm that doesn’t have completely tropical characteristics, but it’s close enough to earn a name and the same treatment from the National Hurricane Center. The seven subtropical storms that formed in the Atlantic this year broke the previous record of five set back in the late 1960s.
Florence and Michael are unfortunate examples of storms breaking through an otherwise dull season. Each of these storms were able to reach their full intensity and turn into tragedies because they found a brief pocket of favorable conditions for strengthening that none of the other storms managed to encounter.
Hurricane Florence started out near the Cabo Verde Islands in the last few days of August, and spent the week or so that followed cycling between strong and weak. This revolving allowed it to take an odd track through the central Atlantic and survive several hostile environments before reaching the right combination of warm water, low wind shear, and moist air that allowed it to explode into a monstrous hurricane.
The real story of Florence, of course, was the water instead of the wind. The storm was essentially a repeat of Hurricane Harvey the year before, but over the East Coast instead. The storm got trapped under a ridge of high pressure and, with nothing to push it along, it just sat and dumped copious amounts of rain over the Carolinas.
Hurricane Michael was a far speedier storm. Michael formed in the western Caribbean in early October and moved swiftly over the next couple of days toward the Gulf of Mexico. The system grew from a tropical storm to a borderline category five hurricane in just two days. Anyone who tuned out the news over the weekend prior to the storm’s arrival woke up on Monday to a dire situation.
Michael exploded once it reached the soupy, untouched waters of the Gulf of Mexico, allowing the storm to continue strengthening right up until landfall. If the storm had a few more miles of water before the eye came inland east of Panama City, Florida, it could have reached maximum winds of 160 MPH. It’s still possible that a reanalysis of Michael over the coming months will find that the storm did just that.
The countries that border the Atlantic Ocean have now endured several back-to-back historic hurricane seasons. What could be in store for next year? One potential factor is a developing El Niño in the eastern Pacific Ocean. The warm waters of an El Niño can trigger a chain of events that tempers tropical activity in the Atlantic through increased wind shear. These stronger winds rip the tops off of thunderstorms that try to form, putting a stop to potential seeds for tropical cyclone development. Those effects aren’t always a sure bet, though, and as we saw this year (and so many times before), it only takes one or two storms to turn an unremarkable hurricane season into a historic tragedy.