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Why Hurricane Michael's Power Caught Forecasters Off Guard
Hurricane Michael was a sly storm, one that seemed almost unexceptional at first. It followed its predicted path with seeming obedience, but then burst into sudden fury as it approached the Florida Panhandle, reaching wind speeds at the cusp of Category 5 strength and leaving mud and rubble in its wake.
It was, in other words, a hurricane: the product of multitudinous forces that blend heat, wind and moisture into a potent threat, with a whopping dose of chance thrown in. Influences on the formation, direction and strength of hurricanes can involve faraway events like dry air from Saharan dust storms, the heated waters of El Niño in the Pacific, the undulations of the jet stream.
Increasingly, climate change is part of the dangerous mix as well.
Little wonder that modern weather modeling got its start in chaos theory, which acknowledges that small changes can lead to enormous effects so that, as one founder of the field put it, the flap of a butterfly’s wing in Brazil might set off a tornado in Texas.
In the case of Michael, the National Hurricane Center forecast the storm’s path with great accuracy, but its sudden intensification as it approached land was harder to predict. Millions of residents were caught off guard as Michael escalated from a tropical storm to a major hurricane in just two days, leaving little time for preparations.
Why Michael surprised everyone
“This is really an amazing event,” said Andrew Dessler, a professor of atmospheric sciences at Texas A&M University. “It came out of nowhere and really intensified rapidly.”
Michael, he said, did not follow the common behavior of storms, which tend to weaken as they reach the shore because of interaction with the land. “It had the pedal to the metal all the way until it hit the coast,” he said.
Hurricane Michael’s sharp increase in strength as it approached Florida was due in part to its low barometric pressure, said Phil Klotzbach, a research scientist in the atmospheric science department at Colorado State University.
Low barometric pressure increases a storm’s intensity, and the barometric pressure within Hurricane Michael early Wednesday was just 925 millibars. There have only been a half-dozen storms that struck the United States with lower barometric pressure, the most recent being Katrina, Andrew and Camille — and all six “were devastating storms,” Klotzbach said.
While prediction of a storm’s path has grown increasingly accurate, the ability to predict rapid intensification has lagged somewhat, said Haiyan Jiang, an associate professor in the department of earth and environment at Florida International University.
The strengthening occurred despite pronounced wind shear in the region that might have been expected to weaken the storm, she added.
“The shear was high, so nobody expected it was going to intensify this rapidly,” Jiang said.
Where the power comes from
The biggest factor in the power of hurricanes is the difference between the temperature of the surface of the water they pass over and the coolness of the upper atmosphere.
Warm water is “basically the battery for, the fuel, that hurricanes feed off of,” said Jennifer Francis, a professor of atmospheric sciences at Rutgers University’s department of marine and coastal sciences.
The temperature of water in the Gulf of Mexico has been warmer than usual for this time of year, which helped make Hurricane Michael a powerhouse.
It is too early to say how much global warming contributed to Michael’s ferocity. But as the world heats up, we can expect stronger storms, said Adam Sobel, an atmospheric scientist at the Lamont-Doherty Earth Observatory of Columbia University. Together with Kerry Emanuel of the Massachusetts Institute of Technology and others, he has researched how warming waters increase storms’ wind force.
Even without the scientific calculations, he said, the connection makes intuitive sense. Hurricanes “form in places where the ocean’s surface is warmest,” and so warming oceans should mean stronger hurricanes — at least, among those that survive challenges like wind shear and reach maturity and strength.
“The rich get richer,” he said.
There is some evidence that this connection is already occurring, he added. “Michael could have occurred 100 years ago,” he said, “but there’s evidence that the probability of it is increasing as the climate warms.”
Climate change complicates the picture
Climate change disrupts weather systems in more ways than one. A major report from the United Nations group known as the Intergovernmental Panel on Climate Change predicted weather crises around the world if nations fail to take serious action to curb the greenhouse gases that are causing the planet to grow hotter.
Such long-term projections of climate change are, in many ways, more dependable than short-term predictions of weather — which have, of course, improved greatly as well. The large forces at play in climate change, like the effects of accumulated carbon dioxide in the atmosphere, have been noted for more than 100 years. Modern models predicting warming have been borne out in thousands of studies showing phenomena like rising ocean temperatures and shrinking glaciers.
When it comes to hurricanes, climate scientists have said that the effects on storms are already being seen in heavier rainfall, as a warmer atmosphere holds more moisture. Sea level rise is beginning to bring higher storm surge, as well.
Slower storms, heading farther north
The wanderings of hurricanes vary widely, steered by wind currents and high- and low-pressure systems; some storms race across oceans and land while others dawdle. Hurricane Emily in 1987 reached a forward speed of nearly 60 mph over the North Atlantic, faster than a running cougar. This year’s Hurricane Florence and last year’s Harvey stalled for days, dumping record amounts of rain and causing devastating floods.
Michael moved at an unexceptional speed of about 12 mph across the Gulf of Mexico. But climate change could be affecting the pace of other storms.
Research by Francis of Rutgers suggests that warming in the Arctic could be weakening the jet stream in ways that, in the summer, can deprive weather systems like hurricanes of the atmospheric push that keeps them moving; those factors, she said, may have been at play in the unusual pokiness of Harvey and Florence.
As the Atlantic Ocean heats up along the Eastern Seaboard, Francis said, we can expect to see late-season storms persist longer and the East Coast become “much more friendly to tropical storms” like Hurricane Sandy, which carved a path of destruction in the Mid-Atlantic in 2012.
There’s more we don’t know
While there is growing scientific agreement on many of the theories about the effects of climate change on storms, there are also areas where consensus has yet to develop. “There’s still stuff we don’t know about hurricanes,” Klotzbach said.
Dessler, for example, said he was not certain that Arctic warming was having a pronounced effect on slowing storms like Harvey and Florence.
Still, he said, “the idea that hurricanes are going to stay exactly the same seems far-fetched.”
Whatever the changes wrought by climate change turn out to be, Dessler said, “Bad things are going to happen. I don’t know exactly what they’re going to be, but they’ll be things we don’t like.”
He drew an analogy to our wardrobes. “You’ve selected your clothes to fit you now,” he said, just as we are adapted to the present state of the global climate system. Whether we gain weight or lose it, he said, “no change makes your clothes fit better.”