The unforgettable thing about record-setting Hurricane Michael will always be how rapidly it became a near-Category 5 storm, perfectly timed for a sneak attack on the Florida Panhandle.
On Tuesday morning, Floridians knew a storm was coming but not how strong it would be. As of 5 a.m., Michael was a strong Category 1 hurricane with a minimum pressure of 973 millibars, a measure of atmospheric pressure indicating that air is rising in the storm, pulling winds toward its center. The official forecast took the storm up to mid-Category 3 at landfall.
But 24 hours later, Michael was already far stronger: It now had 140 mph winds and a pressure falling sharply. The wind speed increased 45 mph in just 24 hours, representing a leap from Category 1 to Category 4 — and the storm wasn’t done intensifying.
Pressure would ultimately fall to 919 millibars, one of the lowest measures of any hurricane at landfall in the United States — and the winds responded by increasing to 155 mph right as the storm struck the coast. This was a borderline Category 5 storm, and it’s clear that the only reason Michael didn’t quite cross that threshold was because it was crossing beaches by that time instead.
This process of “rapid intensification” — extremely dangerous near a coastline — is something we keep seeing lately. Technically, it is defined by the National Hurricane Center as an increase in wind speeds of 35 mph or more in 24 hours.
All of the worst hurricanes of the past two years — Harvey, Irma, Maria, Florence and Michael — intensified even more rapidly than this. Maria increased a stunning 80 mph in wind speeds, leaping from a Category 1 to a Category 5 storm in 24 hours, not long before its catastrophic landfall in Puerto Rico that ultimately led to thousands of deaths.
Harvey and Michael didn’t strengthen quite so much, so fast, but they rapidly intensified in the crucial hours before making their final continental U.S. landfalls.
Rapid intensification means that a storm has found itself in an ideal environment — one usually characterized by warm waters and little adverse wind shear. Michael accomplished its feat while crossing waters in the Gulf of Mexico that ranged from 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) warmer than normal. That represents an enormous reservoir of extra potential hurricane energy, beyond what the warm Gulf already provides this time of year.
In fact, according to hurricane expert Kerry Emanuel of the Massachusetts Institute of Technology, Michael could have become even stronger if not for some adverse wind shear, potentially reaching full Category 5 strength. That’s what happened in a model of the storm that Emanuel ran in real time. “With no shear, Michael would have intensified substantially faster,” he said.
Climate scientists have begun to focus on hurricane rapid intensification as an increasingly prevalent feature in the world we’re entering. Simply put, with warmer seas, storms ought to be able to pull this off more often.
In a recent study in the Journal of Climate, researchers found more rapid intensifications in a simulation of a human-warmed world, and also that this would prove a key pathway toward more intense hurricanes in general.
“The rapid intensification of these storms, which was part of what made them so dangerous and devastating, is something models are telling us global warming should make more common globally over the present century,” said Gabriel Vecchi, one of the authors of the study and a climate scientist at Princeton University. ″However, I don’t think I’m in a position to say — one way or the other — whether global warming played an important role in Michael’s rapid intensification,” he cautioned.
“Our present thoughts and efforts need to be with the people affected by Michael,” Vecchi continued in an email. “But after that we need to plan for the longer term in a way that acknowledges the combined impact [that] greenhouse gas emissions and other human factors (like urbanization near coasts) have on increasing our future risk to hurricanes.”