Hurricane Michael’s fury may have seemed sudden, but surprising? Not especially, at least to forecasters.
As the storm crossed the Gulf of Mexico bound for the Panhandle coast, hurricane forecasters warned repeatedly that the system could strengthen. Seven days before it hit, when the storm was still churning across the southern Caribbean and odds remained low at just 30 percent, they warned conditions around the gestating storm could become “less hostile,” meaning worse for the Panhandle.
Michael transformed into a tropical depression before dawn Sunday. By 11 a.m. Monday — more than two days before landfall — it was forecast to become at least a major storm, at least a Category 3. With the exception of a brief pause in strengthening noted at 4 a.m. Tuesday, warnings were consistent, and dire.
What forecasters did fail to diagnose were Michael’s quick explosions of power, a process called rapid intensification that occurs when hurricane winds climb 35 mph in 24 hours. Michael underwent three, very fast, between Monday morning and landfall Wednesday afternoon. It wound up at the top of the Cat 4 scale, with 155 mph sustained winds at landfall.
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“That’s one of our biggest forecast challenges and we’re pretty up front that rapid strengthening is not something the models do really well,” said Mike Brennan, the chief of the hurricane specialists unit at the National Hurricane Center in Miami.
That uncertainty gets built into information shared with emergency managers, he said, though the message can sometimes get lost on the public in the pre-storm frenzy, especially after four Cat 4 hurricane strikes in the U.S. in just over a year.
So how to fix it? The National Oceanic and Atmospheric Administration launched a 10-year project to improve intensity forecast after the brutal 2004-05 hurricane season spawned Katrina and Wilma. The project was extended, given the poor results: out of 39 instances of rapid intensification during the 2017 season, forecasters got six correct.
Part of that research opened up debate over where to focus attention and limited resources: on the ocean or the atmosphere? Increasingly, and with Michael as the latest example, the answer is ocean.
While sea surface temperatures are included in models — warm waters that evaporate quickly can send that energy into a storm to provide fuel — what’s not well-crunched is how temperatures might change along a hurricane’s path and how deep they go and what role the Gulf’s Loop Current plays.
“The Loop Current and the warm eddy system...sustain very high heat content in water. It’s primarily in the central and eastern Gulf of Mexico and that’s where a lot of this occurs,” said University of Miami Rosenstiel School of Marine and Atmospheric Science oceanographer Nick Shay.. “It can provide higher octane for a hurricane to intensify particularly when the atmospheric conditions are favorable.”
October, when the season undergoes a second spike is, in fact, the worst month for hurricanes in Florida, when the state has been hit or narrowly missed more frequently than any other month, driven largely by storms bursting from the Caribbean.
That’s because in the Gulf, it’s not yet fall. Steamy ocean waters remain. This year, temperatures hovered a few degrees above normal, which some have blamed on climate change. Since the 1950s, the Gulf has in fact only warmed by about 1 degree Celsius, which doesn’t seem like a lot but can trigger exponential changes, said Ben Kirtman, an atmospheric scientist at Rosenstiel and a lead author on the United Nation’s International Panel on Climate Change 2014 assessment.
“When we have one degree of warming we have much bigger increases in evaporation in the atmosphere and we know water evaporation is fuel for the storm,” he said. “There’s a lot of potential other factors, but that’s the simplest one.”
It’s impossible to blame a single storm like Michael on climate change. But the earth’s system is being primed for more frequent intense storms, he said.
“We have a warmer ocean, a warmer atmosphere and all those things come together to increase the probability of a storm like Michael happening,” he said. “That’s our scientific understanding. We don’ t have much more depth than that.”
Typically storms that reach the Gulf of Mexico this time of year are already major hurricanes, Brennan said. If seasonal wind shear doesn’t keep them in check, they tend to undergo eyewall replacements as they move across the Gulf, which can weaken winds. But Michael didn’t strengthen until it was on the edge.
Waters in the northern Gulf were about 2 to 3 degrees Celsius above normal, which forecasters worried would fuel the storm. There was wind shear, Brennan said, but it didn’t seem to slow it.
“Almost in the last 12 hours everything lined up,” he said. “It’s a real dire scenario to have an intense hurricane very near the coast. That’s one of our worst-case scenarios.”
What the models likely failed to accurately account for was the depth and variance of warm waters, Shay said, and the role played by the Loop Current. The current winds around the Gulf, shifting and acting like a conveyor belt for everything from algae to warm water.
“That brings high heat content deep water into the Gulf,” he said.
As it bends north toward the coast, it can shed an eddy packed with warm water.
“It’s a recipe for disaster,” he said. “I know the climate people want to emphasize that storms are getting more intense, but I want to remind people about just where Michael formed was where Camille formed in 1969.”
Historically storms that pass over the Loop Current, and the eddies it creates, become monsters: Gilbert in 1988 passed over an eddy as well as Lili in 2002, Charley in 2004 and Katrina and Rita in 2005. All underwent rapid intensification.
“So we’ve seen the signature before,” he said.
Rather than oversimplify the problem and just blame warming oceans, he said, for forecasters it’s more critical to understand whether the loop current is getting deeper, which would create a bigger obstacle for deeper cold oceans to reach the surface and slow the flow of fuel.
Michael then got a final jolt from the atmosphere when an air stream created the perfect outflow at the top of the storm, like a chimney or a giant exhaust fan.
“My point is you gotta look at the longer-term ingredients in the Gulf of Mexico and see if this is a climate change event or is it that just in this particular case with Michael, all these key ingredients all came into play at the same time.”
To better understand the dynamics, Shay sent a team of researchers into the storm with NOAA and Air Force hurricane hunters armed with sensors to drop into the storm. Some will measure temperature, salinity and current at 4,500 feet deep. They also released drifters to measure surface conditions. The information they gather will explain how the ocean mixed before, during and after the storm, and how more exactly the ocean fuels hurricanes.
“This is essentially going to confirm what we already believe,” he said.
“A lot of meteorologists believe it’s only atmosphere,” he added. But “there’s a whole group of us who believe you have to understand what lies beneath.”