Its growth from an unassuming tropical storm on Sunday to a 155-mph beast flirting with Cat 5 status was unexpected by meteorologists who watched astonished as Michael’s minimum sea level pressure ticked down to a mind-blowing 919 millibars at landfall.
That’s lower than 1992’s Hurricane Andrew and 2005’s Hurricane Katrina, ranking Michael 3rd in records dating to the 1800s for lowest minimum pressure at landfall, according to Colorado State University hurricane expert Phil Klotzbach.
Pressure is a measure of a hurricane’s intensity, the force of suction formed as winds spiral toward cloud tops blistering high into the atmosphere. The lower the number, the more violent the storm.
“In terms of what I know about hurricanes, this isn’t supposed to be happening,” said Hugh Willoughby, a distinguished research professor at Florida International University and former director of NOAA’s Hurricane Research Division. “I thought it would be a strong Cat 3 overnight then weaken to a Cat 2 before landfall.”
Michael gained Category 4 strength with 130 mph winds as of 2 a.m. Wednesday. Eleven hours later it had gained 25 mph in a rapid intensification that put it just 2 mph below a Category 5.
It is the first Category 4 hurricane to hit the Panhandle, and the first Category 4 to make landfall in the Continental U.S. in October since 1954’s Hazel. Michael also had the fourth highest wind speeds at landfall behind only the 1935 Labor Day Hurricane (185 mph), 1969’s Camille (175 mph), and Andrew’s 165 mph, Klotzbach said.
A storm’s pressure doesn’t correlate exactly with wind speeds and depends on the surrounding environmental pressure as well as the size of the storm, said Brian McNoldy, a senior research assistant at the University of Miami’s Rosenstiel School of Atmospheric Science.
“There is a degree of randomness involved with a storm like Michael and I don’t think we fully understand what happened yet,” said Falko Judt, a research meteorologist with the National Center for Atmospheric Research. “What really caught us off guard is hurricanes usually weaken when they approach the land there.”
Michael intensified up to the moment it pounded into Florida’s vulnerable Emerald Coast about 15 miles southeast of Panama City near 1:30 p.m.
“I can’t remember another storm that did this,” Judt said.
Michael started as a typical October hurricane.
While much of the Gulf Coast and Eastern Seaboard experience a winding down of hurricane season in October, Florida remains vulnerable — a sore thumb surrounded by warm waters, and a target for anything that spins up in the Gulf of Mexico or Caribbean Sea.
Since 1851, Florida has endured 36 hurricanes that made October landfalls, including 10 major hurricanes, which is considered a Category 3 or higher. That’s five times higher than runner-up Louisiana, which has experienced seven October hurricane hits, including three major hurricanes.
This October is different though. It follows Florida’s warmest September on record, which helped heat Gulf of Mexico waters to a storm-friendly 86 degrees.
And warm Gulf waters can run deep, giving Michael plenty of fuel.
“I was surprised it got so strong, but we’ve seen this before when the atmospheric conditions are somewhat favorable and the ocean conditions are really favorable,” said Nick Shay, professor of Ocean Sciences at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. “When there is heat for the storm and the circulation is good and the shear is tending to relax, it’s a recipe for disaster, and that’s what we are having.”
Forecasting a hurricane’s intensity is still a challenge. While hurricane-track forecasts have improved 50 percent over the past 15 years and were the most accurate last year since the modern mapping of storms began, intensity forecasting hasn’t kept pace.
Part of the difficulty in forecasting intensity is minute interactions between the ocean and atmosphere that remain a mystery, said National Center for Atmospheric Research meteorologist Rosimar Rios-Berrios.
A hurricane gains strength as heat from the ocean is transferred into the atmosphere to energize clouds, which convert it into wind power.
“It’s a process that is very difficult to observe and difficult to predict,” Rios-Berrios said.
Shay said Michael did go over a pool of cooler water in the Gulf, but it wasn’t enough to weaken it significantly, and it soon entered another warm area as it approached the coast.
Wind shear, which can work to tear apart hurricanes, relaxed at about the same time.
“It’s just a confluence of awful factors,” said Bob Henson, a meteorologist and climate writer for The Weather Company, an IBM Business.
In a social medial post late Tuesday, Henson said satellite images of Michael were “jaw dropping.”
“This is the real deal,” he said. “Hurricanes that intensify overnight just before reaching land are the worst nightmare of forecasters and emergency managers.”
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