Hurricane Irma thrives on fateful mix of 'ideal' conditions
By Steve Gorman
(Reuters) - Hurricane Irma, a deadly, devastating force of nature, rapidly coalesced from a low-pressure blip west of Africa into one of the most powerful Atlantic storms on record, following an unhindered atmospheric path and fed by unusually warm seas.
A combination of many factors, experts said on Friday, set the stage for Irma's formation and helped the storm achieve its full thermodynamic potential, creating the monster tropical cyclone that wreaked havoc on the eastern Caribbean and may inflict widespread damage on Florida.
"It got lucky," said John Knaff, a meteorologist and physical scientist for the National Oceanic and Atmospheric Administration (NOAA). "This storm is in the Goldilocks environment for a major hurricane. It's bad luck for whoever is in its path, but that's what going on here."
Brian Kahn, an atmospheric scientist and cloud specialist for NASA's Jet Propulsion Laboratory, called the ocean conditions that spawned Irma "absolutely ideal."
Balmy water temperatures along Irma's trajectory ran deep beneath the surface and slightly higher than normal, by as much as a degree Fahrenheit in places, providing ample fuel for the storm's development, scientists said.
Irma also encountered little if any interference in the form of wind shear - sudden changes in vertical wind velocity that can blunt a storm's intensity - as it advanced at about 10 to 18 miles per hour, an ideal pace for hurricanes.
Its fortuitous path of least resistance was essentially ordained by a well-placed atmospheric ridge of high pressure that steered the storm by happenstance through some of the Caribbean's warmest waters as well as an area mostly devoid of wind shear.
The result was a gargantuan storm that rapidly grew to a Category 5, the top of the Saffir-Simpson scale of hurricane strength, with sustained winds of 185 miles per hour, the most forceful ever documented in the open Atlantic.
It also ranks as one of just five Atlantic hurricanes known to have achieved such wind speeds during the past 82 years.
CROWDING OUT HURRICANE JOSE
Irma lashed Cuba and the Bahamas as it drove toward Florida on Friday after hitting the eastern Caribbean with its tree-snapping winds, torrential rains and pounding surf, killing at least 21 people and leaving catastrophic destruction in its wake.
By Friday night, Irma's winds had diminished to 155 miles per hour (250 kph) and it was downgraded to a Category 4 storm, though still considered extremely dangerous.
The tiny islands directly crossed by Irma, like Barbuda and St. Martin, did little to weaken it, but the storm's intensity could be measurably diminished by a close encounter with a larger land mass, such as Cuba, en route to the U.S. mainland, scientists said.
So powerful is Irma that it may end up dampening another Category 4 storm in its wake, Hurricane Jose, which like Irma originated off the west coast of Africa and was following a similar course.
Waters already plied by Irma could be left a bit cooler, robbing Jose of potential energy, and "outflows" from Irma could produce wind shear for Jose, said Scott Braun, a research meteorologist for NASA's Goddard Space Flight Center.
Knaff said it was no surprise that the advent of Irma coincided with the precise peak of the Atlantic hurricane season.
Much less certain is the role of global climate from human-induced atmospheric increases in heat-trapping "greenhouse" gases, scientists said.
There is consensus that climate change has raised sea levels, which is likely to exacerbate hurricane storm surges. Rising ocean temperatures have been clearly documented as well.
Research is divided on whether global warming will make tropical cyclones more frequent, though data from climate modeling suggests a higher probability for stronger, wetter hurricanes in the Atlantic when they do occur, said Tom Knutson, a climate research scientist for NOAA.
"We think, based on model simulations that climate change is having an effect, making storms slightly more intense with higher rainfall rates, but these changes are not huge and we cannot yet clearly detect them in observations," he said.
(Reporting by Steve Gorman in Los Angeles; Editing by Mary Milliken)
