Global Courant 2023-04-14 16:29:19
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Thunderstorms usually dissipate when the rain runs out or cold air is drawn in. The petrol is running out. But not Wednesday
Author of the article:
The associated press
Seth Borenstein
Published April 14, 2023 • Last updated 5 minutes ago • read for 3 minutes
FILE: Trees bend in tropical storm winds along North Fort Lauderdale Beach Boulevard as Hurricane Irma strikes the southern part of the state on Sept. 10, 2017 in Fort Lauderdale, Florida. Photo by Chip Somodevilla/Getty Images
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In some ways it was the Florida Man of Storms – not quite knowing when to say when.
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Usually, thunderstorms go out after the rain wears off or cold air is drawn in. The petrol is running out. But not Wednesday, when the storm that hit Fort Lauderdale had a gas station nearby — the warm and humid Gulf Stream.
The end result was more than 10 inches of rain that drenched and flooded Fort Lauderdale in six to eight hours. That was among the top three in major U.S. cities over a 24-hour period, behind Hilo, Hawaii’s 27 inches in 2000 and Port Arthur, Texas’s 26.5 inches in 2017, according to weather historian Chris Burt.
While it could happen elsewhere in the U.S. coast, Florida has the right topography, lots of warm water nearby and other favorable conditions, said Greg Carbin, chief of the forecasting division of the National Oceanic and Atmospheric’s Weather Prediction Center. Administration.
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Just two days before the downpour, Weather Prediction Center forecaster David Roth told colleagues that conditions were similar to those on April 25, 1979, when 16 inches of rain fell on Fort Lauderdale, Carbin said.
What parked over Fort Lauderdale on Wednesday was a supercell — the type of strong thunderstorm that can produce deadly tornadoes and hail, plowing across the Great Plains and Mid-South in a savage, fast-moving but short path of destruction, several meteorologists said.
Typically, such a cell would “self-extinguish” in maybe 20 minutes or at least keep moving, Carbin said. But in Fort Lauderdale, the supercell was in a lull between opposing weather systems, Carbin said. It took six to eight hours.
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“You had this extreme heat and humidity just pouring into the cell and because it had a bit of a spin to it, it essentially acted like a vacuum and sucked all that moisture back into the main core of the system,” said Steve Bowen, a meteorologist. and chief science officer for GallagherRe, a global reinsurance broker. “It essentially kept reigniting itself.”
What was important, said former NOAA chief scientist Ryan Maue, was “the availability of warm ocean air from the Gulf Stream was essentially infinite.”
Other factors included a strong low-pressure system, with counterclockwise winds, blowing away into the toasty Gulf of Mexico, Maue and Carbin said. There was a temperature difference between the slightly cooler land in Florida and the waters of the Gulf Stream of more than 80 degrees. Add to that wind shear, which is when high and low altitude winds flow in opposite directions, helping to add some spin.
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Many of those conditions are not unusual in themselves, including the location of the Gulf Stream. But when they combined in a precise way, it behaved like a continuous nutrient loop that dropped rain in amounts that the National Weather Service in Miami called a 1 in 1,000 chance.
“We continue to see more and more of these millennial” weather extremes in major cities, Bowen said. “The whole definition of normal is changing.”
The physics states that a warmer climate holds more moisture in the air, about 4% more for every degree Fahrenheit (7% for every degree Celsius). But warming also increases the intensity of storms that amplify that moisture level, said Pennsylvania State University climate scientist Michael Mann.
And that moisture falls as rain.
One-day downpours have “increased in frequency and magnitude over the past several decades and will continue to increase in both in the coming decades,” University of Oklahoma meteorology professor Jason Furtado said in an email. “These heavy rainfall coupled with sea level rise on the Florida coast should serve as important wake-up calls to South Florida residents about the serious risks climate change poses to them.”
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