The rain seemed never to stop.
Thunderstorms usually run out when they run out of rain or absorb cold air. They’re running out of fuel. But the storm that hit Fort Lauderdale on Wednesday had a gas station close at hand — the warm, humid Gulf Stream.
That’s why the city was flooded with almost 65 centimeters of rain in six or eight hours. That makes it one of the worst downpours to hit a US city in 24 hours: the Hawaiian city of Hilo received 69 centimeters of rain in 2000, and the Texas city of Port Arthur received 67 centimeters in 2017, according to Chris Burt, a weather historian.
While this could happen elsewhere along the U.S. coasts, Florida has the right topography, ample nearby warm water and other favorable conditions, noted Greg Carbin of the National Oceanic and Atmospheric Administration of the United States. UNITED STATES.
Just two days before the flood, meteorologist David Roth of the Weather Prediction Center had warned his colleagues that conditions were beginning to resemble April 25, 1979, when 40 centimeters of rain fell in Fort Lauderdale, Carbin said.
A cell like that would normally implode after about 20 minutes, or at the very least keep moving, Carbin said. But the Fort Lauderdale supercell found itself caught between two opposing weather systems, allowing it to rage for so long.
“You had all this heat and all this moisture feeding the cell, and because it had some rotation, it was basically acting like a vacuum cleaner and sucking all of this moisture back into the core of the system,” meteorologist Steve Bowen said. Essentially, she was endlessly resuscitating herself. »
Crucially, said former NOAA chief scientist Ryan Maue, was “an almost endless availability of warm ocean air from the Gulf Stream.”
A powerful low pressure system with counterclockwise winds over the Gulf of Mexico also contributed to the problem, MM said. Maue and Carbin. There was also a small temperature difference between the Florida mainland and the slightly warmer waters of the Gulf Stream. Windshear, which occurs when winds blow in opposite directions at high and low altitudes, added some rotation to the storm.
Taken individually, most of these conditions are not unusual, including the location of the Gulf Stream. But coming together in a peculiar way, they created a continuous feeder loop that dumped amounts of rain that the National Weather Service in Miami called a “1 in 1000 chance.”
These extreme weather events, which are only expected to occur once every 1,000 years in major cities, are becoming more frequent, Bowen said.
“It’s the whole definition of normal that’s changing,” he said.
Physics says that a warmer climate traps more moisture in the air (about 7% for every degree Celsius). But warming also increases the intensity of storms, which amplifies the level of humidity, explained Michael Mann, a climatologist at Pennsylvania State University.
This moisture then falls as rain.
One-day thunderstorms “have increased in frequency and magnitude over the past few decades and will continue to increase over the next several decades,” Jason Furtado, a meteorologist at the University of Oklahoma, wrote in an email. This heavy rainfall coupled with rising sea levels along the Florida coast is an important ‘warning call’ for South Florida residents regarding the serious threats climate change poses to them. »
