What parked over Fort Lauderdale on Wednesday – bringing record rainfall for the Florida city – was a supercell, the powerful type of thunderstorm that can spawn killer tornadoes and hail in a fierce path of destruction , fast but short, several meteorologists said.
The end result was more than 25 inches of rain flooding and flooding Fort Lauderdale in six to eight hours. This ranked in the top three in major US cities over a 24-hour period, trailing 68.5 centimeters in Hilo, Hawaii, in 2000 and 67.3 centimeters in Port Arthur, Texas, in 2017, according to historians. the weather.
“For context, over a six-hour period, the amount that fell is about a 1 in 1,000 chance of occurring in any given year,” said Shawn Bhatti, a meteorologist with the National Weather Service in Miami. .
While it may happen in other places on the U.S. coast, Florida has the right topography, plenty of nearby warm water and other favorable conditions, said Greg Carbin, forecast branch chief at the center. National Oceanic and Atmospheric Administration weather forecasting service.
Just two days before the downpour, Weather Prediction Center forecaster David Roth told colleagues that conditions lined up like April 25, 1979, when just over 40 centimeters of rain fell on Fort Lauderdale. , Carbin said.
Warm ocean air provided fuel
Normally, a cell like that would “shut down” 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.
“You had this extreme heat and humidity that was just powering the cell, and because it had a bit of a spin, it was basically acting like a vacuum and sucking all that moisture into the main core of the system,” said Steve Bowen, meteorologist and scientific director of GallagherRe, a global reinsurance broker. “He kept turning back on, basically.”
What was essential, 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, moving into the Gulf of Mexico, Maue and Carbin said. There was a temperature difference between the slightly cooler lands of Florida and the Gulf Stream waters over 80 degrees. Add to that wind shear, which occurs when winds flow in opposite directions at high and low altitudes, which helps add spin.
‘The normal is changing’: more one-day showers seen
Many of these conditions in themselves are not unusual, including the location of the Gulf Stream. But when they combined in a precise way, it acted like a continuous power loop.
“We continue to see more and more of these millennial weather extremes” in major cities, Bowen said. “The whole definition of normality is changing.”
Physics indicates that a warmer climate holds more moisture in the air, about four percent more for every degree Fahrenheit (seven percent for every degree Celsius). But warming also increases the intensity of storms, amplifying that level of humidity, said Michael Mann, a climatologist at Pennsylvania State University.
And this moisture then falls as rain.
One-day showers have “increased in frequency and magnitude over the past few decades and will continue to increase for the next two decades,” University of Oklahoma meteorology professor Jason Furtado said. in an email. “These heavy rain events coupled with rising sea levels on the Florida coast should serve as a ‘wake-up call’ to South Florida residents about the serious risks climate change poses to them. .”
Meanwhile, affected residents continue to clean up.
In Fort Lauderdale’s Edgewood neighborhood, Christopher Alfonso and Tony Mandico, neighbors for 50 years, said their homes were likely total losses.
“This storm… slammed into us for hours and hours and hours,” Alfonso said. Pointing to the tightly packed houses with tiny courtyards, he said: “All that asphalt, that concrete, no grass – there was no room for [the water] go.”
Airlines have been forced to cancel or change flights to and from Fort Lauderdale airport, which was closed until 5 a.m. Friday. Southwest has canceled about 50 departures through Friday morning, and the number could increase, a spokesperson said. The airline allows customers to book flights to and from Miami and Palm Beach at no additional cost, he said.
More than 650 flights were canceled in Fort Lauderdale on Thursday, according to FlightAware.
Meanwhile, schools in Broward County were to remain closed on Friday.