‘It just kept reigniting itself’: How a supercell storm pounded Fort Lauderdale for hours | CBC News

What parked over Fort Lauderdale on Wednesday — leading to record rainfalls amounts for the Florida city — was a supercell, the type of strong thunderstorm that can spawn killer tornadoes and hail in a fierce, fast-moving but short path of destruction, several meteorologists have said.

The end result was more than 63.5 centimetres of rain drenching and flooding Fort Lauderdale in six to eight hours. That ranked among the top three in major U.S. cities over a 24-hour period, behind 68.5 centimetres in Hilo, Hawaii, in 2000 and 67.3 centimetres in Port Arthur, Texas, in 2017, according to weather historians.

“For context, within a six-hour period the amount that fell is about a 1 in 1,000 chance of happening within a given year,” said Shawn Bhatti, a meteorologist with the National Weather Service in Miami.

While it could happen in other places in coastal America, Florida has the right topography, plenty of warm water nearby and other favourable conditions, said Greg Carbin, forecast branch chief at the National Oceanic and Atmospheric Administration’s Weather Prediction Center.

Just two days before the downpour, Weather Prediction Center forecaster David Roth told colleagues that conditions were lining up similar to April 25, 1979, when just over 40 centimetres of rain fell on Fort Lauderdale, Carbin said.

Warm ocean air provided fuel

Normally a cell like that would “snuff itself out” 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 warmth and moisture that was just feeding into the cell, and because it had a bit of a spin to it, it was essentially acting like a vacuum and sucking all that moisture back up into the main core of the system,” said Steve Bowen, a meteorologist and chief science officer for GallagherRe, a global reinsurance broker. “It just kept reigniting itself, essentially.”

What was key, 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, churning away in the toasty Gulf of Mexico, Maue and Carbin said. There was a temperature difference between the slightly cooler land in Florida and the 80-degree-plus Gulf Stream waters. Add to that wind shear, which is when winds are flowing in opposite directions at high and low altitude, helping to add some spin.

‘Normal is changing’: more 1-day downpours seen

Many of those conditions by themselves are not unusual, including the location of the Gulf Stream. But when they combined in a precise way, it acted like a continuous feeding loop.

“We continue to see more and more of these thousand-year” weather extremes in major cities, Bowen said. “The whole definition of normal is changing.”

Physics states that a warmer climate holds more moisture in the air, about four per cent more for every degree Fahrenheit (seven per cent for every degree Celsius). But warming also increases the intensity of storms amplifying that moisture level, said Pennsylvania State University climate scientist Michael Mann.

And that moisture then falls as rain.

One-day downpours have “increased in frequency and magnitude over the last several decades and will continue to increase in both in the coming decades,” University of Oklahoma meteorology Prof. Jason Furtado said in an email. “These heavy rainfall events coupled with sea level rise on the Florida coast need to serve as significant ‘wake-up calls’ for the residents of South Florida about the severe risks that climate change poses to them.”

Meanwhile, affected residents continue to clean up.

In the Edgewood neighbourhood of Fort Lauderdale, Christopher Alfonso and Tony Mandico, neighbours for 50 years, said their homes are likely total losses.

“That storm … just poured down on us for hours and hours and hours,” Alfonso said. Pointing to the tightly packed homes with tiny yards, he said, “All this asphalt, concrete, no grass — there was no place for [the water] to go.”

Airlines were forced to cancel or change flights to and from the Fort Lauderdale Airport, which was closed until 5 a.m. on Friday. Southwest cancelled about 50 departures through Friday morning, and the number could grow, a spokesperson said. The airline is letting customers rebook on flights to and from Miami and Palm Beach at no additional charge, it said.

More than 650 flights were cancelled at Fort Lauderdale on Thursday, according to FlightAware.

Meanwhile, Broward County schools were to remain closed Friday.