4. Jet Stream
Something else is making these storms linger and dump more rain, a powerful river in the air 5 to 9 miles in the sky: the jet stream.
It swings west to east across the northern latitudes. It’s formed when warm air from the tropics meets colder and denser air from the Arctic.
There’s energy in this collision of hot and cold. The greater the temperature difference, the more energy. When the Arctic is colder, the jet stream flows faster, wiggling across North America like a freshly caught fish grasped with two hands.
But, because of climate change, the Arctic is warming two to three times as fast as the rest of the world. The temperature difference between north and south is less — less energy for the jet stream. It’s as if the atmosphere has loosened its grip on that undulating fish, allowing it to flap back and forth more slowly and dramatically.
“As the jet stream gets weaker and weaker, the stream tends to make these larger north and south swings,” said Jennifer Francis, a senior scientist with the Woods Hole Research Center. “And they tend to cause very slow-moving weather regimes.”
In some cases, the jet stream can bend as far south as Mexico, picking up moisture from the Gulf and then shooting it like a firehose in the Southeast, as it did in Louisiana in March 2016, dropping 20 inches of rain in some areas.
Or it can weaken, allowing lumbering storms in the South to linger. This happened on September 25, and it caught people across this suddenly waterlogged city by surprise, including Mandi Mezin.
She had driven from Atlanta to Charleston for a bachelorette party. Off Rutledge Avenue, she found herself surrounded by flooded streets. She drove onto a sidewalk to save her car. “I’ve never seen flooding like this,” she said.
Brian Walter, a UC Santa Cruz researcher studying flooding and inequality in North Charleston, surveyed the flooding downtown on his bike — safer than a car, because it wouldn’t stall in the waters on President Street.
“It’s frustrating and disappointing,” Walter said of the rain bomb’s handiwork. “It’s hard not to see it as a premonition.”
5. Another Storm
A lingering storm is a troublemaker. When the jet stream and other high-altitude winds are nearby, they blow the tops off thunderheads.
But when these high-altitude winds aren’t around, thunderheads form like popcorn kernels. Rising 5 miles and higher, they fill with ice crystals and water droplets. They might drop water below them in the form of a summer squall, but they need something else to do some serious damage, a trigger.
Like a tropical storm. When these forces collide, the ice crystals and water droplets smash into each other, generating electricity, and then this mass of energy and water falls, lighting the sky in inch-wide bolts that are so hot the air around them suddenly explodes, creating thunder.
Kenneth Kunkel, a scientist at the North Carolina Institute for Climate Studies in Asheville, and his colleagues studied data from 3,104 U.S. weather stations and recently came up with a surprising discovery about these extreme weather events.
They looked at precipitable water levels in the atmosphere. Imagine a column of air from the ground to the sky. Precipitable water is a measure of how much water vapor in that column can turn to rain.
But Kunkel and his colleagues found that the precipitable water measurement only went so far. In an extreme event, water vapor rapidly condenses and falls as rain, releasing energy in the form of heat. This extra heat adds more fuel to storms, whisking more water vapor into the tempest. It creates a powerful cycle, and soon everything below is sopping wet.
Kunkel’s study found that extreme rain events turbocharged the water vapor so much that twice as much rain fell as the precipitable water measurement. And that as the climate warms and holds more moisture, storms could have 15 to 25 percent more rain.
For people caught in the September 25 downpour, immediate needs trumped these larger trends.
Dane Rebang, 28, stood at America and Amherst streets on Charleston’s East Side waiting for an Uber ride to take him to work on Daniel Island after his low-riding car got swallowed in the flood.
He’s lived on the street for two years and knows storms can turn the neighborhood thoroughfare into a water-filled canal in no time. But he was taking a nap when the rain started Friday, and in less than an hour the street outside his apartment was knee-deep in water.
“Now I’m left standing here in staph water because my car is underwater,” he said.