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Why derechos are so devilishly difficult to predict



At 8:30 a.m. On Monday, Mark Licht was sitting at his home outside of Ames, Iowa, on a conference call with fellow agronomists and meteorologists from across the state. Iowa had been through a dry spell and the western half of the state was hit by severe drought. What the farmers needed was a big storm, thought Light, a grow systems specialist at Iowa State University. The weather forecasters on the call told him that South Dakota and Nebraska were just starting. But they said it didn’t look like it had the energy to make it to Iowa. Everyone kept their fingers crossed and hung up.

At around 10.15 am, Licht received an email from the group; The storm looked like it would hold together.

Less than an hour later, he heard storm warning sirens from the nearest town. He went out. It was sunny, hardly a cloud in the sky. The air was still and the humidity choked. “That̵

7;s weird,” he thought. But when he checked the radar, he saw a huge mass rolling in his direction at about 60 mph.

He took his family down to the basement and ten minutes later the storm was over them. It was raining so hard that you couldn’t see more than a few meters in front of you. Winds so strong they could cut a tree in half. When Licht and his family showed up about 45 minutes later, the steel shed where their cars were parked had completely collapsed. “We were in the middle of one of the most destroyed storm paths,” says Licht. “It will be a long process to deal with the damage, but we’re lucky it wasn’t any worse.”

Iowa knows summer storms. But the one that sped through the Midwest on Monday, covered 770 miles in 14 hours, built 10 million acres of crops, twisted grain elevators, and turned off electricity for hundreds of thousands of people for days was a rare type of storm known as a derecho.

The term means “straight ahead” in Spanish and was coined in the late 19th century by a Danish professor of physics at the University of Iowa who, in contrast to the associated circular winds, used it to describe the “straight blow of the prairie” tornadoes. For a derecho to meet the National Oceanic and Atmospheric Association’s definition today, it must travel at least 240 miles and move at a speed of at least 58 miles per hour. This week’s derecho hit top speeds of around 112 mph in Cedar Rapids, Iowa, about two hours east of Licht’s house.

“Derechos are those long-lived, fast-moving walls of super-thunderstorms,” ​​says Paul Huttner, a meteorologist who monitors the weather for Minnesota Public Radio. These are regular but infrequent events that happen once or twice a year in the Midwest. Derechos come in two variants: linear and progressive. Monday’s storm was a progressive derecho that moves faster, is more compact, and offers more punch than its outstretched siblings. And this, says Huttner, “was a real fool.”

Derechos are often referred to as “inland hurricanes” because the ground experience – whipping, destructive winds, sideways rain – is similar to that of a Category 1 hurricane. But they are completely different weather phenomena. Derechos are driven by various factors and behave more like a wildebeest onslaught than a hippopotamus puff. And, unlike the course and severity of hurricanes, which scientists can predict well days or even weeks in advance, predicting where and when a derecho will form remains one of the most difficult tasks for weather forecasters.


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