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Derecho turns sky green, sweeps through 5 states with 90 mph winds

Hardest hit were Iowa and South Dakota, where winds reached hurricane force

The sky turned green as a derecho, a widespread wind storm, moved through South Dakota and other parts of the Great Plains on July 5. (Video: Reuters)
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The Midwest and the Plains states don’t get hurricanes. They get derechos — sprawling thunderstorm complexes that can travel hundreds of miles and cover multiple states with the impact of a 100-mile-wide tornado. Parts of South Dakota and Iowa, as well as Nebraska, Minnesota and Illinois, faced a derecho on Tuesday. It even turned the skies green in some areas.

Central states and Southwest to swelter as new heat dome builds up

Winds gusted as high as 96 mph in Huron, S.D., the same location that faced 90 mph wind gusts during an equally severe derecho back on May 12. Tuesday’s derecho also brought a gust to 91 mph in Agar, S.D., and 99 mph in Miner, S.D. In Dewey County, S.D., softball-sized hail accompanied winds gusting to 84 mph; the National Weather Service received reports of broken windows, and a machine shed and grain bins destroyed.

The Weather Service received scores of damage reports from South Dakota to Illinois from the violent complex of storms. The winds knocked down power lines and trees, some falling onto homes and vehicles. Tens of thousands of utility customers lost power.

Ahead of the derecho, thousands witnessed skies turn an ominous shade of neon green, the heavens appearing borderline phosphorescent. While green skies are sometimes byproducts of thunderstorms, few meteorologists could remember having seen skies reflect that peculiar hue.

How strong were the winds?

For a thunderstorm squall line to be classified a derecho, “Damage must be incurred either continuously or intermittently over a swath of at least 650 km (~400 mi) and a width of approximately 100 km (~60 mi) or more.” That’s according to the American Meteorological Society. The National Weather Service also stipulates that, in addition to widespread 60 mph winds, a few significant wind gusts, or those topping 75 mph, must be thrown in as well.

Tuesday’s episode more than fit the bill. Here’s a look at some of the greatest observed wind gusts:

  • 99 mph, Howard, S.D. Storm report noted frequent gusts of 70 mph or greater for between 20 and 30 minutes
  • 96 mph, Huron, S.D.
  • 91 mph, Agar, S.D.
  • 87 mph, Ree Heights, S.D.
  • 85 mph, Wall Lake, S.D.
  • 84 mph, Timber Lake, S.D.
  • 82 mph, Butte, Neb.
  • 79 mph, Hartley, Iowa
  • 70 mph, Independence, Iowa
  • 64 mph, Magnolia, Minn.

In Sioux Falls, S.D., the Weather Service reported winds gusted over 58 mph for 45 minutes straight, with a peak gust of 80 mph.

A first for many

Hundreds posted on social media before, during and after the storms to remark on how bizarre the sky scene was as the storms rolled in. Among them were scores of veteran storm chasers, many of whom had never experienced a sky that color.

The infamous green sky that comes with some severe thunderstorms has to do with what the thunderstorm is holding — water, and lots of it. It’s believed that big raindrops and hail scatter away all but the blue wavelengths, allowing primarily blue light to penetrate through and beneath the storm cloud.

During the late afternoon and evening, the light is already skewed red/yellow by the setting sun. The combination of yellow and blue light produces green, although experts in atmospheric optics still have some uncertainties about the overall process.

Even by that metric, however, the colors exhibited by storms over Siouxland and along the Interstate 29 corridor of South Dakota, the James River Valley and northwestern Iowa were unlike any in recent meteorological memory.

One Twitter user even joked about making it a new paint color, referring to it as “Derecho Green.

What made the derecho?

This was a progressive derecho — or one that rides along a temperature boundary. Progressive derechos are common in summertime patterns dominated by a high-pressure heat dome. In this case, the heat dome was anchored over the Mississippi Valley, and thunderstorm complexes known as “ridge runners” rode up and around it.

Central states and Southwest to swelter as new heat dome builds up

The northern periphery of heat domes is a breeding ground for progressive derechos. That’s where there’s a stout south to north temperature contrast — or a boundary for thunderstorms to straddle — and plenty of wind energy aloft. Heat domes shunt the jet stream to the north, meaning the river of swiftly-moving upper-atmospheric winds was racing over the Northern Plains with plenty of momentum for thunderstorms to tap into.

More than 60 million people were under alerts for the heat, which helped fuel Tuesday’s derecho. Scientists have determined rising temperatures from human-caused climate change may be intensifying these storm complexes.

Hints of a derecho-climate change link, ten years after 2012 storm

Thunderstorms had been in the forecast for days, but it wasn’t until a morning thunderstorm complex over western South Dakota caught the attention of Storm Prediction Center forecasters that the severity of the situation became apparent. At 11:51 a.m. Central time, the agency drew a level 4 out of 5 risk for severe weather on their forecast map, writing, “A derecho with embedded significant severe wind gusts appears probable from central to eastern South Dakota into southwest Minnesota and northern Iowa into this evening.”

On satellite imagery, the derecho could be seen ingesting warm, humid air from the east.

By the end of the day, the derecho had traveled about 500 miles. Virtually all customers who had lost power had it restored as of Wednesday morning, according to data from PowerOutage.us.

Although it’s unclear if another derecho will form on Wednesday, strong to severe thunderstorm are again forecast to erupt overtop the heat dome along a corridor from the northern Rockies to the Mid-Atlantic. With a heat dome lodged over the zone from the Plains to the Carolinas, nearly 80 million people are under excessive heat warnings or heat advisories Wednesday.

Jason Samenow contributed to this report.

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