A massive, violent derecho slammed the Upper Midwest, from Iowa to Indiana, yesterday afternoon and evening. The fast-tracking storm generated widespread wind gusts of 70-80 mph, left three quarters of a million without power, and caused two fatalities.
Here we capture the science and power of this storm in ten images.
1. The view from space
We start our tour from the grandest possible perspective, that of geostationary satellite. Derechos often present a massive, elliptical appearance in the infrared imagery. All of that cloud ice was ejected upward in narrow but powerful cloud updrafts, spreading horizontally at the stable tropopause at an altitude of nearly 60,000 ft.
2. Arrow of destruction.
Beneath dense cloud layers, weather radar reveals where the action is: Focused along a spearhead-shaped, fast-moving squall line of intense thunderstorms. This is called a bow echo. And if you imagine a cocked hunting bow, the arrowhead marks the zone of intense, forward-blasting wind, a swarm of downbursts.
3. Electrical mayhem
Within the derecho’s cloud canopy is a powerful dynamo – tens of thousands of electrical discharges. These cloud-to-ground strikes are so dense, they outline the shape of the bow echo! Lightning activity this vigorous requires powerful updrafts, copious amounts of supercooled water, and dense pockets of ice particles including snow and graupel.
4. Downburst On doppler.
By measuring the frequency shift of microwave energy pulses, wind speed can be accurately ascertained within a bow echo. This image shows the approach of the bow echo through the eyes of Doppler radar. The radar is located at Davenport. The arc of the storm approaches from the northwest. The pocket of pink is a wall of 65-70 mph wind bearing down on Iowa City, created by a cluster of downbursts.
5. This bow delivers a blow.
Here is the bow echo, up close and personal. The color scale indicates composite radar reflectivity, which shows not just the strongest radar return near the surface, but anywhere in the vertical column up to 60,000 ft. There is no mistaking the forward blast of downburst winds south of Dubuque, which distorts the leading edge of the squall line into its characteristic bow shape. Also embedded in this bow echo were large mesovortices, which could be seen swirling counterclockwise in the time-lapse radar imagery.
6. Ground truth.
Here is what the derecho looked like at the ground, on the approach. The low cloud bank is called a shelf cloud, which marks the gust front or leading edge of the wind surge. This view is looking toward the west in Cedar Rapids. Just behind the shelf is the “wall of wind” and torrents of heavy rain mixed with hail.
— Zachary Jay Stewart (@zachjaystewart) June 30, 2014
This is what derechos do best – snap and uproot trees, millions of them. This image was taken at Winterset, Iowa, a particularly hard-hit location. It will take days to clear out the debris, repair roofs, and mend the shredded electrical grid. It’s been scientifically demonstrated that a single derecho can inflict the same level of wind damage as a landfalling hurricane.
8. Damage control.
This morning, official damage reports are still arriving at NOAA’s Storm Prediction Center, but the outline of the derecho on this damage map is unmistakable. Derechos often produce a linear, fan-shaped envelope of dense wind damage. Field surveys are underway to tease out possible tornado tracks. Weak to moderate intensity tornadoes are often embedded in a bow echo, developing in small meoscyclones along the bow’s leading edge.
9. The word got out.
NWS’s Storm Prediction Center made an accurate call outlining the threat area, many hours before storms congealed into a bow echo. SPC outlined a moderate risk area, with 30%-45% probability of damaging winds, and also a heightened awareness for tornadoes. The timely package of outlooks and watches was key to survival, since this derecho took dead aim at several urban centers including Chicago and Milwaukee.
10. Lots of colors are never good
When you click on a NWS homepage, an image like this does not portend fair weather. This is a snapshot from 4:15 p.m. CDT, showing all active warnings along the southern end of the bow echo. One under-appreciated hazard of derechos is widespread flash flooding. The flood zone sets up along the bow echo’s southern flank, where intense cells train (pass one after another) across the same region.
Here’s a video recap of the storm from Reuters: