A damaging derecho threatens to barrel across the Midwest on Wednesday
An outbreak of severe thunderstorms is erupting across parts of the Midwest, with Chicago, Columbus and Dayton, Ohio in the crosshairs of what could become a long-lasting damaging wind event known as a "derecho."
Such events are relatively rare and difficult to predict, but computer models and weather observations are showing the potential for a derecho (pronounced de-Ray-cho) to coalesce across central Illinois into north-central Indiana and Ohio through Wednesday night, before barreling southeastward, possibly reaching Washington, D.C. by Thursday morning.
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Derecho events are defined as "a widespread... straight-line windstorm." Such storm systems may or may not contain tornadoes as well. Events like this can bring winds at or above hurricane force, and knock out power to hundreds of thousands of people, depending on their path and intensity.
In June 2012, for example, a derecho tore through the Middle Atlantic states, leaving more than a million customers in the dark for days and even shutting down parts of Amazon's then nascent cloud computing division as well as Instagram and Pinterest.
The moderate risk area encompasses around 17.2 million people.
According to the Storm Prediction Center (SPC), there is a moderate risk of severe thunderstorms across a broad region from Iowa to Indiana and Ohio. The moderate risk area encompasses around 17.2 million people.
In that area, from western Illinois through northern Indiana and Ohio, there is a 45 percent probability of damaging thunderstorm winds or wind gusts of 58 miles per hour higher within 25 miles of a point through Wednesday night.
Moderate risk is the second-highest risk level on the SPC's storm outlook scale.
The highest chance for tornadoes, some of which could be intense (rated EF-2 or higher on the Enhanced Fujita Scale), looks to be in northwest Illinois, extreme southwest Wisconsin and northeastern Iowa, although this may change as the storm system evolves on Wednesday afternoon.
About 13 million people live in an area that has been given a 10 percent chance of seeing an EF-2 tornado or greater within 25 miles of a given point, including the Chicago metro area, which has not experienced a major tornado close to its downtown area in many years. Contrary to popular belief, the city's skyscrapers and proximity to Lake Michigan do not, in fact, hinder tornadoes or even prevent them from occurring in or close to downtown.
The National Weather Service forecast office in Chicago is predicting a complex evolution of the storm threat for northeastern Illinois.
"The region is primed for a significant severe weather event, most notably but certainly not limited to a widespread wind damage event, but details about the location of the stronger convection still remain a bit uncertain...," forecasters wrote in an online discussion.
One of the wild cards in the forecast is how far north a warm front will progress, since this will separate extremely humid air from drier air to the north. Areas along and south of the front stand a higher chance of seeing severe storms.
Some of the cities that have been hit by tornadoes in the past decade include Miami, Nashville, Fort Worth, Salt Lake City and St. Louis.
This is a developing story.
Andrew Freedman is Mashable's Senior Editor for Science and Special Projects. Prior to working at Mashable, Freedman was a Senior Science writer for Climate Central. He has also worked as a reporter for Congressional Quarterly and Greenwire/E&E Daily. His writing has also appeared in the Washington Post, online at The Weather Channel, and washingtonpost.com, where he wrote a weekly climate science column for the "Capital Weather Gang" blog. He has provided commentary on climate science and policy for Sky News, CBC Radio, NPR, Al Jazeera, Sirius XM Radio, PBS NewsHour, and other national and international outlets. He holds a Masters in Climate and Society from Columbia University, and a Masters in Law and Diplomacy from The Fletcher School at Tufts University.
