‘Major Severe Weather’ Forecast for U.S. Midwest Today

A powerful storm system capable of toppling trees, knocking out power and tying up air traffic will probably start later today with an outbreak of strong tornadoes across northern Illinois and Iowa.

Almost 51 million people from Iowa to the Atlantic are under threat from “major severe” thunderstorms and high winds, said the U.S. Storm Prediction Center. About 12 million of them, including Chicago residents, are in the area most at risk.

“We have a powerful weather system taking shape in the Midwest,” Russell Schneider, director of the center in Norman, Oklahoma, said by telephone. “This is a very significant weather day for Chicago and the upper Midwest, one of the more severe weather days they will see this year, so people need to have a plan.”

There is a chance the storms will grow into a derecho, a rare event characterized by winds of at least 58 miles (93 kilometers) per hour creating a line of damage at least 240 miles long. A derecho swept through the Midwest into the mid-Atlantic a year ago, knocking out power to 5 million people from Chicago to the District of Columbia and killing 22, according to government data.

Today’s storm will probably cause widespread power failures and disrupt air traffic, especially at Chicago’s O’Hare International Airport and in Detroit, Schneider said.

Tornado Threat

“There is a significant tornado threat across northern Illinois and Iowa and that threat will quickly become a damaging wind and hail threat,” he said.

At noon local time, 95 flights had been canceled at O’Hare, according to FlightAware, a Houston-based airline tracking service.

Derechos are rare because they require a number of smaller storms “to work together,” Schneider said. More than 75 percent of derechos occur from May to August and they’re most likely to happen along an axis from the southern Great Lakes southwest into Texas, according to the storm center.

Today’s storms are also expected to bring heavy rains, according to the National Weather Service. Flood watches, warnings and advisories stretch in an unbroken string from northern Illinois to Massachusetts.

There is a chance of severe weather later today and overnight along the Interstate 95 corridor from Philadelphia to Washington, said Tom Kines, a meteorologist with AccuWeather Inc. in State College, Pennsylvania. Those storms won’t be the main threat, however.

Tomorrow’s Threat

A second round of bad weather, sparked by the same system crossing the Midwest today, will strike at the mid-Atlantic tomorrow, Kines said.

An area from central New Jersey to northern Virginia, including Philadelphia, Baltimore and Washington, has a 45 percent chance of high winds, hail and possibly tornadoes, according to a storm center forecast.

Kines said New York City and Boston may be spared the worst weather. The New England states will benefit from cool onshore winds that will help stabilize the atmosphere, keeping the severe thunderstorms from forming, he said.

Severe thunderstorms and the tornadoes that sometimes accompany them caused $15 billion in insured losses in 2012 and $25 billion in 2011, according to the Insurance Information Institute in New York.

From 1992 to 2011, thunderstorms and tornadoes accounted for the second-highest amount of catastrophic loss in the U.S., $130.2 billion, topped only by hurricanes and tropical storms with $161.3 billion, the institute said.

To contact the reporter on this story: Brian K. Sullivan in Boston at bsullivan10@bloomberg.net

To contact the editor responsible for this story: Dan Stets at dstets@bloomberg.net

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