‘Major Severe Weather’ Forecast for Upper 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 through tomorrow, said the U.S. Storm Prediction Center. About 12 million people, including Chicago residents, are in an area considered to be at high risk.

A tornado watch, meaning the deadly storms are likely, has been issued until 9 p.m. local time for parts of Iowa, Minnesota, Wisconsin and Illinois, and “potentially explosive thunderstorm development is possible over the next few hours,” the center said.

“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.

Storm’s Timing

The most severe weather is expected to start in the Chicago area about 5 p.m. local time, the National Weather Service said. Gusts as high as 100 mph are possible.

Today’s storms will probably cause widespread power failures and disrupt air traffic, especially at Chicago’s O’Hare International Airport and in Detroit, Schneider said. Airlines are being warned that extreme turbulence is possible and 3-inch (8 centimeter) hail is possible on the ground and in the air.

At 2:30 p.m. local time, 108 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.

Flood Alerts

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. A second round of bad weather, sparked by the system crossing the Midwest today, will strike 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.

NYC Rain

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.

New York may still receive 2.5 to 3.5 inches of rain, with some isolated areas getting as much as 5 inches through tomorrow night, according to the weather service.

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.