“Dark clouds approaching at Chicago O’Hare.” – Twitpic from sunriseweather

By Bill Deger, Meteorologist

Jul 11, 2011; 10:00 AM ET

“A wind gust to near 75 mph was measured at Midway Airport… Downed trees and power lines along with power outages have been reported.”

A wide area along the Interstate 70 and 80 corridors will be at risk for damaging thunderstorms today and tonight as a front crashes into a steamy air mass.

Already this morning, nasty thunderstorms with a history of producing hurricane-force wind gusts were ripping through Iowa and northern Illinois, tearing down trees and power lines and damaging homes.

These thunderstorms were plowing into Chicago just in time for the morning rush. A wind gust to near 75 mph was measured at Midway Airport at 8:15 a.m., while a 63-mph gust was recorded at O’Hare International Airport minutes earlier.

Flights were being delayed more than an hour.

A roof was reportedly ripped off a structure near Libertyville, Ill. Other reports coming in site “flattened” corn fields near Nelson and Polo, Ill., and a semi-truck blown onto its side in Rockford, Ill.

AccuWeather.com’s affiliate, WBBM Newsradio 780 in Chicago, reported skies so dark it almost looked like midnight. Metra train service on the northwest side of the city was stopped as the thunderstorms approached.

The thunderstorms will continue roaring eastward into Michigan and northern Indiana this morning before more pop up farther south later today, as afternoon temperatures rise well into the 90s.

The massive surge of hot and humid air will be the fuel for the strong storms, which will erupt ahead of a cold front today and this evening. The corridor at risk for these storms extends from the northern and central Plains into the Midwest.

Storms will likely form in clusters and have the ability to “bow out” into line segments and rapidly push south and east. Similar thunderstorms late Sunday spread a path of wind damage and even spawned a few tornadoes from the Dakotas into Minnesota.

Also similar to Sunday, the bulk of the activity is expected late in the afternoon and evening, with many areas not even experiencing them until well after dark tonight.

Metropolitan areas that will need to be on the lookout for ominous skies today and tonight include Omaha, Neb., Des Moines, Iowa, Indianapolis, Ind., and Cincinnati, Ohio.

Damaging wind gusts past 70 mph, capable of bringing down large trees, will occur in some of the storms. Similar wind speeds could also damage structures such as farm mills and silos.

Large, pelting hail is also likely in the stronger storms, especially in those forming on the central Plains. Hail larger than the size of quarters has the potential to damage cars and even sensitive crops.

Torrential, flooding rain and frequent lightning are always a threat from any thunderstorm, whether outdoors, at home, or in a car.

If you encounter a thunderstorm while driving, slow down and, if necessary, pull over and wait out the worst of the storm. Heavy rain, hail and high winds produced by storms will create near-zero visibility for a time, making driving dangerous.

People planning on being outdoors, whether it be for business or leisure, should stay up to date and be ready to head to safety at the first sign of a storm.

While the threat for tornadoes will be isolated, some severe thunderstorms have the potential to spawn them. At first warning of a possible tornado, be sure to head to a basement or the lowest floor of the building you are in.

Locally strong thunderstorms will also be on the prowl across other parts of the country to begin the workweek today.

Typical afternoon summertime storms will erupt over Georgia, Florida and the Gulf coast region, while monsoonal moisture will trigger widespread late-day storms over the Southwest and Rockies. Both of these areas could experience a few isolated damaging storms.

AccuWeather.com Meteorologist Heather Buchman contributed to the content of this story.