We begin with the severe storm rolling across the nation right now, weather experts putting out a kind of all points bulletin. One in five americans in the path of what could become a weather... See More
We begin with the severe storm rolling across the nation right now, weather experts putting out a kind of all points bulletin. One in five americans in the path of what could become a weather phenomenon called a derecho, a 240 mile stretch of wicked wind. Ginger zee is standing watch with the people of chicago right now. Ginger? Reporter: We's just hearing the first rumles of thunder over chicago. Tonight millions in some of the bigger cities in the midwest included in this highest risk zone. Reporter: Tonight, the atmosphere is full I charged. Primed for violent storms. And we are right here. I'm just looking up at the atmosphere really fired up already here. Just outside chicago, the high risk issued for cities like davenport, iowa, all the way to fort wayne and including chicago, illinois. A high risk is just off the charts. That's when we're really sounding the bell as loudly as we can to let folks know they really need to pay attention and they need to take this seriously. Reporter: The national weather service outside chicago abuzz. We could see those thunderstorms gather together and coalesce into a series of lines and segments that would produce damaging wind gusts. Reporter: The chaos in the father could produce a dangerous weather phenomenon known as a derechoes. It's calling care? Ominous shelf clouds like these. Reporter: Winds can blast up to 100 mile per hour. It takes on a bow shape because air rises, cools then falls behind them and then pushe front of the line out. They can be deadly and certainly as damaging as these tornados. Last year's derecho tore through 12 states, with it blindingdown pours from illinois to d.C., winds gusting over 100 mile per hour and a huge part of the nation peppered with lightning. Trees were plucked and chucked into homes and cars. This street in ohio blanketed in power lines. It left $1 billion in damage and killed 13 people. I need to show you those big areas that are on alert tonight. So from eastern iowa all the way through chicago, into northwest indiana and michigan. It has the potential all the way through parts of ohio and tomorrow into parts of the east coast. Ginger, I know you said people should stay indoors but what if you are caught outside? We want everyone to be in shelter always. If you are outside, in your car, you want to be in your car, put yourself there and seatbelt of course, put that on, put the head down, the flashers on and cover your head from debris. If you can get to shelter, please do. Everybody is warped to do just that. Thank you so much. Ginger zee reporting and standing watch tonight.
This transcript has been automatically generated and may not be 100% accurate.