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In this image, raindrops are seen on a vehicle window as a pedestrian walks in the rain in Los Angeles, Jan. 31, 2019. FREDERIC J. BROWN/AFP/Getty Images

Severe thunderstorms and tornadoes will sweep the United States in the coming days, threatening almost one million square miles across 18 states.

Ahead of the tumultuous spring weather, thunderstorms were forecasted for parts of the country including Iowa, South Dakota, Michigan and Ohio on Thursday. Indianapolis was hit by a severe storm Thursday afternoon, leaving over 40,000 without power. Parts of Illinois also witnessed golf ball-sized hail.

The National Weather Service (NWS) said Tuesday that Central U.S. is in the path of the storms that could bring tornadoes and large hail, affecting 50 million people.

“There are several rounds of potential severe weather. Right now, we have slight and enhanced risk of severe weather for the next three days and into next week as well,” said Marc Chenard, a meteorologist with the NWS.

The first round of the severe weather is forecasted to begin from western Texas to southern South Dakota on Friday afternoon. The storm could bring tornadoes and hail and also result in flash flooding in some area. Parts of Nebraska could also witness multiple tornadoes Friday evening. Cities that could be worst affected Friday include Midland in Texas, Guymon in Oklahoma, Goodland in Kansas and Sterling in Colorado.

AccuWeather meteorologist Jake Sojda said, "Heavy to locally severe storms may erupt near the northern boundary of warm air over northern Iowa and southern Minnesota during Friday.”

The storms will then move over the plain Friday night and shift eastward Saturday. The storms will reach Mississippi and the Gulf Coast on Sunday, bringing isolated tornadoes. The severe weather may affect as for as northern Louisiana.

A new round of the severe weather will then begin Monday. It will emerge from the Rockies and move over the High Plains. It will move eastwards, affecting states such as Texas, Iowa and Nebraska.

"It's hard to nail down where the most severe weather and tornadoes will hit over the next few days. Pay attention to the weather, pay attention to weather alerts, pay attention to local warnings. Just pay attention!" AccuWeather senior meteorologist Paul Walker said.

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