US Midwest put on tornado watch

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A severe storm threatens to bring tornadoes to a vast swathe of the Midwestern US, forecasters are warning.

States ranging from Texas to Minnesota have been put on alert, with tornado experts saying storms on Saturday could be a "life-threatening event".

On Friday a first tornado swept into Norman, Oklahoma, site of the US national Storm Prediction Center (SPC). No serious injuries were reported.

US tornadoes have already killed at least 39 people in 2012.

Nick Miller, from BBC weather, explains what conditions caused the tornadoes.

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