US Midwest put on tornado watch

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 injuries were reported.

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

An outbreak of deadly twisters hit the states of Indiana, Kentucky, Ohio, Georgia and Alabama in early March.

At the start of April the Dallas-Fort Worth area was badly hit, with hundreds of flights being disrupted but no-one injured or killed.

'Dangerous day'

The worst storms were predicted to hit Kansas and Oklahoma on Saturday, forecasters said.

The SPC issued a storm warning for a "high risk of severe thunderstorms" in eastern Nebraska and far-western Iowa, as well as south-central Kansas into west-central Oklahoma.

Parts of Texas, Minnesota and Illinois were also put on storm alert.

"We're quite sure tomorrow will be a very busy and dangerous day in terms of large swathes of central and southern plains," National Weather Service spokesman Chris Vaccaro told the Associated Press. "The ingredients are coming together."

Those ingredients include strong jets of wind moving in from the west mixing with moisture-rich air moving across from the Gulf of Mexico.

The difference in wind direction is expected to increase the possibility of tornadoes.

- BBC