A cross-country tornado snow blitz threatens 100 million Americans with the ‘most severe’ storm outbreak of the year. ‘Dangerous, damaging and life-threatening’ weather conditions spanning half a million square miles will smash the US into mid-month.

On Thursday, the storm was bringing heavy snow and strong winds to the Sierra Nevada and heavy rainfall over parts of coastal southern California.

Dozens of tornadoes are reported from thunderstorms that lit up the radar from Wisconsin to Mississippi in the early hours Saturday.