Harsh weather moved west on Monday as a polar vortex was expected to grip the Rockies and the northern Plains after winter storms pummeled the eastern US over the weekend, killing at least 10 people, including nine victims in Kentucky who died during flooding from heavy rains. Over 1000 people stranded by floods had to be rescued.

The National Weather Service warned of “life-threatening cold” into Tuesday, with temperatures in northeastern Montana predicted to dip as low as 45° below zero (-42.7°C) with wind chills down to 60 below (-51°C).