What to know about winter storm hitting North America

A huge winter storm has brought snow, ice and and freezing temperatures to a broad swathe of the US, with more than 60 million Americans under weather alerts.

Much of the US is waking up to below freezing temperatures on Monday morning, with some of the coldest conditions expected in Kansas, while the Washington DC area is awaiting unusually heavy snowfall.

Forecasters say the extreme weather - named Storm Blair - is being caused by the polar vortex, an area of cold air that circulates around the Arctic.

As of early Monday morning, 1,400 US flights had been cancelled for the day and 800 delayed, according to FlightAware.com.