A huge swathe of the US was blasted with ice, snow and wind on Monday as the polar vortex that dipped south over the weekend kept much of the country east of the Rockies in its frigid grip, making many roads treacherous, forcing school closures, and causing widespread power outages and flight cancellations.

The immense storm system brought disruption to areas of the country that usually escape winter's wrath, downing trees in some Southern states, threatening a freeze in Florida and causing people in Dallas to dip deep into their wardrobes for hats and gloves.

Washington DC received heavy snow as President-elect Donald Trump’s victory was certified. Taking advantage of the rare snowstorm in the nation’s capital, revellers engaged in a snowball fight in front of the Washington Monument as flags flew at half-staff in memory of former President Jimmy Carter.

Ice and snow blanketed major roads in Kansas, western Nebraska and parts of Indiana, where the National Guard was activated to help stranded motorists.

The National Weather Service issued winter storm warnings for Kansas and Missouri, where blizzard conditions brought wind gusts of up to 72 kph. The warnings extended to New Jersey into early Tuesday.

The polar vortex of ultra-cold air usually spins around the North Pole, but it sometimes plunges southward into the US, Europe and Asia. Studies show that a fast-warming Arctic is partly to blame for the increasing frequency of the polar vortex extending its grip.

School closings were widespread, with districts in Indiana, Virginia, Kentucky, Missouri and Kansas cancelling or delaying the start of classes on Monday. Classes were also cancelled in Maryland, where Governor Wes Moore declared a state of emergency on Sunday and announced that state government offices would also be closed the next day.

More than 2,500 flights were cancelled and at least 6,500 more were delayed nationwide as of Monday morning, according to tracking platform FlightAware. (AP)