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Some areas of Utah are expecting up to two feet (60 cm) of snow
By Madeline Halpert-BBC News, Washington
A powerful storm has spawned tornadoes in the southern US and brought heavy snowfall and other extreme weather across the US and parts of Canada.
The storm has injured several people in Texas and damaged buildings.
Around 9,000 US flights were delayed on Monday and Tuesday as the storm churned across the country, according to FlightAware.com.
The weather has also closed down roads in some western states that are expecting up to 2ft (60cm) of snow.
The storm could affect residents in nearly every state in the contiguous US, experts said.
“It’s a fairly vigorous storm system,” said Rich Otto, a meteorologist with the Weather Prediction Center. “This one is a little larger than your average winter storm.”
“Folks should stay home during the day [on Tuesday] if they live up north,” he added.
The worst of the weather began on Monday night and was expected to continue into Wednesday, hitting the central and northern High Plains, a region of the US that includes eastern Montana, Wyoming, South Dakota and Nebraska, Mr Otto said.
These states are expected to see gusty winds over 40mph (64km/h) and heavy snowfall.
More than 20 million people were under severe weather advisories in the US on Tuesday.
Blizzard warnings from the National Weather Service were also in effect for all of those states, as well as Colorado. Those are triggered by winds of at least 35mph accompanied by heavy snowfall.
Heavy snowfall combined with strong wind gusts will create low visibility and “treacherous” travel conditions, Mr Otto said.
It is expected that some parts of the region could get one to two inches of snow per hour.
The storm has also spilled into Canada, with heavy snow and high winds hitting the country’s southern Prairie region, including the provinces of Saskatchewan and Manitoba, on Tuesday and Wednesday.
Upper Midwestern states are also expected to get a quarter to half an inch of freezing rain on Tuesday. Meanwhile, the lower Mississippi Valley faces the possibility of severe thunderstorms, hail and flash floods, according to the National Weather Service.
Texas, in the south, was hit by a series of storms that spawned multiple tornadoes in the northern part of the state, including one in Wise County, near Dallas, that damaged homes and businesses, according to local authorities.
At least seven people in Texas were injured by the severe weather, according to local government officials.
A twister hit Wayne, Oklahoma, on Tuesday morning reducing at least one family home to rubble, but leaving residents unscathed, according to local media.
The extreme weather is expected to linger through much of this week, experts said, as it pushes eastward into the central Plains by Thursday.
The storm system began to hit the US last Friday, bringing powerful winds to California.
The Sierra Nevada mountain range in northern California reported 5ft of snow in some areas over the weekend, forcing some highways to close and triggering avalanche warnings in the area through Monday night.