Heavy snow and freezing temperatures swept through the Upper Midwest on Monday, with parts of Minnesota, Montana and Wisconsin getting around a foot of powder, triggering flight cancellations and hundreds of car crashes as the roads turned into ice. More than 100 schools were closed and Veterans Day activities were called off because of the sudden onset of winter weather.

Seven million people were under winter storm warnings from Montana to Michigan ahead of the arctic blast of frosty temperatures that will reach the East Coast later this week. By late afternoon, Whitefish, Montana, had been hit by 14 inches of snow, while Webster, Wisconsin, and Isanti and St. Augusta in Minnesota had 11 inches, according to the National Weather Service. Several others areas had five to 10 inches of snow.

The snow snarled traffic in many places: Police responded to more than 400 car crashes in Forest Lake, Minnesota, while a car that slid off the road during the first snow in Duluth led to a 16-car pile-up when other drivers couldn't stop. Public transportation was also temporarily halted in Duluth, where officials advised people not to drive unless necessary. Over in Dalton, Colorado, a semi-trailer loaded with turkeys skidded off the interstate.

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Schools and air travel were affected, too. More than 100 schools were closed Monday in the Minneapolis area, according to NBC affiliate KARE, a;though many were expected to reopen Tuesday after a few hours' delay. And Rapid City Regional Airport in South Dakota was closed because of the weather, while nearly 200 flights were canceled at Minneapolis-Saint Paul International Airport.

"There were 175 flights canceled today, pretty extensive delays through this afternoon and lasting into this evening," Patrick Hogan, a spokesman for the Mnneapolis airport, told NBC News. "Our hope is that by 6 a.m., the airfield will be in pretty good shape."

Forecasters expect the snow to continue to fall in Minnesota, Wisconsin and Michigan through Wednesday.

An annual Veterans Day barbecue, which typically welcomes 300 to 400 people in Eau Claire, Wisconsin, and is primarily held outdoors, has already been canceled, NBC affiliate WEAU reported.

A cold surge will accompany and follow the snow, with two-thirds of the country seeing below-average temperatures by Friday, The Weather Channel reported. Nearly half of Oklahoma, and even parts of Texas, were under freeze or hard freeze warnings Monday, while states to the west, like Nevada and Idaho, were dealing with wind gusts above 60 mph.

But temperatures had already plunged in some places: In Rapid City, South Dakota, the temperature fell from 53 to 16 degrees, and in Springfield, Colorado, it dropped 22 degrees in a few minutes.

It's the duration of the cold air that's unusual this early in the season; Minneapolis could stay below 32 degrees for seven days straight. The weather is part of a system being pushed in by the remnants of Typhoon Nuri, which hit Alaska's Aleutian Islands with hurricane-strength winds over the weekend.