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Temperature anomalies at about 5,000 feet, showing the Arctic outbreak (purple and blue). Graphic via Weathermodels.com

If you're 25 or younger and live within a large swath of the Midwest — from Minneapolis to Chicago — this week will feature the coldest temperatures of your life as a lobe of the polar vortex plunges south out of the Arctic.

Why it matters: The frigid outbreak, which has already begun in parts of Minnesota and Wisconsin, will be life-threatening and disruptive for tens of millions of people this week. Wind chills on Wednesday morning in parts of Illinois, Wisconsin and Minnesota are likely to approach the minus 50s°F to minus 60s°F. Such conditions can cause frostbite in just minutes and will likely force the cancellation of schools across the region.

Details: Weather forecasters across the Midwest have begun using words and phrases like "historic" and "all-time coldest" in association with the Arctic outbreak heading for the region. From Tuesday to Thursday, temperatures about 5,000 feet above the surface, which forecasters look to as an indicator of potential surface temperatures, may reach the coldest values ever observed in the lower 48 states.

  • In Minneapolis, Milwaukee, Madison, Des Moines, Green Bay, Chicago, Indianapolis, Detroit and Chicago, the cold will peak between Tuesday and Thursday, but it will remain extremely cold throughout the week.
  • In a harbinger of things to come, International Falls, Minn., known as the nation's "icebox," saw its temperature plummet to minus 46°F on Sunday, the fifth-lowest temperature it has ever recorded.
  • A fresh snowpack, courtesy of a storm moving through the region earlier in the week, will enhance the potential for record cold readings, as snow-covered ground is more efficient at reflecting solar radiation back into the atmosphere during the day and keeping it colder at night.

By the numbers: The National Weather Service is warning that wind chills will plunge to the lowest readings since the mid-1990s across the Upper Midwest.

  • The NWS is forecasting a low temperature in Chicago of minus 20°F on Wednesday morning. This would set a daily record low and be one of the coldest temperatures ever recorded in the city. The last time it was this cold in Chicago was Jan. 18–20, 1994, the Weather Service said.
  • A low of minus 20°F would be within 7°F of the city's all-time coldest temperature, which was set in January 1985.
  • If Chicago’s high temperature remains below minus 11°F on Wednesday, it would be the city's coldest afternoon high temperature on record. In fact, the city could see temperatures remain below 0°F for 60 hours or so, a rarity even for such a cold location.
  • About 200 million people in the U.S. will see temperatures dip below freezing during the coming week, based on model projections.

The bottom line: The NWS forecast office in Des Moines put it starkly: "This may be the coldest air in a generation."

  • The Arctic outbreak is associated with a piece of the tropospheric polar vortex, which has broken off and spun south out of the Arctic. The stratospheric polar vortex — which is a whirl of low pressure at upper levels of the atmosphere over the pole — was knocked off-kilter in early January, increasing the odds of cold outbreaks in the U.S. and Europe.
  • The globe as a whole, however, is still much warmer than normal, and scientists say the cold snap in parts of the U.S. in no way invalidates the overwhelming scientific evidence showing the planet is warming over the longer term due to the burning of fossil fuels for energy.

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