The most intense cold snap to affect the United States in 20 years descended upon parts of the Midwest on Monday and is expected to linger through the week. Meteorologists forecast that temperatures may dip to nearly negative 40 degrees Fahrenheit (which is also negative 40 degrees Celsius)—negative 60 degrees with wind chill—in Minnesota and as far west as Montana. "We haven't seen temperatures this low since January of 1994," says Jeff Masters, director of meteorology at the popular website Weather Underground. States as far south as Alabama and Georgia will also experience chillier temperatures than they've seen in years. "The atypical part of this weather pattern is some of these really cold temperatures are going to be pressing into parts of the country that don't typically see that magnitude of cold in early winter," said Robert Oravec, a lead forecaster with the National Weather Service in College Park, Maryland. Chilled in Canada Experts say the unusually cold temperatures are caused by air chilled in northern Canada being blown south into the U.S. "That air has been sitting for days in northern Canada, in almost 24-hour darkness, and over snow-covered ground that radiates heat out to space very efficiently," Masters says. Over the weekend, the core of that cold air mass—a whirlpool of frigid air called a "polar vortex"—was shunted southward by the jet stream, a band of strong wind that blows in the upper level of the atmosphere. "The jet stream acts as a kind of boundary between cold air in the north and warm air to the south," Masters explains. Kink in the Jet Stream The jet stream typically blows relatively straight west to east, but every so often a kink forms that changes its direction. "If you get a really good kink, it allows cold air from Canada to spill southward," Masters says. "The shape of this kink is a little bit unusual. We haven't seen something like this in about 20 years." Oravec of the National Weather Service adds: "In this case, we're in such a weather pattern in which the polar vortex is being pushed pretty far south, almost into the Great Lakes area." Masters, who lives in Detroit, says he plans to weather out the cold snap at home, and advised others to do the same. The ASPCA also advises that pet owners keep their animals indoors when the mercury dips. Dogs can lose their scent very easily in snow, and may become lost if they get off their leash. If your dog must be outdoors, ASPCA recommends owners wipe off the pooch's legs and stomach after coming back inside to prevent it from licking antifreeze, salt, or other potentially dangerous chemicals from its paws. The good news: meteorologists predict that this cold snap will not last very long and that temperatures will return to normal by the end of the week. "It's a very intense, but relatively short-lived, cold air outbreak," Masters says. Follow Ker Than on Twitter.

Explaining Record Cold Snap

The most intense cold snap to affect the United States in 20 years descended upon parts of the Midwest on Monday and is expected to linger through the week. Meteorologists forecast that temperatures may dip to nearly negative 40 degrees Fahrenheit (which is also negative 40 degrees Celsius)—negative 60 degrees with wind chill—in Minnesota and as far west as Montana. "We haven't seen temperatures this low since January of 1994," says Jeff Masters, director of meteorology at the popular website Weather Underground. States as far south as Alabama and Georgia will also experience chillier temperatures than they've seen in years. "The atypical part of this weather pattern is some of these really cold temperatures are going to be pressing into parts of the country that don't typically see that magnitude of cold in early winter," said Robert Oravec, a lead forecaster with the National Weather Service in College Park, Maryland. Chilled in Canada Experts say the unusually cold temperatures are caused by air chilled in northern Canada being blown south into the U.S. "That air has been sitting for days in northern Canada, in almost 24-hour darkness, and over snow-covered ground that radiates heat out to space very efficiently," Masters says. Over the weekend, the core of that cold air mass—a whirlpool of frigid air called a "polar vortex"—was shunted southward by the jet stream, a band of strong wind that blows in the upper level of the atmosphere. "The jet stream acts as a kind of boundary between cold air in the north and warm air to the south," Masters explains. Kink in the Jet Stream The jet stream typically blows relatively straight west to east, but every so often a kink forms that changes its direction. "If you get a really good kink, it allows cold air from Canada to spill southward," Masters says. "The shape of this kink is a little bit unusual. We haven't seen something like this in about 20 years." Oravec of the National Weather Service adds: "In this case, we're in such a weather pattern in which the polar vortex is being pushed pretty far south, almost into the Great Lakes area." Masters, who lives in Detroit, says he plans to weather out the cold snap at home, and advised others to do the same. The ASPCA also advises that pet owners keep their animals indoors when the mercury dips. Dogs can lose their scent very easily in snow, and may become lost if they get off their leash. If your dog must be outdoors, ASPCA recommends owners wipe off the pooch's legs and stomach after coming back inside to prevent it from licking antifreeze, salt, or other potentially dangerous chemicals from its paws. The good news: meteorologists predict that this cold snap will not last very long and that temperatures will return to normal by the end of the week. "It's a very intense, but relatively short-lived, cold air outbreak," Masters says. Follow Ker Than on Twitter.
Photography by Jeff Roberson, AP

Behind Record U.S. Cold Snap: Canadian Air and a Jet Stream Kink

Temperatures may dip to nearly negative 40 degrees Fahrenheit in parts of the country this week. Here’s why it’s so freakishly cold.

Read This Next

Why kids need their own COVID-19 vaccine trials

A black-footed ferret has been cloned, a first for a U.S. endangered species

3-ton parts of Stonehenge may have been carried from earlier monuments

Go Further

Subscriber Exclusive Content

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet