Winter’s first polar vortex blast, already taking shape in the Arctic this weekend, targets the Lower 48 next week. By Tuesday, temperatures below zero will plunge south into the northern plains and Midwest. Over the course of a few days, the cold air will blast across the country to the Northeast.

The northern tier has already seen a taste of what this winter has to offer – in fact, the region is already experiencing a significant cold snap. On Thursday morning, the temperature in Billings, Mont., dropped to minus-3. It was the first time the location saw a temperature below zero in 698 days, since Jan. 9, 2015. Almost the entire state of North Dakota is under a wind chill advisory – the National Weather Service is calling for temperatures that feel like minus-35.

Next week’s cold blast will dive farther south and east.

Weather forecast models are suggesting temperatures will nosedive in the Midwest. All of Minnesota and Wisconsin – plus the Chicagoland area – could see overnight lows plummet into negative territory: minus-15 in Minneapolis, minus-10 in Milwaukee and minus-5 in the Chicagoland area around Wednesday or Thursday.

The forecast is 20 to 35 degrees below average for this time of year.

This National Weather Service map shows the forecast for Saturday. Image from National Weather Service

What will likely be the coldest air since last February will barge into the mid-Atlantic and Northeast late next week. Daytime temperatures from Washington to Boston will struggle to climb above freezing. Overnight lows will surely be in the single digits and teens, if not below zero in parts of New England.

Through Thursday, 75 percent of the Lower 48 will have experienced a temperature below freezing, including Texas, the Deep South and the Pacific Northwest, based on National Weather Service forecasts.

The frigid air tied up in this polar vortex blast has its origins in Siberia and northern Canada. It will be the coldest air of the season so far for most of the United States. Future cold blasts may be more potent, but we haven’t experienced this since last February.

The polar vortex is not a new thing – it’s a weather term that was popularized in 2014, though it’s always been something meteorologists knew of and referred to among themselves.

It’s a very large, extremely cold air mass over the Arctic (the Antarctic has one, too). The concentrated area of cold air is bound by the jet stream, which is a current of fast-moving air at very high levels of the atmosphere. When the jet stream is strong and keeps the polar vortex area bottled up north, temperatures can fall to minus-100 degrees.

The vortex is always present – even in the summer. But winter is when it really comes alive – not only is Arctic air colder because of the lack of sunlight, this is also when the jet stream plunges south. When that happens, it allows the cold air to spill south, like a freezer with the door left open.

Some of the most historic cold-air outbreaks of the past 30 years have been caused by the polar vortex diving south.