A brutal cold snap has arrived in the central and eastern parts of the United States and Canada just in time for Valentine’s Day weekend, dragging south some of the coldest air we’ve seen so far this winter. Low temperatures on Saturday morning plummeted to double digits below zero in the Upper Midwest, with lows on Saturday and Sunday dipping near or below zero in the Northeast and Mid-Atlantic. This kind of cold weather, while common in February, is still dangerous nonetheless.

The elephant in the room with a cold snap like this is the dreaded polar vortex, a term that took the Internet by storm back in 2014 when intense, relentlessly cold temperatures gripped just about everyone east of the Rocky Mountains for a couple of weeks. A convenient scapegoat and mainstay of scary headlines, the polar vortex is nothing new (scientists discovered its existence in the 1800s) and it’s not always responsible for cold weather in the winter. It is winter, after all. 

The polar vortex is a persistent counterclockwise circulation around the North Pole that largely keeps the coldest air confined to the far northern latitudes, acting sort of like a barrier between us and a bad couple of days. Sometimes this circulation will break down a bit, allowing a weakness in the barrier that lets blasts of very cold air flood south from the Arctic regions of Canada into the rest of Canada and the United States. Even more rarely, like back in 2014, the main circulation in the polar vortex will swing south, creating an epic bout of frigid weather.

Thursday morning’s run of the GFS model shows a trough in the middle levels of the atmosphere dipping over parts of the northern United States, bringing along with it the first brutal cold snap of the winter. Image credit: Tropical Tidbits

This weekend’s cold snap isn’t the main circulation of the polar vortex, but rather one of those troughs dipping south; meteorologists on social media will occasionally refer to it as a “lobe” or just a piece of the polar vortex heading our way. The above image is what it looks like at the 500 millibar level of the atmosphere, which usually sits about 18,000 feet above the ground. Colder colors show lower heights (roughly equating to lower air pressure), while warmer colors show higher heights (roughly equating to higher air pressure).

Looking at the weather model image above, you can see that the main circulation stays up over far northern Canada, while that little tendril of cold briefly swoops south over the Great Lakes before visiting the Northeast and retreating back into Canadian airspace. This is the source of our cold weather this weekend.

Thursday morning’s run of the GFS model showing low temperatures in the central U.S. on Saturday morning. Image credit: WeatherBELL

How cold will it get? Pretty doggone cold. The worst of the Arctic outbreak unfolded across the Upper Midwest—centered on Minnesota—on Saturday morning. Minneapolis reached -6°F overnight, and other places could plummet to -20°F, which will approach record lows in some spots, but certainly not the coldest temperatures ever recorded. Wind chill values on Saturday morning were even colder. 

It’ll be cold in the Northeast today, Saturday, but tomorrow, Valentine’s Day, will be the heart of the outbreak across the most populous region in the country.

Thursday morning’s run of the GFS model showing low temperatures in the eastern U.S. on Saturday morning. Image credit: WeatherBELL

Thursday morning’s run of the GFS (American global) weather model show that temperatures will close in on the single digits in Washington, D.C., with lows approaching the zero mark in Philadelphia, New York City, and Boston. Away from the coast, communities in New England will experience low temperatures well below zero, approaching double digits at higher elevations. The model even paints possible temperatures as cold as -20°F or worse in New York’s Adirondack Mountains.

High temperatures in the Northeast on Sunday will struggle to climb out of the single digits and teens.

While those air temperatures are bad enough, the wind will make it even worse. The wind chill is what the combination of cold air and wind feels like on your skin. Wind blows away the bubble of warm air over your skin, making your body work even harder to stay warm. Frostbite, hypothermia, and even death are possible if you’re not properly protected from both the cold and the wind.

Thursday morning’s run of the GFS model showing wind chill values on Saturday morning. Image credit: WeatherBELL

The same run of the same weather model shows wind chill values below zero into the Mid-Atlantic, with wind chills of -20°F to -30°F or worse possible in New England. This kind of cold is extremely dangerous for even just a couple of minutes, and both humans and animals could be quickly injured or killed if not properly protected.

Make sure you bundle up nice and tight if you have to go out this weekend, and please remember to bring your pets inside.