India, Jan. 24 -- A snowstorm and harsh winter weather have impacted at least 177 million people across the US, with several Northeastern and Mid-Atlantic states under watches or warnings for ice and snow. More than 200 million were under cold weather advisories or warnings.

As a winter system lasts through the weekend, forecasters are again pointing to the polar vortex, a large-scale atmospheric pattern that can determine how far south Arctic cold is able to travel.

The polar vortex is a vast pool of frigid air that circulates high above the Arctic, spinning in the upper atmosphere. It exists because temperatures near the North Pole are far colder than those farther south, creating powerful winds that rotate around the region. Contrary...