Dangerously low temperatures spread over most of the United States in January, but warmer temperatures are on their way. The sub-zero temperatures were caused by a polar vortex, which occurs when the jet stream around the polar region is displaced and pushes cold air to the south.

While the coldest temperatures seem to have passed, extreme cold is lingering as the polar vortex finishes a second push into the U.S. The effects are expected to center in the Midwest, stretching from Iowa to Kentucky over time. However, things are expected to warm up starting Jan. 21. Some states may experience rain following the increased temperatures.

Read more on winter weather expectations across the U.S. here.