A wave of extreme cold continues to sweep across most parts of the United States on Tuesday due to the snowstorm “Fern,” which has been described as one of the most severe in recent decades. The storm has so far resulted in at least 21 deaths and has left more than 800,000 homes without electricity.

According to the U.S. National Weather Service, wide areas — particularly in the Great Lakes region in the north of the country — have recorded a sharp drop in temperatures, falling to below minus 20 degrees Celsius. In parts of the states of Minnesota and Wisconsin, temperatures dropped to below minus 30 degrees Celsius.

The weather service warned that the cold wave is expected to intensify in the coming days with the advance of a polar air mass, especially in the central United States, where wind chill temperatures could reach around minus 45 degrees Celsius.