Record low temperatures are descending on the American midwest. The region is long familiar with bitter winters, but this is an exceptional cold snap that forecasters warn could be life-threatening. The extreme chill comes after a snowstorm that hit the area overnight on Monday.

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Here’s more on what weather experts are expecting and how officials are preparing for the brutal freeze.

Temperatures across the upper midwest will be 20 to 40 degrees below average, according to the National Weather Service (NWS). Below zero temperatures will set in on Tuesday, but Wednesday is expected to be the coldest day. Wind chills in northern Illinois could fall to -55F (-48C), which the NWS called “possibly life threatening”. The mercury in the Minnesota “twin cities” of Minneapolis and Saint Paul could hit a low of -28F, and aren’t expected to rise above zero until 11am Friday. Milwaukee, the large industrial city on Lake Michigan in Wisconsin, is also forecast to hit a low of -28F, with a wind chill as low as -50F, beating the record of -26F degrees set in 1996. As of 7.15am local time on Tuesday, wind chills colder than -50F had already been observed in several towns in Minnesota and North Dakota.