Bitterly cold air from Siberia has brought dangerously frigid and likely record-setting temperatures to the eastern half of the US.
Temperatures are 20-40F below normal for February from the Mid-Atlantic to the South.
Schools in Chicago have closed and trains in the north-east corridor have been affected by the cold.
Warnings of extreme cold are also in effect in the Canadian provinces of Ontario and Manitoba.
Frigid temperatures are expected to continue into Friday.
Record lows have already been broken in Kentucky, where it was -8F (-22C) on Thursday morning in Paducah and most of the state was below 0F (-17C).
Weather forecasters believe the cold air will help break more than 100 daily record lows, NBC News reports.