The weather service says to prepare for, "Very cold wind chills as low as 20 degrees below." "The dangerously cold wind chills as low as 20 below zero could cause frostbite on exposed skin in as little as 30 minutes," adds the weather service. Understanding cold weather alertsCold weather advisory: Be Aware. A cold weather advisory is issued when seasonably cold air temperatures or wind chill values, but not extremely cold values, are expected or occurring. An extreme cold warning is issued when dangerously cold air temperatures or wind chill values are expected or occurring.