A Siberian cold front dropped the temperature at Denver International Airport from 42 degrees to 5 degrees in an hour.

Temperatures at Denver International Airport dropped to -24 degrees Thursday morning, according to the National Weather Service in Boulder, marking the coldest day in December since 1990 and one degree away from that record.

The high today is -2 degrees. The last time Denver had a high temperature below zero was Feb. 5, 2014, when the high was -1 degrees, according to the National Weather Service in Boulder, and today’s high is the lowest since Dec. 21, 1990, when the high was -7 degrees.

The wind chill will be the worst offender, though, as that could reach as low as -50 degrees across the plains and mountains and from -20 to -30 degrees in the Interstate 25 Urban Corridor.