A shroud of cold air descends on the U.S.
From NOAA, an animation showing a wave of cold during the Martin Luther King Jr. holiday weekend last month:
A drop in the jet stream sent temperatures across the United States plummeting over the Martin Luther King Jr Holiday weekend. The pronounced change in temperatures can be seen in this weather data from NOAA/NCEP's Real-Time Mesoscale Analysis. Areas colored blue are below freezing. The diurnal cycle of heating and cooling can be seen over time, but the pattern is clear: much of the U.S. is pretty cold.
While you're at it, you might as well check out other videos on the NOAA Visualizations YouTube channel. Some good stuff.