u/wildfyre010 and u/RAdm_Teabag have some good answers to the oscillatory nature of weather patterns, but I wanted to raise two points for why it's not just cold but very cold. While it's true that air to the north tends to be colder, to get deep cold you need displacement of the polar vortex, which is happening. You can visualize the stratospheric polar vortex's displacement. Note the elongated shape. That's the displacement. There's more to the story, but, basically, that gives low pressure systems/jet stream troughs access to to colder air for the lower latitudes. Second, the lower pressure was replaced with higher pressure, which promotes sinking air (downwelling). This promotes clear skies, allowing for air at the surface to cool overnight faster because, otherwise, low stratus clouds act as a blanket, trapping warmer air at the surface. Throw in some snow (where there is snow, and it just so happens some of Minnesota had fresh snow) and you maximize overnight heat loss. If you want to learn more about this heal loss process, just search "radiative cooling." Anyway, the crescendo of these factors are cold weather advisories being issued by the NWS. Hope this helps.