About as Cold as It Gets

I thought I would take a moment to put the current Utah cold spell into some context.  I have a rule called the 20/20 rule, which says that anytime the 700-mb (10,000-ft) temperatures are below -20ºC or above 20ºC we are unusually cold or warm, respectively.  This morning's 700-mb temperature in the sounding collected at the Salt Lake City airport was -21.9ºC, so we are in rarified air.

Another way to picture this is based on return period. As shown in the panel at upper left, for this time of year, an event this cold has a return period of about once every 10 years (or even rarer) over northern Utah.

Source: NWS

It's pretty easy to find some remarkably low temperatures if you pull up observations from around the area.  For instance, Alta-Collins has been below 0ºF since about midnight on the 4th and has just begun to rebound reaching -9ºF at 11 am MST this morning.

Source: MesoWest

Cold, but you can find colder.  The Peter Sinks, limestone sinkholes in the northern Bear River Range east of Logan, dropped to -34ºF the night before last.  Now that's cold.

Addendum @ 6:20 PM:

My bad.  The minimum temperature at Peter Sinks the night before last was actually -42ºF.  Bloody cold!