High Pressure over the Rocky Mountains, and its associated inversion, has trapped moisture at low levels, including along the Snake River in southern Idaho. As a consequence, fog is prevalent at low levels, and cold temperatures are allowing for the development of freezing fog. (Click here for a National Weather Service advisory from the Pocatello WFO).
The GOES-R IFR Algorithm shows high probabilities over the Snake River Valley where the Fog/Freezing Fog was occuring — Twin Falls and Rexburg both reported freezing fog during the animation above. The Brightness Temperature Difference field, below, also captures the areal extent of the low stratus deck.