It has been one of the snowiest winters on record in Utah, which has the state motto, "The greatest snow on earth".
However the snowfall can sometimes be so great that the avalanche danger suddenly becomes very high and a state of emergency known as "Interlodge" is declared in the Little Cottonwood Canyon, location of Alta and Snowbird ski resorts.
The Utah Department of Transportation along with several private and public entities operate a "state of the art avalanche forecasting and mitigation program" and declares Interlodge when a combination of snow, wind and temperatures occur.
Unusually, it was declared during the day on Friday after a natural avalanche hit the main highway up to the resorts. The main message was for skiers to leave the slopes and to not try to drive anywhere and in both cases get to a building. It was lifted a little before 7pm.
The resorts eventually posted that they'd received another 20-21 inches (50cm) of snowfall in 12 hours during the day with snowfall rates peaking at 5 inches (13cm) per hour.
Alta has a special Interlodge explanation video: https://www.alta.com/stories/interlodge