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It appears that some very strong cold pools formed in the canyons overnight, and these cold pools are persisting in those areas that are still topographically shaded and haven't yet seen the sun. The 10:45 am web cam shot of Alta shows this well. This camera is sited to the west (left) of the two warm observations in the image above. They are all in the sun. Areas the canyon near the north facing slopes are in the shade or have only recently seen some sun. The 12ºF observation is taken near the lower right corner of the image.
Perhaps someone can comment on the veracity of these observations, but I have seen situations like this previously when it is clear and very dry near the winter solstice when the sun angle is quite low. Often you can find all sorts of cold pockets in areas where it is shaded and the air is calm. A screaming descent of Snowbird today must be interesting.