Low Cloud Magic

'Twas a cold but beautiful Christmas Eve in upper Little Cottonwood Canyon.  Alta was solidly above the clouds through our departure at 2 PM.  The photo below taken at about 11:30 AM from Gunsight shows the low clouds lingering over the Salt Lake Valley and in portions of the lower canyon.  

By about 2:15 as we were headed down canyon, patch low clouds had reached the Snowbird base area and producing just enough ice crystals to produce a halo.  Very pretty.  

Photo: Libby Snethen

The clouds were likely forming just beneath an elevated inversion that had its base near 700 mb (10,000 feet), as suggested by the HRRR analysis for 1800 UTC (11 AM MST).  

We normally think of inversions as being near the valley floor, but they are typically elevated when we are downstream of an upper-level ridge.  They descend with time as the ridge moves overhead.  That will happen over the next couple of days.  In fact, if you look at the HRRR forecast for 1800 UTC (11 AM) tomorrow, we don't have true inversion conditions, but there is a strong stable layer with a base near 850 mb.  Essentially, Mother Nature is warming the air aloft while cutting off an increasingly shallow cold pool near the valley floor.  

Pollution though will probably be slow to build as traffic tends to be lighter on Christmas, although I suspect you will see a bit of haze tomorrow, in contrast to todays pristine airmass.