Thursday, December 19, 2019

Pre-Holiday Warmth

After a great run of snowfall, we're in the midst of a break in the action this week.  This has led to the formation of a strong inversion over the Salt Lake Valley.  Yesterday afternoons sounding shows the inversion base at about 850 mb, or about 650 feet above the airport.  Above this level, temperatures increased about 5˚C. 

Source: SPC
Not surprisingly, air quality is deteriorating and is currently in the middle of the "moderate" category. 
Source: Utah DAQ
Above the gunk, wonderful conditions can be found in the mountains.  Yesterday's high at the base of Alta reached 32.5˚F.  Mountain temperatures will climb further today and tomorrow and the inversion will strengthen, or, if you prefer, tighten its noose.  By 5 PM Friday, free-atmosphere temperatures at 700 mb (about 10,000 ft) are expected to reach 0˚C. 

However, with a trough approaching from the west, southwesterly flow is expected to increase Friday night and Saturday.  The NAM forecast for 5 PM Sunday afternoon is calling for 700-mb (10,000 ft) winds of about 50 knots over the Salt Lake Valley. 

Thus, while the inversion will hold through Friday, it will very likely be gone by Sunday afternoon.  Prior to that, on Saturday, it's tough to say.  Right now I'd lean toward a top-down scouring of the cold pool with the benches possibly breaking out for part of the day Saturday, but a shallow cold pool could remain at lower elevations.  Time will tell. 

Where the inversion does mix out, temperatures will be mild for December.  I suspect we could see highs in the 50s at some bench locations Saturday and on the valley floor on Sunday. 

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