The Dreaded Inversion

Thanks to the active pattern that has dominated over the past several weeks, we haven't had much in the way of poor air quality so far this winter, but that is changing temporarily this week.

The setup was the passage of an intense but shallow cold front on Sunday, which dropped temperatures in the Salt Lake Valley to well below average.  At the Salt Lake City Airport, the min/max temperatures on Monday were 9/20 and on Tuesday 10/25, compared to a 30-year average (1991–2020) on those days of 10/25.  

At the same time, temperatures aloft have begun to climb as a ridge builds over the area.  Early Tuesday morning, temperatures on Mt. Baldy above Alta were about -2˚F.  This morning they are around 10˚F.  

A look at this morning's sounding from the airport shows a 4.2˚C (7.5˚F) inversion based at about 825 mb, or about 1500 feet above the valley floor.  

Source: SPC

As the inversion developed yesterday, pollutants began to build up in the valley.  I have friends visiting from Innsbruck and took them to Alta where we had a great day introducing them to the Greatest Snow on Earth.  However, a late afternoon trek out the High -T showed the pollution lurking below.  

Indeed, the time series of PM2.5 at Hawthorne Elementary shows concentrations rising into the low end of moderate air quality yesterday afternoon and then continuing to climb overnight to 25.5 ug/m2 before subsiding slightly to just over 20 in the early morning hours.  

A saving grace so far is that the event is young and the base of the inversion is elevated.  That means that instead of pollutants being trapped very near the valley floor, they are dispersing through a layer that is about 1500 feet deep.  The sounding above showed that the atmosphere was well mixed in that layer, at least for now.  

The GFS, however, forecasts the movement of an upper-level ridge over Utah over the next two days, with the ridge axis parked right over us by 0000 UTC 3 February (5 PM MST Thursday).  

This will warm temperatures aloft and strengthen and lower the inversion.  The GFS forecast sounding for 1200 UTC 3 February (5 AM MST Friday) shows a strong inversion based right on the valley floor.

Only a weak mixed layer is expected to develop Friday afternoon (not shown).  Thus, we will see increasing PM2.5 concentrations this week that I suspect will persist until Saturday.  An approaching trough may weaken and scour out the valley cold air on Sunday or Sunday night.  Let's hope it comes through and gives us a refresh of mountain snow.