Stirred Not Shaken

As discussed in the previous post, a weak upper-level trough was expected to move over northern Utah yesterday, but it was unclear if it would crack the inversion (I was skeptical).  Indeed, looking at things this morning, I'll call it "stirred not shaken." Impacts on the pollution are varied.  In the Avenues foothills, there was clear improvement and evidence that the pollution is shallower, as sometimes happens in a weak partial mixout event in which things are eroded from the top down.  

On the other hand, a look at satellite imagery shows evidence of fog or low clouds in portions of the Salt Lake Valley, Tooele Valley, and the area from Bountiful to Layton.  Instead of being bright like the mountain snow, the fog and low clouds look grey, which may reflect that it is thin, patchy, or simply mixed with all of the smog that is still stuck on the valley floor. 

Source: College of DuPage.  Image from 1611 UTC/0911 MST 16 Jan 2026.

As I worked on this post this morning, some of these low clouds moved over campus.  

Air quality during this episode has varied considerably during the day.  At Hawthorn Elementary, one can see the PM2.5 levels peaking each afternoon.  Peak values worsen each day through the 14th (Wednesday).  There was slight improvement yesterday.  

Source: MesoWest

At Rose Park, one sees this same behavior, but peak values are somewhat higher and the nighttime declines are not as large. PM2.5 concentrations this morning are just a smidge lower than the previous two days.  

Source: MesoWest

The question now is what will happen this afternoon.  Temperatures this morning are actually several degrees C cooler above the valley floor than they were yesterday.  At about 7000 ft elevation, for example, it's 1.8°C this morning, whereas it was 5.0°C yesterday.  Thus, the inversion is weaker.  This makes it more susceptible to surface heating during the day.  One possibility is that with a bit of surface heating we start to vent the valley floor and we mix out some of the gunk.  A best case scenario is we vent everything.  I think that's unlikely, although perhaps some areas see some improvement.  

Perhaps more likely is that the pollution remains very stingy over and near the Great Salt Lake.  The lake is a bit colder than the surrounding land surface in situations like this and there can be a lens of cold, polluted air that remains entrenched over it.  That cold and polluted air tends to move into the Salt Lake Valley during the day.  Sometimes it mixes out for a bit only to have the polluted lake air push in during the day.  

We will see how this pans out.  Definitely a tough forecast.  Hope for a full mix out.