On Saturday afternoon, we were still "in the clear" and the afternoon sounding showed only weakly stable conditions over the Salt Lake Valley up to almost 700 mb.
Basically, we went from good to unhealthy for sensitive groups in about 24 hours, which is a remarkable rise. Concentrations dropped just a bit thereafter, but sit near 30 ug/m3 this morning.
I used to assume it would take 3.5 days to go from clean air to unhealthy for sensitive groups, but that's clearly not the case in this event. Why is unclear, at least to me.