A high pressure front that settled over northern Utah means a return to colder temperatures and pollution-trapping air inversions heading into the weekend.
Indeed, the Utah Division of Air Quality rated all areas of the state, with the exception of southwestern Utah's Washington County, as "yellow," or moderate for breathability on Thursday and Friday.
Forecasters warn that will likely not improve, and could worsen, as urban valley smog increases through the weekend.
The National Weather Service predicted high temperatures Thursday will dip into the mid- to upper-30s after Wednesday's forecast for highs around 40 degrees. Overnight lows were pegged in the upper-teens.