Poor air quality conditions out west steal headlines again this weekend as Code Yellow (Moderate), Orange (Unhealthy for Sensitive Groups) and Red (Unhealthy) PM 2.5 were recorded across the Pacific Northwest, Pacific Southwest and Rocky Mountain states. The worse conditions, in terms of PM2.5, were confined to northeastern Utah and south central Oregon, which peaked at Code Red levels on Saturday and again for Utah and southeast Idaho on Sunday (top courtesy AIRNow). Surface ozone levels were also a problem over the weekend in Utah, reaching Unhealthy levels each afternoon (middle courtesy AIRNow). Concurrently occurring out west was a parked powerful and persistent west coast ride of high pressure (bottom left, courtesy NOAA HPC). Much of the aforementioned experienced sunny and dry conditions due to High Pressure's characteristic of strong atmospheric subsidence or descending- stagnant air. Sunday evening's vertical profile in Boise, Idaho is indicative of this type of regime, showing a strong temperature inversion (environment warming with altitude- redline in image) near the surface (bottom right, courtesy SPC). These conditions coupled with weak surface winds may have contributed to the elevated PM2.5 and surface ozone. Fortunately, a mid-week low pressure system is forecast to sweep through the west which could disrupt the poor air quality trend.