We are now fully in the grips of a monster upper-level ridge that will keep the mountains dry and the valley inverted. On the positive side, that means warm, sunny days in the mountains. On the negative side, we are staring directly down the barrel of what will be a very poor air quality episode.
Meteorologists use a diagram known as a Skew-T to examine upper-air soundings collected by weather balloons. In these charts, lines of constant temperature are "skewed" (blue dotted lines below). Yesterday morning's sounding showed a shallow inversion at low levels. The surface temperature was 26ºF -3.3ºC and the temperature at the top of the inversion near 800 mb was about 0ºC. Above this level, at 700-mb near the crest of the Wasatch Range, the temperature was about -4ºC.
PM2.5 levels prior to midnight at the Hawthorn air quality site climbed well into the unhealthy for sensitive groups category. They decreased some overnight, but popped up in the past hour to the unhealthy for sensitive groups threshold. 24-hour averages (blue dots) are now above EPA Clean-Air Act standards.
Temperatures aloft will continue to rise through late Tuesday when the ridge axis is centered over the Intermountain west and the ridge is at maximum intensity. At that time, forecast free-atmosphere 700-mb temperatures (near 10,000 feet) are near 4ºC.
Bottom line: This is going to be an outlier inversion event in terms of meteorological strength and duration, a situation further exacerbated by the extensive snow cover on the valley floor. From an air-quality perspective, it will easily be the worst event of this season, with the situation worsening through the week.