The first bad-air spell of the year is building in northern Utah.
Weather conditions are ideal for creating an inversion, said Rob Gillies, director of the Utah Climate Center in Logan. High-pressure, combined with snow cover on the ground, traps exhaust from cars and industry in the basin valleys.
"There's a ridge moving in from over the West Coast," he said. "It's not here yet. But it's coming."
A pollution buildup has been in the forecast for days. So far, though, only the air-quality monitors in northern Provo are registering high pollution levels.
The state Air Monitoring Office this week issued a "red" pollution advisory for Utah County for Wednesday through Friday. It warns "people in sensitive groups" -- the young, the old and people with heart and lung problems, in particular -- to avoid outdoor exertion.
In Salt Lake, Davis, Weber and Cache Counties, air-quality forecasters see pollution levels building to "yellow," or moderate, on Friday.
"We're expecting things to move in that direction," said Bryce Bird, manager of the Division of Air Quality's Air Monitoring branch.
The state's air experts review weather forecasts twice a day and publish three-day outlooks for air quality. And the picture is constantly changing, said Bird, pointing to the state's air-quality Web site as a good information source: www.airquality.utah.gov.
The National Weather Service's Salt Lake City office says the haze will move in Friday and stick around through Sunday.
On Thursday, the temperature will be 40 in Salt Lake City. The rest of the week, highs will be in the mid-30s. Nighttime lows will be in the low 20s.
In St. George, skies will be sunny, with highs in the upper 50s and lows around freezing through the weekend.
