There is some good news: A weak storm system is expected to arrive today and churn the mix up a little. But it won't be enough to break the inversion's grip, and no significant relief is expected until Wednesday, at the earliest.

"The inversion isn't going to go away, but it could lift the ceiling and ease things up a bit," said Bob Dalley, director of the state's air quality program. "The problem is, it's going to clamp right back down on Sunday."

Friday, Dalley noted, "was as bad as it's been during the whole inversion. We had extremely high hourly values and an extremely high 24-hour value. So any relief, at this point, would be appreciated."

Low visibility vexed Salt Lake City International Airport on Friday, forcing some commercial flights to land elsewhere. At times, visibility dropped to just 300 feet, said Alvin Stuart, superintendent of airport operations.

Stuart couldn't say whether smog added to the airport's troubles, only that foggy conditions kept some planes from landing or departing on time. The visibility was worst between 8 and 10 a.m. Friday, and after sundown.

The Division of Air Quality issued a red no-burn advisory for today in Cache, Weber, Davis, Salt Lake and Utah counties and also asked residents to drive as little as possible. Health advisories have been posted for all Wasatch Front residents to limit outdoor exposure and exercise.

In Cache County, the warning targets sensitive groups, such as children, the elderly and those with respiratory issues.

Dalley says that this marks the longest prolonged inversion along the Wasatch Front in a number of years, and if it gets to four weeks it would equal the longest stretch of bad air in recent history.

But the National Weather Service is forecasting a storm system to arrive around mid-week that, forecasters hope, will provide enough oomph to at least start clearing the gunk out.

"It looks like there could be some deep mixing with a little moisture," said Dalley. "It might be enough."

jbaird@sltrib.com