The National Weather Service's Salt Lake City office is forecasting a high of 91 degrees today and Saturday and lows around 60, about 3 degrees below the average.

The high at the airport Thursday was 97.

"Won't be a whole lot of change," said forecaster Chris Young. "It might be a little better."

Beginning late Saturday, the chance of thunderstorms climbs to 30 percent and lingers through Wednesday.

But southern Utah probably won't see any temperature relief until Sunday, when a 5-degree decrease puts temperatures closer to 100 in St. George.

Salt Lake City has seen only a trace of precipitation this month, which means a greater likelihood of lightening-caused wildfires.

Lightening strikes, coupled with hot, dry conditions in California have fanned the wildfires that have pumped smoke into the northern Utah valleys. And the smoke, windless heat and ordinary pollution are fueling a buildup of ground-level ozone in the afternoon.

The poor afternoon air quality has prompted the Utah Division of Air Quality to issue "red" air-quality warnings for the past few days. Advisories remain in place through Saturday and will stay there unless the wind kicks up and disperses the pollution.

The very young, the very old and people with heart and lung problems are particularly vulnerable to the health impacts of high ozone. But many doctors also urge healthy people to avoid working outdoors and exercising heavily during the high-pollution hours of the afternoon - from the lunch hour to about 6 p.m.

fahys@sltrib.com