Low Pressure System to Sweep Out Heat, Smog

That hot and heavy feeling descended upon Los Angeles again Thursday as bright sunshine teamed up with airborne pollutants to bring high temperatures and first-stage smog alerts to inland portions of Southern California.

Because of temperatures ranging from 87 at the Civic Center to the upper 90s in the valleys, the thickest layer of brown air this year put a stranglehold on the Los Angeles Basin, blocking from view not only the local mountains but much of the downtown skyline as well.

But weather forecasters say a low-pressure system should sweep through the region today, bringing relief to both the brow and bronchi for the weekend.

First-stage smog alerts for ozone were called in the San Fernando, Santa Clarita, San Gabriel and Pomona-Walnut valleys Thursday, as well as in the Riverside and San Bernardino areas.

Air quality conditions considered “unhealthful” were reported in all but the high desert, according to Ron Ketchum, a spokesman for the South Coast Air Quality Management District.

It was the second day in a row of heavy smog as first-stage alerts were declared in the San Gabriel, San Bernardino and San Fernando valleys Wednesday. The smog alerts Thursday were the fourth of 1988.

By the middle of May, Ketchum said, the South Coast air basin normally suffers at least nine to 10 smoggy days, with first-stage alerts or worse for ozone, an invisible pollutant formed by a combination of nitrogen oxides, hydrocarbons and sunlight.

Nitrogen Dioxide Pollution

The visibility was impaired by nitrogen dioxide pollution, which is generated by factories and auto emissions, the AQMD said.

Mike Smith of WeatherData Inc., which provides forecasts for The Times, said the ridge of high pressure that has hovered over the Southland since Tuesday will give way to a weak low-pressure system that should restore a cool, onshore flow and cut high temperatures by 5 to 10 degrees today.

Early morning fog and clouds will cover coastal areas in the morning hours today and Saturday, but the haze should burn off by noon.

Thursday’s low temperature of 65 degrees at the Civic Center set a record for the highest minimum for the date, the National Weather Service said. The old record was 64 degrees set in 1984.

Temperatures today are expected to hit 78 at the Civic Center to 90 in parts of the San Fernando and San Gabriel valleys.

Saturday and Sunday should also be seasonably warm, with temperatures reaching 82 at the Civic Center and again near 90 in the valleys.

No rain is in the forecast for the weekend, although the low-pressure front will probably drop some moisture on Northern California, Smith said.

Ketchum said the high temperatures that often accompany smog are not the cause of the pollution. Heat only creates the inversion layer that holds the smog over the basin.

During first-stage alerts, Ketchum said strenuous outdoor activity, such as running, jogging or other exertion requiring excessive mouth breathing, should be curtailed. Young children, the elderly and those with respiratory problems should also take extra precautions, he said.

The AQMD also urges people to cut back on their driving during alert conditions because automobile exhaust contributes to ozone levels.

First-stage alerts are called when pollutants such as ozone, nitrogen dioxide and carbon monoxide reach 200 on the AQMD’s Pollutant Standard Index.