LOS ANGELES – Smog agency officials have banned the use of some home fireplaces starting at midnight tonight, as an unusual smog pattern prompted air pollution warnings in the downtown Los Angeles area and most of the San Fernando Valley.

“This is the first time AQMD has issued this type of order under this program,” agency spokesman Sam Atwood told City News Service. “It’s our first mandatory no-burn order for the affected area.”

Starting at midnight tonight and lasting until midnight Sunday night, wood-burning fireplaces cannot be used in the affected area.

First-time violators could be fined $50, and with fines going up to $500 for repeat offenders. Such “no burn alerts” are common in Northern and Central California, but rare in the Southland.

The affected area includes the central San Fernando Valley — generally north of the Ventura (101 and 134) freeways, east of the San Diego (405) Freeway, south of the Foothill (210) Freeway and west of the Glendale (2) Freeway.

In the L.A. basin, wood fires are banned generally east of Fairfax Avenue, north of Slauson Avenue, and west of the Long Beach (710) Freeway.

People may enter their zip codes on a web site — www.aqmd.gov/noburn — to see if burning is against the law, The ban takes effect at midnight tonight and expires 24 hours later.

Atwood said the AQMD enacted new air pollution rules that took effect in November, 2011, that gave it the power to temporarily idle fireplaces and other wood fires. Tonight’s ban is the first under that program, and it includes articifical logs and wood pellets.

An unusual weather pattern was blamed for the no-burn order, and credited with giving the Inland Empire fabulously clean air but setting up the coastal area and valleys for widespread airborne crud.

The National Weather Service said a strong inversion layer and gentle offshore winds will prevail over the Los Angeles basin today and Sunday, causing dense overnight fog and warm daytime temperatures.

The marine layer is capped by a layer of hot, dry air at an altitude of under 400 feet above sea level, the NWS said at noon. That warmer air is 11 degrees warmer than the surface temperatures, and acts like a cap.

“Dense fog formed under the inversion, and it will be slow to clear,” the NWS warned for Los Angeles today and Sunday.

The weak Santa Ana is triggering canyon winds of just 10-20 miles per hour, just enough to blow air pollution towards Santa Monica Bay, but not strong enough to blow it further.

That prompted the South Coast AQMD to issue a smog forecast for air that can be unhealthy for people in sensitive groups for Sunday in Metropolitan Los Angeles and the San Fernando and Santa Clarita valleys.

Air quality is designated “unhealthful for sensitive groups” when Air Quality Indices are from 101 to 150. At this level, active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion, the AQMD warns in its daily smog forecasts.

Greater L.A. is expected to see an AQI of 105 Sunday.

The Riverside, Big Bear and Coachella Valley areas are predicted to have good air quality Sunday, with AQI levels as low as 40.