Yet another inversion is expected to clamp down over Southern Oregon this weekend, trapping cold, stagnant air in the Rogue Valley.

Yet another inversion is expected to clamp down over Southern Oregon this weekend, trapping cold, stagnant air in the Rogue Valley.

The National Weather Service office in Medford issued an air stagnation advisory warning that pollution could increase. The advisory, issued Friday afternoon, will be in effect from 10 p.m. Saturday until midnight Wednesday.

Jackson County Department of Environmental Health has called for burning restrictions to protect air quality, which was listed as moderate Friday afternoon. Saturday will be a "yellow day," with no burning allowed in noncertified woodstoves and no visible smoke allowed from any woodstove.

A ridge of high pressure is expected to begin building Saturday, the advisory said. The warm air aloft will trap cold air in the valleys, setting up strong inversions.

Pollution, especially tiny particles that can cause breathing difficulties, collects in the trapped air. Because pollution could rise to levels that are unhealthy for sensitive groups, the county limited burning, officials explained.

National Weather Service meteorologist Frederic Bunnag said inversions are common in the region in December and January, and have been particularly prevalent this year.

With high pressure preventing storms from sweeping through the area, conditions are abnormally dry, but it's not yet a drought, he said.

A front will make its way into the region on Thursday. It probably won't bring much rain, but will freshen up the air, at least temporarily. Another inversion could take shape next weekend, too, Bunnag said.

This weekend's inversion will cause dense overnight fog that should burn off by midday to reveal partly sunny skies over the Rogue Valley most days. In Grants Pass and Roseburg, the gray likely will last all day.

Temperatures will remain warm on the ridges, but will dip to around freezing each night in the valleys. That will bring a risk of heavy frost and black ice that can leave roads slippery. Potentially treacherous conditions should melt away by midday, Bunnag said.

The county updates its wood-burning advisory daily. Call 776-9000 to get the latest information.

Reach reporter Anita Burke at 776-4485, or e-mail aburke@mailtribune.com.