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 has issued an air stagnation advisory, warning that pollution could increase. The advisory 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 this afternoon. Saturday will be a "yellow day" with no burning allowed in non-certified wood stoves and no visible smoke allowed from any wood stove.

A weak cold front will blow through tonight, causing some mixing in the atmosphere, but then a ridge of high pressure begins to build by Saturday night, said the advisory. The warm air aloft will trap cold air in the valleys, setting up strong inversions.

National Weather Service meteorologist Frederic Bunnag said such conditions are common in the region in December and January, and have been especially 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 will likely 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.

— Anita Burke