High pressure is building across the region, trapping fog and air pollution in the Rogue Valley.
The National Weather Service office in Medford issued an air stagnation advisory early this morning, warning that a strong inversion is forming. The advisory will be in effect from 6 tonight until noon Saturday.
A layer of warm air aloft will trap cold air in valleys across southwestern Oregon, the forecast explains. Areas of freezing fog are expected overnight.
Little wind is expected in most parts of the Rogue Valley, so fog will linger and tiny particulates from wood burning and car exhaust will collect in the trapped air, the advisory says.
Jackson County air quality officials have declared today a "yellow" day, noting that pollution could approach unhealthy levels. Restrictions ban burning in non-certified woodstoves and require no visible smoke from certified woodstoves.