Air quality is expected to worsen as high pressure builds over the region today, trapping colder and increasingly polluted air in the valleys of Southern Oregon, the National Weather Service warned.

Air quality is expected to worsen as high pressure builds over the region today, trapping colder and increasingly polluted air in the valleys of Southern Oregon, the National Weather Service warned.

The weather service's Medford office issued an air stagnation advisory that went into effect at 4 a.m. today and will remain in effect until 4 p.m. Saturday.

Jackson County air quality officials have declared today a "yellow day" for woodstove burning, restricting the use of noncertified woodstoves as heat sources. Monitors in the Medford area show that airborne particulate levels are increasing and health officials said unhealthy levels could occur during the air stagnation advisory.

The high pressure, which is predicted to settle in today through Wednesday, may weaken a bit on Thursday, but forecasters said a system strong enough to break up the inversion and blow the stale air out of the valleys isn't expected until Saturday.

— Anita Burke