An air stagnation advisory is in effect from 4 p.m. Christmas Day to 7 a.m. Saturday, with pollution levels will gradually increasing, reports the National Weather Service in Pendleton.
The affected areas include the Tri-Cities, Prosser, Connell, Hermiston, Walla Walla, Waitsburg, Dayton, Sunnyside and Yakima.
An air stagnation advisory indicates that due to limited movement of an air mass across the region, pollution will increase, the weather service said.
The Christmas Day forecast calls for patchy freezing fog before 10 a.m., then patchy freezing fog after 4 p.m. It will be mostly sunny, with a high near 35 and north wind 3 to 5 mph. Tonight will be partly cloudy, with a low around 21. Calm wind.