Expect another cold night in the Tri-Cities with a low of about 11 degrees early Friday morning, according to the National Weather Service.
A lack of clouds and near calm winds are leading to no air movement to allow heat to accumulate during the day and escape into space, it said.
An air stagnation advisory also has been issued as a ridge of high pressure builds over the region through early next week, trapping pollutants in the lower valleys.
The weather service warns that diminishing air quality could be a problem for people with respiratory issues.
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Lows below 20 degrees could continue into early Saturday morning, with a low of 14 degrees forecast that morning.
Patchy freezing fog is forecast overnight from Friday night through Monday night.
The overnight low early Thursday morning in Pasco was 11 degrees at the Pasco airport.
In north Richland the low was 12 degrees and in South Richland it was 13 degrees. In Kennewick the lows were a little warmer.
The low in Prosser was 11 degrees.