Visibility is as low as 1/4 mile in parts of Canyon County on Thursday morning, according to the National Weather Service. It was about 3 miles in Boise at 6:30 a.m., but the fog could get worse as the morning goes on, a meteorologist said.
A dense fog advisory is in effect in the Lower Treasure Valley until 11 a.m. The sun is expected to burn off the fog by late morning, as it did Wednesday morning.
The fog is one of the byproducts of inversion conditions. Cold air and pollutants are trapped in the Valley below warmer air aloft.
A weak cold front is expected to come through Friday morning. That may bring light rain or snow and help scour out the stagnant air. The mountains could get 1 to 2 inches of snow.
The high Sunday was 39, and that's the projected high for Monday. The forecast for Friday is a high of 40. But temperatures will drop over the weekend; expect highs of about 32.
Some good news: Sunny skies over the weekend. Inversion conditions may set up again on Monday morning, and there could be dense fog again.