Thick fog, limiting visibility to a quarter-mile or less, is being forecast through 11 a.m. Saturday.
With a predicted low of 19 to 24 degrees expected early Saturday, the fog could freeze on roadways and make them slick. The temperature had already lowered to freezing at 32 degrees by 6:30 p.m. Friday, according to the National Weather Service.
Authorities urge motorists to slow down to meet the conditions and to leave plenty of room between your vehicle and the one in front of it.
A four-vehicle crash on Meridian Road near Kuna on Friday evening was blamed on fog. Minor injuries were reported, according to the Ada County Sheriff's Office.
Saturday's high is predicted to reach 36 degrees.