The incident occurred at about 12:30 p.m. on the overpass above state Highway 29, near downtown Napa.
There were no other witnesses, and the suspect fled on foot afterward, Detective Don Winegar said.
The suspect is described as Hispanic, 30-35 years old, 5 feet 11 inches tall and about 160 pounds, police said. He has a thin build, black hair 2 or 3 inches in length, a mustache and a pointed nose, police said.
He was wearing baggy black Dickies pants, a red short-sleeve cotton T-shirt, white tennis shoes, plaid boxer shorts visible above the waist of his pants and a thin chain necklace, police said.
Anyone with information is asked to call Winegar at (707) 258-7879.