While driving down hwy 19 hill just west of Henderson a large bird nearly touched my windshield as it passed over the hwy.

It looked like a Boreal Owl

Jeff, the photo appears to be that of the boreal owl as you indicated. According to Kenn Kaufman, the boreal is an elusive owl of the north woods and rarely wanders south. However, two winters ago during the "irruption" of Great Gray Owls, some boreals were seen in northern Minnesota (due to their food shortage of voles). Just received the Fall issue of THE LOON, journal of the Minnesota Ornithologists' Union, in which they report on the birds seen in Minnesota during the December 2006 - February 27th period. Not a single boreal was reported during that period. What was the size of the owl you observed? (Yes, it's tough to gauge while driving!)
We believe the owls of our area are experiencing difficulty finding food...the ice crust on the snow is impenetrable for an owl.. That's why they have to wait until a mouse or vole surfaces, or else take on squirrels and birds. Searching for mice is what brings the owls to the roads and highways, where they often meet their untimely deaths. Great observation!!!!! (See owl photo in Photo section, FEATHERS.)