What is probably the worst squall I've driven through happened when we were going through Bozeman Pass. It was more exciting than I want a simple highway cruise to be. Oh, AND the road was down to one lane in each direction. Nobody tried to pass me, at least. And there was something that suspiciously looked like ice or snow on the shoulder at the high point.
Down from the pass and into Butte, the weather cleared, but we could see heavy clouds in front of us. So we decided to call it a night.
Now we're off. Some cute girl owes me a steak dinner. ;>