Two occasions, both in New Hampshire. I was driving through the Franconia Notch State park at about 50 mph one winter night. Clear and about 0 degrees. Fairly typical for that time of year. Clear night too. Suddenly, everything went white. I couldnt see an inch from any of my windows. Wind had created an instant white out with me moving and cars in front and behind, and a guardrail somewhere on my right. Didnt know if I should have hit the brakes or not, or if anyone else had. Lasted maybe 30 seconds, probably less. All was well, but scared the crap outta me.
The other time I was hiking in the same area on Memoral Day. It was around 65 at the base of the Franconia Ridge, and I climbed with my wife and two others to walk along the spine of the ridge above treeline. Once we reached the top of the mountains, we turned north. I think we were about half way along that three mile stretch when I heard what sounded like a thousand freight trains. I had heard it before and had everone get down. A blast of wind came from the east (almost always goes west to east) at what I estimate to be 60 mph or more. Temperature went from 50 to below freezing in moments. My wife and I had clothes to deal with it, but our friends did not listen to us and brought nothing in the way of protective clothing. Oh, I forgot the freezing rain. We had to move, or we would certainly die of exposure. Moving along as quickly as we could, we continued north in sustained winds of 45 mph or more until we reached the northernmost and highest peak. There a trail led down to the Greenleaf Hut, on the shoulder of the mountain. We had to get there below treeline before the ground got so cold that ice coated it. If that happened, we could not climb down, and we certainly would be found dead by the park rangers later. We beat it. There was a thermometer outside that read 21 degrees. We walked into the Hut, which is more a proper building with kitchen and living space. I did not know why people were staring at us until I removed my backpack and more than an inch of ice shattered and flew everywhere from off my body and backpack. We were encased in ice and did not know it. Now that was spooky. Several people died in the White Mountains that day. We were lucky.