While driving east on I-70 in the Rocky Mountains, my passenger saw a flash of light in the sky north of the highway. I was driving, so I couldn't focus on the area, but when I did glance over, I witnessed on of these flashes. A total of three were seen by three people. These were very bright, circular flashes in the clouds. They were separate from one another and extremely bright. The only explanation we could think of was lightning. However, there was no impending weather in the area, no thunder, and it was below freezing out. I am aware that lightning can happen in the winter, but these flashes were perfectly round and super bright. It was surreal.