Those are called "lenticular clouds" They're caused when the air flow is just right so when it flows over Mt. Rainier , the air gets pushed upward where it cools and condenses into clouds. Depending on how smooth the flow is, you can get some amazing clouds formations as we've seen so far today. Here is more information on what causes them from an earlier blog entry. It's usually a sign of rain within 24 hours because typically the moist flow that precedes a storm around here is the perfect set up for these clouds.