Driving home this afternoon, I saw this cloud formation. It’s an ‘atmospheric wave’ which is formed by the atmosphere rising and falling like ripples in a pond. When the air rises above a threshold, its temperature drops below the point where visible moisture condenses. As it flows downward, it warms again and the moisture evaporates.
This type of formation can occur in the lee side of mountains or ahead of an approaching front under the ‘right’ conditions. This wave is one ahead of a front since there is rain in the forecast later this evening.
Pilots of smaller aircraft are wary of areas where these form since turbulence can be severe to extreme. I can tell you from personal experience as a pilot that it is not much fun trying to keep the wings level in even moderate turbulence.
UPDATE: The forecast was accurate – our chance of rain is now 100% at 7:50PM PDT.