They are cumulonimbus mammatus--and they appeared Sunday evening around 6:30 over Olympia. What a spectacular show at the end of a tumultuous day of rain and wind and sun. Cumulunimbus is their offifcial Latin name cumulus (heaped) + nimbus (rain) + mammatus to describe the  bulging pouches hanging down from the underside of the cloud. 

They are distinct and you are not going to confuse them with another cloud. 

Mammatus as in mammary gland. They are referred to as "cloud boobs," which makes them easier to remember, but doesn't sound very scientific. Mammatus is not a cloud type (or species or variety) but is a "supplemental feature" of  cirrocumulus, altostratus, altocumulus and stratocumulus but is most often seen in cumulonimubs. Mammatus form when pockets of cold air sink within the cloud. They linger if the content of the water droplets or ice crystals in the mamma (that's the noun) is high.

These clouds are not precursors of tornadoes. They usually appear at the end of a storm and gradually, gracefully evaporate...as you can see in this gallery of photos I took on my after-dinner walk.

Now that Daylight Savings Time has arrived, dont' forget to take an evening cloud-spotting stroll in your neighborhood.