Until I moved to Washington state, I'd never even heard the phrase "pineapple express." But then one October, when it had been raining and cold all week, the weather turned. While the air was still heavy with moisture, it was balmy and even tropic, with a pronounced wind. That was my first pineapple express.

The pineapple express refers to a weather phenomenon wherein warm moist air from the Central Pacific, or more specifically, from Hawaii (hence the name) comes blowing up along the coast of California, right on through Washington and Montana. In western Washington, the pineapple express translates to unseasonably warm weather, and warm but constant rain with strong winds. It's sometimes a cause of flooding, but it's not like this region isn't used to lots of water falling from the sky.

In Southern California, the pineapple express means buckets of endless rain falling for days—and nowhere to go. In Southern California, the Pineapple Express brings floods and mudslides, and houses sliding from the cliffs into the sea. In the desserts of California and Nevada, it means fast-moving flash floods, washing away everything in their path. In Montana, heavy rain and warm air melting snow can create sudden flash floods as well. The graphic above is a NASA image showing the pineapple express that hit Southern California in December of 2010. From the 15th to the 22nd of December, California was deluged with record-breaking storms dropping from 12 to 24 inches of rain on coastal slopes. You can read more here, and here.