The culprit was a phenomenon known as an atmospheric river. Atmospheric rivers are long, narrow bands of moisture that move like a river in the sky, carrying water vapor from the equator toward the poles. The animation above shows the signature of atmospheric rivers, including a particularly potent event that doused parts of Washington, Oregon, and Northern California. By the end of the animation (January 18), an atmospheric river system appeared to connect British Columbia and southeastern Alaska to Hawaii. Research has shown that more than 80 percent of western U.S. flood damage over a 40-year period was due to atmospheric rivers.