Imagine a stream of water thousands of kilometers long and as wide as the distance between New York City and Washington, D. C., flowing toward you at 30 miles per hour. No, this is not some hypothetical physics problem—it is a real river, carrying more water than 7–15 Mississippi Rivers combined. But it is not on land. It's a river of water vapor in the atmosphere. Atmospheric rivers (ARs) are narrow corridors of water vapor transport in the lower atmosphere that traverse long swaths of the Earth's surface as they bind together the atmospheric water cycle (Figure 1). The characteristic (indeed defining) dimensions of these ARs are (1) integrated water vapor (IWV) concentrations such that if all the vapor in the atmospheric column were condensed into liquid water, the result would be a layer 2 or more centimeters thick; (2) wind speeds of greater than 12.5 meters per second in the lowest 2 kilometers; and (3) a shape that is long and narrow, no more than 400–500 kilometers wide, and extending for thousands of kilometers, sometimes across entire ocean basins.