An atmospheric river—a long, narrow conveyor belt of rainstorms—flows about 1.5 kilometers above the ocean surface and can extend thousands of kilometers toward land from out at sea, carrying as much water as 15 Mississippi Rivers. It strikes a coast as a series of storms that move from west to east with prevailing winds and arrive for days or weeks on end. Meteorologists have had some difficulty predicting the amounts and types of precipitation from these systems and, therefore, possible flooding.

A weather sensor network for California, to be completed in 2014, should allow forecasters to predict upcoming storms and floods with much greater precision. Satellite radars can track airborne water vapor well over the ocean but not so well over land. They also do not give a good assessment of winds within the corridor of water vapor, which affects how quickly the rain moves inland. Furthermore, the amount of flooding is strongly influenced by how wet or dry a region's soil is before and during the storms, which can only be accurately measured by sensors embedded in the ground. Knowing how much of the precipitation will fall as rain or snow is also important because rain causes more immediate flooding, whereas snow may cause delayed flooding.

The new warning system will provide all that information and more. The system's centerpiece will be four unique “atmospheric river observatories,” located about 400 kilometers from one another. The units, each about the size of a dump truck, look upward and show precise wind speed and direction at several altitudes, the elevation at which precipitation is rain or snow, and the total amount of water vapor above the site. They also indicate standard weather data such as temperature, humidity and atmospheric pressure. Snow radars and soil-moisture sensors are now being deployed at multiple sites across California, reported Michael Dettinger, a research hydrologist at the Scripps Institution of Oceanography, who spoke at the annual American Geophysical Union conference in San Francisco last December.

When the system is complete, the data it generates will be available to the public online, in real time. The system could provide a model for better prediction around the world. Atmospheric rivers can strike the western coasts of most continents and landmasses; in mid-November 2012 a series of atmospheric-river storms caused the heaviest flooding in western England and Wales since the 1960s.