SAN FRANCISCO—The heavy rainstorms that flooded parts of northern California this past week were caused by an "atmospheric river"—a long, narrow conveyor belt of rainstorms that stream in from the Pacific Ocean. Meteorologists were able to predict the storms five days in advance, thanks to a new network of weather sensors recently installed in the state. Although the network is only partially complete, when it is finished in 2014 it should allow forecasters to predict upcoming storms and floods with much greater precision, and could provide a model warning system for flooding on continental west coasts worldwide.

An atmospheric river flows about a 1.5 kilometers above the ocean surface and can extend thousands of miles out to sea, carrying as much water as 15 Mississippi Rivers. It strikes a coast as a series of storms that arrive for days or weeks on end. Each storm can dump centimeters of rain or meters of snow. Meteorologists have had some difficulty predicting the amounts of precipitation, and therefore possible flooding. Satellite radars can track airborne water vapor well over the ocean but not so well over land, according to Martin Ralph, a research meteorologist with the National Atmospheric and Oceanic Administration (NOAA) Earth System Research Laboratory in Boulder, Colo., speaking at the annual American Geophysical Union conference here. Satellites also do not give a good assessment of winds within the corridor of water vapor, which affects how quickly they move rain 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. Snow radars are being deployed in 10 major watersheds. Soil moisture sensors are being installed at 43 sites across California, which will be key to anticipating whether an incoming storm will produce heavy runoff, according to Michael Dettinger, a research hydrologist at the Scripps Institution of Oceanography in La Jolla, Calif., who was also speaking at the meeting. (Dettinger is co-author of a detailed article about atmospheric rivers in the January issue of Scientific American). Some of the snow radars and about three quarters of the soil sensors are already in place.

The centerpiece of the system will be four unique "atmospheric river observatories" located about 400 kilometers from one another. The units, 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. The first observatory is being installed right now in Bodega Bay. The others will be set up in Eureka, Point Sur and Goleta.

The partially completed system already proved its worth two weeks ago. When satellite imagery showed moisture accumulating over the central Pacific researchers drove a mobile observatory into northern California and began compiling data from it with the other sensors that were already operating. Forecasters were able to predict five days in advance that an atmospheric river would begin pummeling the state. True to the forecast, storms started hitting the coast on November 28 and lasted five days, dumping up to 38 centimeters of rain, causing flooding and mud slides. Forecasters used the information to warn residents which rivers might flood when, and the state's Department of Water Resources used it to help decide whether to try to mitigate flooding by opening or closing dams and other structures along California's rivers.

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