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The Advanced Quantitative Precipitation Information project will provide detailed and accurate forecasts about heavy rainfall events using a set of radars similar to this prototype that was temporarily installed in 2016 in the South Bay. Photo courtesy NOAA.

On the heels of northern California’s wettest year on record — with nearly 90 inches of rain — a team of water agencies and weather researchers is installing a new radar system to upgrade flood prediction in the San Francisco Bay Area. For many, it can’t come soon enough.

Take last February, an extremely wet month in an extremely wet winter. Flooding closed Highway 37, which runs along the northern edge of San Pablo Bay, and shrank westbound I-80 to barely a lane in Fairfield, which adjoins the Suisun Marsh. And in San Jose, Coyote Creek over-topped its banks. Fourteen thousand people were ordered to evacuate, and 582 residences, 118 businesses, and about 300 vehicles were damaged or destroyed. Initial estimates put the cost of Coyote Creek flooding at $73 million.

“The new system will tell us how much, when, and where it will rain with greater precision,” said Carl Morrison, executive director of the Bay Area Flood Protection Agencies Association, adding, “It will be hugely helpful to flood agencies.”

Most of last winter’s rain fell from storms called atmospheric rivers — swathes of water vapor that can be hundreds of miles wide and thousands of miles long. Atmospheric rivers start in the tropics and drop rain or snow when they hit land. The “Pineapple Express,” which starts in Hawaii, is a common atmospheric river on the West Coast. Rather than having a relatively fixed course like rivers on land, those in the sky are shaped by other large-scale atmospheric forces and can end up anywhere from southern California to Alaska.

Locally, the risk of flooding is highest at the top and bottom of the Bay: San Pablo Bay in Sonoma and Napa counties, Suisun Bay in Solano County, and the South Bay in Alameda, Santa Clara, and San Mateo counties. Most vulnerable are 200 square miles of low-elevation land near shorelines. Some spots are even as much as 13 feet below sea level. These low-lying lands house infrastructure — including regional systems for water, power, communications, and transportation — valued at more than $50 billion.

Atmospheric rivers cause more than half of the major floods in the Bay Area altogether, and more than 70 percent of those in the North Bay. Predicting when and where flood waters will rise is difficult, though. While National Weather Service (NWS) radar tracks storms regionally, the system was designed with Midwest thunderstorms in mind and often misses rain in the coastal hills of northern California.

The closest NWS radar is on Mount Umunhum, a 3,500-foot peak near Los Gatos. “It’s up too high to see rain falling on the ground in the Bay Area,” explained Rob Cifelli, a hydrometerologist currently detailed with the National Oceanic and Atmospheric Administration‘s (NOAA) Office of Water Prediction.

The new system will boost coverage from Santa Rosa to San Jose, where the first of five radars is already in place. Ultimately, four of the new radars will monitor flood-prone urban areas, while a fifth on the Sonoma coast will track incoming storms. “All flood control agencies in the Bay Area will get information from the project,” the Bay Area Flood Protection Agencies Association’s Morrison said.

Coordinated by NOAA, the project is funded by a $19.8 million grant through Proposition 84 (the Safe Drinking Water, Water Quality and Supply, Flood Control, River and Coastal Protection Bond Act of 2006). The Sonoma County Water Agency is administering the grant for the project, which is formally called the Advanced Quantitative Precipitation Information system. Benefits of upgrading the flood warning system are estimated at $79 million per year, and include avoided flood damage as well as better management of shipping, railroads, and airports.

Similar systems are in place in Dallas-Fort Worth, The Netherlands, and Tokyo. The latter also has an app, and Morrison envisions that Bay Area residents will likewise be able to get flood warnings directly via an app. “It could tell you to avoid I-80 near Fairfield in two hours because there’s going to be flooding,” he said.

Rather than replacing the current system for tracking rainfall and predicting flooding, the new radars will build on it. Other elements include probes and gauges that monitor how wet soil is, and how high and fast streams are running. “The system will integrate all of this to give users a much better picture,” Morrison said.

Besides detecting more low-level rainfall, the new system will also measure it far more accurately. The system’s resolution will be about 200 meters and will update every minute or two, a huge improvement over the current resolution of one kilometer with updates every five to six minutes. “That’s a long time when the water is rising — flood water can come up really fast,” said Cifelli, who is NOAA’s lead on the project. Moreover, the new radars will enable real-time “nowcasts” out to 30 minutes, and NOAA’s new weather model will extend predictions out to 48 hours.

Knowing when and where flooding will occur will help on-the-ground responders like Ray Riordan, director of emergency services for the City of San Jose, which is traversed by eight creeks and a river and has a total of 140 miles of waterways. “It will let us allocate more resources to flood locations, and give us more time to put out flood warnings,” he said. For example, crews will be able to put sand bags on levees and clear out storm drains in areas predicted to flood. And residents will be able to elevate computers and other valuables above the reach of flood waters, move cars out of the flood zone, and, when necessary, evacuate.

New research shows that atmospheric rivers are hitting even harder as the world warms. A 2017 Geophysical Research Letters study found that these storms have intensified over the last 70 years, with three of the 30 that landed in California last winter classified as extreme. And climate change will intensify future atmospheric rivers even more, according to a 2017 Scientific Reports study, making the Bay Area’s new flood warning system even more critical.

Robin Meadows covers water for the Monitor.