Rain: When a good thing becomes too much of a good thing

Atmospheric rivers are a double-edged sword for California. Just a few atmospheric river events can deliver half of a year’s precipitation, which can be a relief for anxious water managers in a drought-prone region. But that much water, when it arrives like a firehose from the sky, can cause flooding that endangers lives and property.

Starting this summer, NOAA and the Sonoma County Water Agency will kick off a four-year, $19-million project to build a new regional radar and advanced rainfall forecast system. Once in place it will give water agencies in the nine-county San Francisco Bay area better warning about potential flooding and water quality impacts from heavy rainfall events.

“Because of the terrain in this area, precipitation often forms in very low levels of the atmosphere, below the level at which existing NEXRAD Doppler radar can see it well,” said scientist Rob Cifelli, NOAA’s project lead. “Our goal is not just predicting how much water will be falling, it’s also forecasting what happens when that water hits the ground and runs off into the bay.”

The system will be based on a new array of lower-elevation, X-band radar units that provide highly detailed information. This data is then fed into an experimental state-of-the-art weather, river and coastal forecasting models resulting in more precise rainfall, runoff and flood forecasting in and around the Bay Area. Versions of the system may be run by local water agencies depending on their needs. 

The first X-band radar will be installed this summer in Santa Clara at the same location where a prototype was tested in 2016.

“What we found is that the radar was able to provide more accurate precipitation estimates in the Santa Clara - San Jose area than NEXRAD did,” Cifelli said. “It was a good test run.”