Solving coastal California’s precipitation puzzle

Precipitation forecasts in California’s complex coastal terrain have challenged meteorologists for years - especially when drenching atmospheric rivers come ashore.

So 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 to give water agencies in the nine-county San Francisco Bay area better warning about potential flooding and water quality impacts from heavy rainfall events.

Current technology does not provide forecasters with enough detailed and accurate information to inform reservoir operations, flood protection, combined sewer-stormwater systems and emergency preparedness when big storms hit, state officials say. That’s because existing, long-range, mountain-top  NEXRAD radars, originally designed to look up into Midwest thunderstorms, are often unable to accurately resolve low-level precipitation hidden in the hilly landscape of coastal California.

The new system will couple an array of lower-elevation, high-resolution X-band radar units with an experimental state-of-the-art weather, river and coastal forecasting models to provide more precise rainfall, runoff and flood forecasting at the urban scale in and around the Bay Area.

“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 from the sky, it’s also forecasting what happens when that water hits the ground and runs off into the bay.”
Atmospheric rivers are a double-edged sword for California. A strong atmospheric river can transport roughly 7 to 15 times as much moisture as the average flow of water at the mouth of the Mississippi. 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 arriving in several storms over just a few days, that much water can also cause flooding and threaten public safety. Scientists estimate that atmospheric rivers are responsible for more than half of major flooding events in the Bay Area, and upwards of 70 percent in the North Bay.

In San Francisco, big storms can overwhelm stormwater management systems, sending millions of gallons of untreated sewage into San Francisco Bay.

The project will be funded by a grant administered by the California Department of Water Resources. For Grant Davis, who left the Sonoma County Water Agency this summer to become the new director of DWR, the August 15 signing of a memorandum of understanding finalizing the project represents the fruit of of several years of planning.

“When I was running the Sonoma County Water Agency, we recognized the potential value of developing this kind of capability for the greater Bay Area,” Davis said. “This is a great example of how local agencies can work together to integrate water supply and flood protection programs.”

The foundation of the Advanced Quantitative Precipitation Information Project, or AQPI, will be a system of four new short-range, high-resolution X-band radars, one medium-range C-band radar and gap-filling precipitation gauges specifically to provide more precise rainfall estimates for atmospheric rivers. The small, high-resolution X-band units—developed by Colorado State University’s Cooperative Institute for Research in the Atmosphere (CIRA) —will be arranged in a networked, regional array to provide continuous, low-level coverage of the Bay Area urban region, augmenting NEXRAD coverage.

The first X-band radar is scheduled to be installed this summer on the roof of a Santa Clara Valley Water District treatment plant. It’s the same location where a prototype was temporarily installed in 2016 near the site of Super Bowl LI.

“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.”

The radar array will feed into a new observation and forecasting system to be developed by NOAA that will produce forecasts from less than 1 hour to 10 days. Forecasts beyond 1 hour will use NOAA’s High Resolution Rapid Refresh weather model to inform flood control and emergency preparations over the short term. Longer-term forecasts will aid in reservoir operations and water supply management.

The team will utilize and evaluate a combination of water models, including NOAA’s new National Water model, River Forecast Center forecasts, and a coastal flooding model being developed by the U.S. Geological Survey. The goal is to predict ocean-based flooding up to 48 hours in advance of an approaching storm.

NOAA, CIRA and Scripps Institute of Oceanography will make a recommendation about who will operate the system after it is delivery, which is anticipated in 2021.

For more information, contact Theo Stein, NOAA Communications, at 303-497-6288 or theo.stein@noaa.gov.