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San Francisco and the Bay Area Could See Up to a 37% Increase in Rainfall During Large Storm Events Over the Next Century

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NEWS RELEASE 
John Coté
415-417-9319
jcote@sfwater.org

Matt Nerzig
Lawrence Berkeley National Laboratory
510-541-7531
mnerzig@lbl.gov

FOR IMMEDIATE RELEASE
April 13, 2022

San Francisco and the Bay Area Could See Up to a 37% Increase in Rainfall During Large Storm Events Over the Next Century

Groundbreaking study that SF commissioned will help the City adopt policies to address powerful future storms, which can now be modeled with much greater accuracy

SAN FRANCISCO – The City and County of San Francisco, in partnership with Lawrence Berkeley National Laboratory, today announced the release of a pioneering new climate study that will provide more detailed information to help the City adapt to the changing climate and the extreme storms it will bring.

The peer-reviewed study, commissioned by the City and led by scientists at Lawrence Berkeley National Laboratory with support from Pathways Climate Institute, Urban Waterworks and City staff, was recently published in the journal Weather and Climate Extremes. It found the effect of climate change on future storms will be significant, leading to more powerful storms unleashing substantially more water. 

Among its findings were that storm-total precipitation associated with the most common type of storm in the Bay Area – an atmospheric river with an extratropical cyclone – may increase by up to 26% to 37% by the end of this century. “We simulated how historically impactful storm events could change if similar events occurred in future climates,” said lead author Christina Patricola (now an Assistant Professor at Iowa State University).

That strength of storm is akin to the massive “bomb cyclone” that combined with an atmospheric river to bring torrential rains and high winds to Northern California and San Francisco in October 2021. That storm was rated as a Category 5, the highest designation in the Scripps Institution of Oceanography Atmospheric River Scale developed for West Coast storms. The storm tied for the third strongest storm since 1950 in the Bay Area and was the strongest storm in the past 26 years of record.

The study also provides a much more localized look at the effects of these projected storms. Prior to this, many climate models had a resolution of 200 kilometers, or 124 miles. One square on the model grid represented the entire Bay Area. Even what had been considered state-of-the-art climate models have resolutions of about 25 kilometers, or roughly 5 ½ miles. Those models can broadly represent West Coast atmospheric rivers but cannot adequately address the complex topography of the Bay Area, which leads to highly variable precipitation rates from one city to the next. This new study has a much more detailed resolution of 3 kilometers, or less than 2 miles, which can provide very local projections for how much rain is expected to fall. 

“Having this level of detail is a game changer,” said Dennis Herrera, General Manager of the San Francisco Public Utilities Commission, which was the lead City agency on the study. “The SFPUC has been at the forefront when it comes to studying climate change and using that data to guide our decisions. This study will help us stay out front on this issue. This groundbreaking data will help us develop tools to allow our port, airport, utilities, and the City as a whole to adapt to our changing climate and increasingly extreme storms. This also makes clear that we’re facing a citywide issue. In practical terms, we simply can’t build a sewer pipe big enough to handle all of this water. We must think holistically about how we build, what we build, and where we build it.”

“This collaboration is one of the first of its kind to apply the climate modeling expertise at Berkeley Lab to inform local decisionmakers,” said Michael Wehner, one of the co-authors of the study. “By tailoring our models to the City’s specific questions, we are able to provide more confident answers in the amount of heavy rainfall that can be expected in a future warmer world.”
 
“We are proud to have partnered with SFPUC and Lawrence Berkeley National Laboratory through our ClimateSF program on this leading-edge study,” said Brian Strong, Chief Resilience Officer for San Francisco. “Understanding the frequency and magnitude of storms in relation to climate change is critical to building a more resilient San Francisco. We now have information at a scale that enables decision making to better protect people, vulnerable communities, and businesses now and into the future.”

This state-of-the-art modeling effort was funded by the San Francisco Public Utilities Commission, San Francisco Office of Resilience and Capital Planning, San Francisco International Airport, and the Port of San Francisco. It used the resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02-05CH11231.

As City departments collaborate to develop shoreline adaptation strategies to address sea level rise, now is the time to add this new information about stormwater into the equation. The City needs solutions to keep rising Bay flooding out, while also allowing stormwater to be conveyed out of the city. 

This study adds to the body of work the City has championed in terms of combined flood hazards such as sea level rise, stormwater, coastal flooding, and groundwater, many of which are included in our Hazards and Climate Resilience Plan that provides concrete actions the City can take to address them.

In addition to physical solutions, the study will help inform adaptations to our planning codes and building codes to better consider future extreme storms and reduce their impact on our neighborhoods, businesses, infrastructure, and the environment. 

Future storms bring the potential for record rainfall and elevated ocean and San Francisco Bay water levels during the storm. Heavy precipitation coupled with elevated Bay water levels are the perfect recipe for flooding throughout the Bay Area, and San Francisco is no exception.

This study is the first piece of ongoing work. Additional analysis is still underway to develop data products and tools that will support the City’s ongoing climate resilience efforts. As part of this work, Pathways Climate Institute, with support from Lawrence Berkeley National Laboratory, is developing a guidebook to help planners, modelers, engineers, and decision makers use the incredible wealth of information that this study provides to better prepare San Francisco for future storms. The guidebook is slated for completion in late 2022.  
 
About ClimateSF
ClimateSF is a comprehensive, multi-agency effort to guide San Francisco’s climate resilience effort, led by the Mayor’s Office and Office of Resilience and Capital Planning, the San Francisco Public Utilities Commission, Port, Department of the Environment, and Planning Department.

About the San Francisco Public Utilities Commission
The San Francisco Public Utilities Commission (SFPUC) is a department of the City and County of San Francisco. It delivers drinking water to 2.7 million people in the San Francisco Bay Area, collects and treats wastewater for the City and County of San Francisco, and generates clean power for municipal buildings, residential customers, and businesses. Our mission is to provide our customers with high quality, efficient and reliable water, power, and sewer services in a manner that values environmental and community interests and sustains the resources entrusted to our care. Learn more at www.sfpuc.org.