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Extreme Precipitation Study


San Francisco Public Utilities Commission, Port of San Francisco, San Francisco International Airport, Office of Resilience and Capital Planning

Pathways, in collaboration with Lawrence Berkeley National Laboratory (LBNL), evaluated how extreme storms may change under future climate conditions. This understanding is a critical data gap for the City of San Francisco and the Bay Area. This project seeks to fill this gap by providing San Francisco agencies and engineering practitioners with the future condition precipitation inputs needed to support long-term resilience planning. These inputs will support multi-billion-dollar projects currently underway, including the SFPUCs Storm Sewer Improvement Program, the Port’s Waterfront Resilience Program, and SFO’s Shoreline Protection Program.

Pathways researched historical Bay Area storm events and synthesized the characteristics of 15 extreme storms that impacted the San Francisco agencies. The evaluation considered storm precipitation totals, peak hourly precipitation rates, maximum wind speeds and direction, barometric pressure, storm surge, storm type, and other characteristics. After vetting with the stakeholders, six historical storm events were selected for modeling and analysis by the Pathways/LBNL team.

LBNL modeled existing historic storm conditions using the Weather Research and Forecasting (WRF) model. The WRF model domain covers a large spatial extent across the Pacific Ocean and the entirety of California, with a 3-km grid resolution over the larger Bay Area. The simulations require the computing resources of LBNL’s supercomputer system – one of the larger super computers in the world. After the storms were suitably calibrated, the storms were modeled under future climate conditions to assess how the storms may change under a warming climate.

In general, the combined atmospheric river + extratropical cycle event produces the most dramatic increase in precipitation intensity, with the short duration precipitation producing super-Clausius Clapeyron behavior. Using the atmospheric river rating system, the results show that future storms could increase in intensity by one to three categories.

Download the Guidebooks:

Volume 1: State of the Science

Volume 2: Future Precipitation Intensity, Duration, and Frequency

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