Advanced Quantitative Precipitation Information

Overview

When major storms approach the San Francisco Bay Area, decision makers need accurate rainfall information to act. Until recently, the regional radar network featured critical gaps in coverage and insufficient detail for sophisticated operations. But that is changing: Advanced Quantitative Precipitation Information (AQPI) is a state-of-the-art system for monitoring rainfall across the Bay Area. The program has deployed a network of 6 advanced weather radars, strategically located to complement the existing radar network by filling gaps in coverage and operating at a higher spatiotemporal resolution. This advanced data, along with radar-derived products such as the mosaic and nowcast, customized precipitation forecasts, and more, are delivered to partners via the newly developed AQPI User Dashboard.

How it Works

Water and wastewater management, flooding, and emergency management are major concerns in the Bay Area. The AQPI program addresses these concerns by providing relevant decision makers with the best possible rainfall information. This advanced data has been used to inform critical decisions, such as those regarding evacuations: AQPI: Winter 2022/2023 Atmospheric River Storms in Santa Cruz County.

The graphic below shows AQPI radar data for a storm impacting the Bay Area on Dec 24-25, 2025. The left panel shows a ~12-hr loop of the AQPI radar mosaic, which integrates AQPI and NEXRAD radar data into one consistent product. The center and right panels are zoomed in on San Francisco at one time to show differences in clarity between the AQPI mosaic data (center) and the NEXRAD-only data (right).

In addition, the customized precipitation forecasts delivered through the AQPI program are used by a large number of partners to plan their staffing and operations at lead times of 1-10 days.

The Future

Working closely with local partners, CW3E has developed a roadmap that outlines program goals over the next several years. These include operations, maintenance, and program enhancements with the following focus areas:

• Improvements in radar-based quantitative precipitation estimates (QPEs)
• Improvements in customized quantitative precipitation forecasts (QPFs)
• Data services (display and delivery via the AQPI User Dashboard)
• Data integration services (training, interpretation, customer service)
• Rapidly-updating streamflow forecasts for small and “flashy” Bay Area watersheds

Shown at right is an example of high-resolution (1-km) weather modeling output from the CW3E West-WRF model showing 15-minute precipitation and wind speed, which is routinely available over the Greater Bay Area during the winter months.

AQPI is also working with NOAA to inform the Radar Next program, which will design and deploy the next generation of weather surveillance radar technology to improve environmental data and products.

And finally, AQPI is supporting a demonstration of value for mobile radar observations in Southern California, specifically near burn scars, during the 2025/2026 and 2026/2027 winter seasons. The 2025/2026 season was limited by equipment delivery delays and dry weather, but a larger campaign is planned for 2026/2027.

The Radar Nework

The completed AQPI radar network includes five X-Band radars and one C-Band radar covering most of the Greater San Francisco Bay Area (figure below). The Santa Cruz X-band was funded and procured separately from AQPI, but is otherwise fully integrated into the AQPI network.

Timeline for Radar Development

As of Winter March 2026, all AQPI radars are now operational. These include X-band radars located in Sonoma, Santa Clara, Contra Costa, San Mateo, and Santa Cruz Counties, as well as the C-band radar located in Marin County. See below for further details of the deployment timeline:

• Santa Rosa X-Band (Sonoma County, hosted by Sonoma County Water Agency)
  Operated a temporary X-band radar since 2018, located near the Sonoma County Airport. A permanent X-Band radar was deployed in March 2023 at the same location.
• Santa Clara X-Band (Santa Clara County, hosted by Santa Clara Valley Water)
  Operated a temporary X-Band radar since 2016, located at the Penitencia Water Treatment Plant. A permanent X-Band radar was deployed in July 2019 at the same location.
• Rocky Ridge X-Band (Contra Costa County, hosted by East Bay partners)
  In December 2022, a partnership of East Bay agencies began deploying an X-Band radar on Rocky Ridge, adjacent to the Las Trampas Wilderness area. The Rocky Ridge radar became operational in Winter 2023/2024.
• Sawyer Ridge X-Band (San Mateo County, hosted by SF Public Utilities Commission)
  Deployed in Fall 2025 with support from SFPUC.
• Santa Cruz X-Band (Santa Cruz County, hosted by the Santa Cruz County)
  Deployed an X-Band radar on a county building in August 2022, funded through a grant from DWR’s Statewide Flood Emergency Response Grant program.
• Mt Barnabe C-Band (Marin County, hosted by Marin County)
  Deployed in Fall/WInter 2025 and located on the peak of Mt Barnabe in western Marin County, the C-Band radar will observe offshore rain bands as they approach the Bay Area.

Background & Partners

AQPI was funded in 2016 by a grant from the California Department of Water Resources (CA DWR) Integrated Regional Water Management Program (IRWM) awarded to Sonoma County Water Agency and participating Bay Area agencies. The National Oceanic and Atmospheric Administration (NOAA) was involved in early development, and the Cooperative Institute for Research in the Atmosphere (CIRA) led deployment of the radar network. A Local Partner Agency Committee (LPAC) has overseen regional collaboration since the inception of this program, and is being superseded by a new governance structure now that the CA DWR grant is expired.

AQPI is led programmatically and scientifically by the Center for Western Weather and Water Extremes (CW3E) at UCSD/SIO. The Cooperative Institute for Research in the Atmosphere (CIRA) at CSU is a key technical partner, contributing to radar network O&M and radar-derived product development. Operational partners include Bay Area water and wastewater agencies, flood control districts, counties, and other organizations relying on timely and accurate rainfall information. AQPI is a pilot project for advanced radar coverage and precipitation information across the state of California.

AQPI is a unique federal, state, and local partnership, receiving support from the National Oceanic and Atmospheric Administration (NOAA), California Department of Water Resources (CA DWR), and local Bay Area sponsors.

AQPI Data

AQPI User Interface: Real-time data for Bay Area stakeholders – includes radar data, mosaic, nowcast, forecast products, archived data, and more.

Contact Information

Jonathan Rutz, CW3E

Program Management and Scientific Lead

Ricardo Vilela, CW3E

Data Management and Web Development

Matt Steen, CW3E

Meteorology and Case Studies