New AQPI C-Band Radar Delivers Impressive Results
April 29, 2026
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 (50-250m, 2 min).
Figure 1. Schematic of AQPI radar network. Radar locations and theoretical coverage areas are represented by stars and circles, respectively, colored red (AQPI C-Band), green (AQPI X-band), or blue (NWS S-band). Color shading indicates (left and center) the lowest radar beam elevation over a given area, which is a proxy for radar accuracy in estimating surface precipitation, and (right) the difference between the lowest radar beam elevation with and without AQPI.
The final piece and cornerstone of this new radar network is a C-band radar, located atop Mt Barnabe in Marin County. It extends direct radar observations well offshore to provide longer forecast lead times, fills gaps in X-band radar coverage, and adds further data where X-band coverage exists. Data from AQPI and NWS radars are woven together to produce the AQPI mosaic, a 250-meter resolution gridded radar reflectivity product, and a radar-based nowcast.
Figure 2. AQPI mosaic 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 (before C-band integration). The center and right panels show AQPI mosaic and NEXRAD-only data, respectively, zoomed in on San Francisco at one time to show differences in clarity.
Thanks to hard work by team members at CW3E and CSU/CIRA, the C-band radar was operational and integrated into the AQPI mosaic just in time for a late-season storm that impacted the Bay Area on 11-12 Apr 2026. In this case, the AQPI mosaic quantitative precipitation estimate (QPE) performed very well relative to its benchmark (the MRMS radar-only QPE product) in matching regional CNRFC rain gauge data. At selected locations, which are not visible to AQPI X-bands due to range limitations and/or blockages, it is likely that a large fraction of AQPI mosaic “skill” can be attributed to the introduction of C-band data into the mosaic product.
Figure 3. AQPI mosaic (left) and mosaic without AQPI (right) at 4:40 PDT on 11 Apr 2026. AQPI radar coverage areas are shown in black. Selected sites not covered by the AQPI network prior to C-band installation are highlighted. The Lake Sonoma Rec Area, although theoretically covered by an X-band radar, is in a blocked area caused by a tree near the radar.
Figure 4. (right) Time series precipitation at selected sites from Fig. 3 based on gauge observations (black), AQPI radar estimates (green), MRMS radar-only estimates (yellow), and MRMS multi-sensor estimates (red).
In summary, the AQPI C-band radar extends high-quality radar coverage to many areas that previously had poor or no coverage, even following installation of the AQPI X-band radars. These include many areas across the North Bay, the adjoining Coast Ranges to the north, and a broad area offshore from which many storms approach. In addition, the C-band radar provides valuable overhead data for areas farther south and east, which also benefit from the longer lead time provided by the offshore observations. In the first storm event analyzed with C-band data, the AQPI mosaic performed very well in areas that had previously been poorly observed, an encouraging early indicator of the completed network’s value.