Atmospheric River Reconnaissance: 2025 Update

Atmospheric rivers (ARs), narrow corridors of intense water vapor transport, provide essential precipitation. On the west coast of the U.S., the absence of ARs causes drought, and ARs are associated with almost all flood hazards. In recent years, a Research and Operations Partnership (RAOP) called Forecast Informed Reservoir Operations (FIRO) has been developed that is focused on water supply resilience and flood hazard mitigation and is conducting several viability assessments at key watersheds in the western U.S. with significant precipitation inputs from ARs. FIRO requires improved AR forecasts to maximize success. With this and other motivations, Atmospheric River Reconnaissance (AR Recon) was developed as an interagency, international RAOP that collects unique observations in the northeast Pacific to improve AR landfall forecasts and associated precipitation forecasts during the cool season (November – March). It began as a pilot study from 2016-2019, which is described in Ralph et al. 2020 (BAMS). Motivated by early demonstrations of value presented in several journal articles, AR Recon observations are now officially called for in the U.S. National Winter Season Operations Plan, with leadership at the Center for Western Weather and Water Extremes (CW3E) in close collaboration with NOAA National Centers for Environmental Prediction (NCEP). The NOAA G-IV aircraft operated by NOAA/OMAO/AOC and several C-130 aircraft operated by the US Air Force 53^rd Weather Reconnaissance Squadron are the core airborne assets used by AR Recon.

AR Recon was also the core of one of the recommendations in NOAA/Science Advisory Boards’ “Priorities for Weather Research” (PWR Report) created in 2021 in response to direction from Congress for the NOAA SAB to make recommendations spanning a decade. More recently, in late 2023, The Atmospheric Rivers Observations and Warnings Act (ARROW Act) was signed into law as part of the National Defense Authorization Act after approval by the US Senate, House and President Biden. It specifies AR Recon be conducted “...over the eastern, central and western north Pacific Ocean, the Gulf of Mexico and the western Atlantic Ocean...,” and that is “Participate in the research and operations partnership that guides flight planning and uses research methods to improve and expand the capabilities of weather reconnaissance over time...” In 2024, for the first time, AR Recon missions were conducted as a demonstration in the western Pacific, from Guam.

The schematic Figure (from Lavers et al. BAMS, 2024) illustrates the broad array of observations used by AR Recon. The foundational observations within AR Recon are dropsondes, which provide atmospheric vertical profiles of temperature, moisture, winds, and pressure at targeted locations over the northeast Pacific, but AR Recon includes many other data streams via robust partnerships. Partners include the Scripps Lagrangian Drifter Laboratory-based NOAA funded Global Drifter Program, prototype data streams for airborne radio occultation based at Scripps, targeted radiosonde releases by CW3E after AR landfall, and others. CW3E partners closely with NCEP Environmental Modeling Center to carry out near real-time data denial experiments to examine the data impact on the Global Forecast System. Data denial efforts are also conducted in partnership with other global operational modeling centers in the off-season. This presentation will cover the accomplishments of the AR Recon RAOP with a focus on results from data impact studies, program evolution, and future plans to continue to advance national weather services in partnership with NOAA based on accomplishments to date.