Theme for 2024:
Anticipating and Planning for California Floods - Past and Future

From Science to Solutions: The Arc of Atmospheric Rivers from Research to Practical Utility and Future Directions

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Abstract:

Marty Ralph has been at the forefront of meteorologists researching ARs impacting California. His 20 years (1993-2013) of research at NOAA laboratories provided the impetus for founding, in 2014, the Center for Western Weather & Water Extremes (CW3E) at the Scripps Institution of Oceanography (SIO), UC San Diego. The establishment of CW3E has accelerated our understanding of ARs and improved the accuracy of weather and hydrological forecasting out 5 to 7 days by the National Weather Service (NWS) for landfalling ARs along the US west coast.

His talk has seven parts with selected highlights below:

• 1998-2006 — Laying the Foundation for ARs
  • The term "atmospheric rivers" was coined by Zhu and Newell in their research papers (1994-1998).
  • The ability to see integrated water vapor (IWV) contained in ARs moving across the Pacific Ocean by using SSM/I sensors on military satellites became available in 1998 from the work of colleague Gary Wick, Ph.D.
  • The offshore structure of ARs was advanced by aircraft dropsonde observations over the Pacific by two NOAA research campaigns: California Landfalling Jets Experiment (CALJET): 1997-1998 and Pacific Landfalling Jets (PACJET): 2000-2001
  • 6 journal papers with AR in title.
• 2007-2013 — Setting the Stage for CW3E
  • Hydrometeorology Testbed (HMT), NOAA originated 2004.
  • Atmospheric River Observatories (AROs): Funded by NOAA-HMT & California Department of Water Resources (CA-DWR) and installed 2008-2015.
  • ARkStorm emergency preparedness scenario for California by U.S. Geological Survey (USGS) completed 2010.
  • 42 journal papers with AR in title.
• 2014-2020 — AR Interest Expands, CW3E Grows, and Forecast Informed Reservoir Operations (FIRO) Launches
  • First International Atmospheric Rivers Conference (IARC) held at SIO in 2016 and held every two years since.
  • AR added to Glossary of Meteorology, American Meteorological Society (AMS), 2017.
  • AR Scale developed to estimate AR strength and potential for beneficial and hazardous impacts, 2019.
  • Lake Mendocino FIRO Final Viability Assessment in Russian River watershed completed in 2021 after six-year research study effort.
  • Atmospheric River Reconnaissance (AR Recon) initiated in 2016 to demonstrate value of aircraft releasing dropsondes into ARs to improve forecast skill.
  • 294 journal papers with AR in title.
• 2021-2024
  • West-WRF Regional Forecast Model tailored to be best AR forecast model.
  • AR Recon expanded to Guam flights and drifting buoys deployed.
  • 233 journal papers with AR in title (2021-2023).
• Climate Change and AR Impacts
  • Climate modeling indicates more damaging ARs in future.
• FIRO Future
  • Leading feasibility assessments of FIRO at six California reservoirs and 14 in Washington and Oregon.
• Future Outlook
  • Continuing research to answer: How far in advance can we predict land-falling ARs?
  • Six year plan to triple CW3E supercomputing capacity to advance machine learning (ML) and artificial intelligence (AI) applications (2024-2030).
  • Northern Hemispheric Global Atmospheric River Reconnaissance Program (GARRP) Demonstration planned for 2026.
  • Atmospheric River Reconnaissance, Observations and Warnings Act (ARROW Act) enacted in 2023 by Congress which directs U.S. Air Force and NOAA to provide aircraft, personnel, and equipment to support and advance the AR Recon mission.

An Overview of ARkStorm 2.0 and Its Implications: California Flood Risk Looms Large in a Warming Climate

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Abstract:

Between 2013 and 2023, California experienced two historically severe droughts and record-breaking wildfire activity leading to hundreds of deaths and net economic losses estimated to be in the hundreds of billions of dollars. Yet over this same period, many parts of the state also experienced their wettest days, months, and seasons on record, which resulted in a number of notable and locally severe flood events. Over this same period, a growing body of scientific evidence has emerged suggesting that the risk of extreme precipitation events and severe flooding in California has likely risen, quietly and in the background, in an era where much public attention has been focused on the impacts and trends in water scarcity. This reality of rising "hydroclimate whiplash" offers context for the ARkStorm 2.0 scenario and associated research, which represents an update of the original (2011) ArkStorm scenario reimagined for the climate change era.

In this talk, I will discuss the design, implementation, and analysis of the new multi-week extreme storm scenario that underpins ARkStorm 2.0. I'll also review the latest science on extreme precipitation and severe flood events in California, both in the specific context of ARkStorm 2.0 and more broadly against a backdrop of climate change, including physical mechanisms and climate modeling caveats. Finally, I will offer my own thoughts on why the risk of water overabundance in the Golden State remains broadly underappreciated in our warming world.

Evolution of Planning for Extreme Flooding in American River Watershed 1947-2024

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Abstract:

Sacramento sits in a flood-prone area at the confluence of the Sacramento and American Rivers with a well-documented history of flooding, as first notably marked by the Great Flood of 1862, during which Governor Leland Stanford famously used a rowboat to reach his inauguration. More recently, the regionally devastating floods of 1986 and 1997 emphasized the continued presence of the flood risk.

A key component of Sacramento's flood defense is Folsom Dam, constructed in 1955 to protect the city and downstream areas from flooding. This presentation highlights the infrastructure and regulatory measures implemented since that time to combat Sacramento's flood risk, which has been repeatedly amplified with new record-setting floods, and more recently, with a growing awareness of the impacts of climate change. Looking forward, the presentation highlights the next phase of measures to protect Sacramento's citizens, including the Folsom Dam Raise and its operational components.

This presentation celebrates nearly a century of creativity, persistence, and resilience by floodplain managers in the face of a life-altering threat, while recognizing there is still more to do.

Sacramento River Watershed Flooding Risks from Future Extreme Precipitation Events

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Abstract:

This session will provide an overview of the evolution of the flood control system in the Sacramento River Basin, developments in California's flood policy that led to the creation of the Central Valley Flood Protection Plan (CVFPP), and a technical summary of the flooding risks posed by future extreme precipitation events based on the 2022 CVFPP Update. Additionally, it will compare the recent Water Year 2023 to historical and projected future flood scenarios.

The presentation will begin with an examination of the historical context, focusing on significant floods that have been pivotal in shaping flood management strategies in the Sacramento River Basin. It will discuss the State Plan of Flood Control (SPFC), highlighting its key components in the Sacramento Basin, which include over 1,100 miles of levees, several major bypasses and weirs, and significant dams such as Shasta, Oroville, New Bullards Bar, and Folsom.

Next, the presentation will shift to the flood policy developments that led to the creation of the CVFPP. This plan is a collaborative effort among local, state, and federal partners aimed at reducing flood risk while promoting ecosystem functions, resiliency, sustainability, and multibenefit projects. The presentation will provide a comprehensive overview of the impacts of climate change, which is expected to bring increased temperatures and altered precipitation patterns, as detailed in the recent 2022 CVFPP Update. An analysis of Water Year 2023 will be presented, comparing it with historical events and projected future events influenced by climate change.

In conclusion, this presentation will provide an overview of the flooding risks in the Sacramento River Basin due to future extreme precipitation events and underscores the importance of proactive flood risk management and the need for ongoing investment in infrastructure and policy development. These measures are crucial to addressing the evolving challenges posed by climate change and extreme weather events, ensuring the safety and resilience of the Sacramento River Basin and its communities.

San Joaquin River Watershed Flooding Risks from Future Extreme Precipitation Events

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Abstract:

The U.S. Army Corps of Engineers (USACE) Sacramento District (SPK), in collaboration with the San Joaquin Area Flood Control Agency (SJAFCA), the Central Valley Flood Protection Board (CVFPB), and the California Department of Water Resources (DWR), is conducting a feasibility study to improve infrastructure and reduce flood risk in the communities of Lathrop and Manteca. These areas are home to 44,000 residents and over 50 critical infrastructure facilities that face flooding risks from the mainstem San Joaquin and Stanislaus Rivers.

The USACE planning process for feasibility studies employs the National Economic Development (NED) procedures to identify the alternative plan with the greatest net national economic benefit while ensuring environmental protection. The NED Plan evaluates future conditions within the study area by accounting for the risks associated with sea level rise and consequences due to population growth. The NED plan does not explicitly account for changes in future inland hydrology within the computation of project benefits.

This study aims to assess the future flooding risks in the San Joaquin River watershed by performing a sensitivity analysis of USACE plan selection to projected inland hydrology in the Lathrop and Manteca Study Area. The analysis will determine if incorporating these projections into the NED framework would result in the selection of a different alternative plan. The findings will provide valuable insights into the robustness of the NED Plan and its ability to effectively mitigate future flood risks under changing hydrological conditions.

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Abstract:

The frequency of multiple atmospheric rivers (ARs) occurring in sequence is projected to increase due to climate change. Sequential AR scenarios are needed to understand regional flooding impacts and inform effective mitigation and response.

This work investigates the flooding impacts of ARkFuture — a modeled month-long sequence of ARs making landfall on the West Coast of the U.S. — specifically focusing on the Truckee Watershed. ARkFuture is the climate change scenario in the ARkStorm 2.0 model, forced by 2072 climate conditions and with an estimated recurrence interval exceeding 400 years. The Truckee Watershed, located on the eastern slope of the northern Sierra Nevada and encompassing Lake Tahoe, has experienced multiple large and damaging historical floods, most recently in 1997 and 2017. With a large area of the watershed at elevations close to the rain-snow line, warming may have major impacts on how the basin responds to future storm events. Moreover, the region has experienced significant population growth in recent decades, increasing impervious surfaces and development in and near the floodplain.

Using a series of hydrologic and operational models, we estimated the local flows and inundation resulting from the ARkStorm 2.0 ARkFuture scenario. We then compared these hydrologic impacts with those from estimates of shorter-duration higher-frequency events such as those used to generate FEMA 100-year flood maps and the historical 1997 flood extent. This comparative analysis sheds light on the unique challenges posed by long-duration events and provides valuable insights for emergency planning and response efforts.

Ultimately, this research serves as a critical case study and demonstration of how the ARkStorm 2.0 scenario can be used in consultation with a group of local stakeholders to facilitate conversations around, and promote community resilience to, climate change impacts on flooding events.

Panel Discussion: How does society plan for anticipated future flood risks?

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• Download Mierzwa presentation [PDF, 1.1 MB]

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Moderator:

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