Existing law establishes the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AR/FIRO) Program in the Department of Water Resources. Existing law requires the department to operate reservoirs in a manner that improves flood protection, and to reoperate flood control and water storage facilities to capture water generated by atmospheric rivers. Existing law requires the department to research, develop, and implement new observations, prediction models, novel forecasting methods, and tailored decision support systems to improve predictions of atmospheric rivers and their impacts on water supply, flooding, post-wildfire debris flows, and environmental conditions.

This bill would, for novel forecasting methods researched, developed, and implemented by the department, require the department to include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts, as defined.

Existing law establishes the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AR/FIRO) Program in the Department of Water Resources. Existing law requires the department to operate reservoirs in a manner that improves flood protection, and to reoperate flood control and water storage facilities to capture water generated by atmospheric rivers. Existing law requires the department to research, develop, and implement new observations, prediction models, novel forecasting methods, and tailored decision support systems to improve predictions of atmospheric rivers and their impacts on water supply, flooding, post-wildfire debris flows, and environmental conditions.

This bill would, for novel forecasting methods researched, developed, and implemented by the department, require the department to include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts, as defined.

Existing law establishes the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AR/FIRO) Program in the Department of Water Resources. Existing law requires the department to operate reservoirs in a manner that improves flood protection, and to reoperate flood control and water storage facilities to capture water generated by atmospheric rivers. Existing law requires the department to research, develop, and implement new observations, prediction models, novel forecasting methods, and tailored decision support systems to improve predictions of atmospheric rivers and their impacts on water supply, flooding, post-wildfire debris flows, and environmental conditions.

This bill would, for novel forecasting methods researched, developed, and implemented by the department, require the department to include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts, as defined.

Existing law declares that all water within the state is the property of the people of the state, but the right to the use of the water may be acquired by appropriation in the manner provided by law. Existing law requires the appropriation to be for some useful or beneficial purpose. Existing law provides that the diversion of floodflows for groundwater recharge, commenced before January 1, 2029, does not require an appropriative water right if certain conditions are met, including that the diversion does not use new permanent infrastructure or permanent construction.

This bill would make a nonsubstantive change to this provision.

SENATE THIRD READING

STR Bill Id:SB 599

Author:(Caballero)

As Amended Ver:April 24, 2025

Majority vote

SUMMARY

Requires the Department of Water Resources (DWR) to research, develop, and implement experimental tools that produce seasonal and subseasonal atmospheric river (AR) forecasts under the Atmospheric Rivers Research and Forecast Improvement Program (Atmospheric Rivers Program).

Major Provisions

1. Requires DWR to research, develop, and implement experimental tools that produce seasonal and subseasonal AR forecasts under the Atmospheric Rivers Program.
2. Defines "seasonal forecast" as a forecast that covers approximately one to six months into the future for the purposes of this bill.
3. Defines "subseasonal forecast" as a forecast that covers approximately two to six weeks into the future for purposes of this bill.

COMMENTS

ARs are long narrow bands of atmospheric water vapor pushed along by strong winds. They transport large amounts of water vapor from the tropics to higher latitudes and strike coastlines globally from California to Europe to Antarctica. They can transport moisture equivalent to more than 25 times the amount of liquid water flowing through the mouth of the Mississippi River.

ARs and their intense precipitation can be both beneficial and hazardous, presenting a special challenge to California's water managers and flood control planners. According to the Scripps Institution of Oceanography (Scripps) at the University of California, San Diego and the National Oceanic and Atmospheric Administration (NOAA), a small number of ARs supply on average between 40% to 50% of California’s precipitation and snowpack annually. A single AR event can provide up to 30% of annual precipitation in Southern California. Problems occur when AR events last for more than a day or when several occur back-to-back such as occurred in January 2023; several ARs in sequence can cause hazards like flooding, mudslides, or ash flows in areas of recent wildfires. NOAA and Scripps research has shown ARs are implicated in most of the extreme precipitation and flooding events in California. Nearly 80% of levee breaches in the Central Valley have been associated with AR events.

A recent study by researchers at the Center for Western Weather and Water Extremes (CW3E) housed at Scripps examined the February 2017 AR event that contributed to the Oroville crisis and found that "California's reliance on precipitation from (ARs) is expected to increase as our climate warms." Similarly, research published in August 2022 ("ARkFuture") shows how climate change is leading to more extreme AR events. ARkFuture models a 30-day "megastorm" event in which "ARs are the primary storm mode" causing extremely high precipitation and associated runoff that is two to four times higher than historical averages (Huang and Swain, 2022). Such an event would result in a "megaflood." Better modeling and forecasting could help to mitigate the risks associated with such an event.

Forecast-Informed Reservoir Operations (FIRO) is a reservoir operations strategy that, in effect, allows reservoirs to be managed in "real-time" so that operators can keep water in the reservoir if the weather forecast shows no imminent precipitation in a reservoir's watershed or, alternatively, release water from the reservoir to free up space for additional flow (thereby maintaining flood protection) when the forecast indicates an AR is headed towards a reservoir's watershed. Enhanced forecasting capability is essential to implementing FIRO. Sonoma Water Agency has piloted and implemented FIRO at Lake Mendocino since 2019 and has been able to increase water stored in that reservoir by nearly 20% while maintaining flood protection. This additional water was immensely helpful in 2021 when the Russian River basin experienced extreme drought conditions.

According to the Author

"[ARs] are a recently recognized phenomenon that bring narrow bands of water vapor that can transport large quantities of water from tropical regions to California where it falls as rainfall. These ARs have grown in intensity and frequency where they create the potential for flooding, coastal damage and intense winds. [This bill] builds upon [DWR's] current research operations to advance extended range AR forecasts, to provide emergency operations ample lead-time for effective decision-making. With more accurate data and longer-range certainty, we can better prepare and protect our communities from catastrophic flood and other severe weather events, improve our water management and storage capabilities, and reduce financial devastation caused by flooding."

Arguments in Support

The Association of California Water Agencies (ACWA) supports this bill and asserts that it will improve weather forecasting to inform decision-making about flood prevention and water management. Further, ACWA maintains that this bill will "increase DWR's AR forecast capacity which would provide emergency operations with ample lead-time for effective decision-making and allow DWR and water managers across the state to better understand atmospheric rivers and their impacts, more effectively manage water resources, and prepare for natural disasters. These investments will save significant funding over time as the state is better able to manage this limited resource and plan for and avert potential disasters."

Arguments in Opposition

None on file.

FISCAL COMMENTS

According to the Assembly Appropriations Committee, this bill has the following fiscal impact:

DWR will likely incur significant ongoing costs to update and expand its work on ARs to include experimental tools that produce seasonal and sub-seasonal forecasts.

For its part, DWR estimates ongoing annual General Fund costs of approximately $5.5 million to implement the bill. DWR's AR/ FIRO Program has a $4 million annual operating budget. Improving the department’s ability to forecast AR events beyond the current four-to-10 day period to sub-seasonal (two-to-six weeks) and seasonal (one-to-six months) periods will require new and expanded research, study, and testing of novel forecasting tools and modeling.

VOTES

SENATE FLOOR: 38-0-2

YES: Allen, Alvarado-Gil, Archuleta, Arreguín, Ashby, Becker, Blakespear, Cabaldon, Caballero, Cervantes, Choi, Cortese, Dahle, Durazo, Gonzalez, Grayson, Grove, Hurtado, Jones, Laird, McGuire, McNerney, Menjivar, Niello, Ochoa Bogh, Padilla, Pérez, Richardson, Rubio, Seyarto, Smallwood-Cuevas, Stern, Strickland, Umberg, Valladares, Wahab, Weber Pierson, Wiener

ABS, ABST OR NV: Limón, Reyes

ASM WATER, PARKS, AND WILDLIFE: 12-0-1

YES: Papan, Jeff Gonzalez, Alvarez, Ávila Farías, Bains, Bennett, Boerner, Caloza, Hart, Celeste Rodriguez, Rogers, Tangipa

ABS, ABST OR NV: Macedo

ASM APPROPRIATIONS: 15-0-0

YES: Wicks, Sanchez, Arambula, Calderon, Caloza, Dixon, Elhawary, Fong, Mark González, Ahrens, Pacheco, Pellerin, Solache, Ta, Tangipa

UPDATED

VERSION: April 24, 2025

CONSULTANT: Pablo Garza / W., P., & W. / (916) 319-2096 FN: 0001390

Date of Hearing: July 16, 2025

ASSEMBLY COMMITTEE ON APPROPRIATIONS

Buffy Wicks, Chair

SBPCA Bill Id:SB 599

Author:(Caballero) – As Amended Ver:April 24, 2025

Urgency: No State Mandated Local Program: No Reimbursable: No

SUMMARY:

This bill requires the Department of Water Resources (DWR) to research, develop, and implement experimental tools that produce seasonal and sub-seasonal atmospheric river (AR) forecasts, as defined, under the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AR/ FIRO) Program.

FISCAL EFFECT:

DWR will likely incur significant ongoing costs to update and expand its work on ARs to include experimental tools that produce seasonal and sub-seasonal forecasts.

For its part, DWR estimates ongoing annual General Fund costs of approximately $5.5 million to implement the bill. DWR’s AR/ FIRO Program has a $4 million annual operating budget. Improving the department’s ability to forecast AR events beyond the current four-to-10 day period to sub-seasonal (two-to-six weeks) and seasonal (one-to-six months) periods will require new and expanded research, study, and testing of novel forecasting tools and modeling.

COMMENTS:

1) Purpose. According to the author:

[This bill] builds upon [DWR’s] current research operations to advance extended range AR forecasts, to provide emergency operations ample lead-time for effective decision-making. With more accurate data and longer-range certainty, we can better prepare and protect our communities from catastrophic flood and other severe weather events, improve our water management and storage capabilities, and reduce financial devastation caused by flooding.

2) Background. ARs. ARs are long narrow bands of atmospheric water vapor pushed along by strong winds. They transport large amounts of water vapor from the tropics to higher latitudes and strike coastlines globally from California to Europe to Antarctica. They can transport moisture equivalent to more than 25 times the amount of liquid water flowing through the mouth of the Mississippi River.

ARs and their intense precipitation can be both beneficial and hazardous, presenting a special challenge to California’s water managers and flood control planners. According to the Scripps Institution of Oceanography (Scripps) at the University of California, San Diego and the National Oceanic and Atmospheric Administration (NOAA), a small number of ARs supply on average between 40% to 50% of California’s precipitation and snowpack annually. Problems occur when AR events last for more than one day or when several occur back-to-back such as those that occurred in January 2023; several ARs in sequence can cause hazards like flooding, mudslides, or ash flows in areas of recent wildfires. NOAA and Scripps research has shown ARs are implicated in most of the extreme precipitation and flooding events in California. Nearly 80% of levee breaches in the Central Valley have been associated with AR events.

FIRO. FIRO is a reservoir operations strategy that, in effect, allows reservoirs to be managed in “real-time” so that operators can keep water in the reservoir if the weather forecast shows no imminent precipitation in a reservoir’s watershed or, alternatively, release water from the reservoir to free up space for additional flow (thereby maintaining flood protection) when the forecast indicates an AR is headed towards a reservoir’s watershed. Enhanced forecasting capability is essential to implementing FIRO. Sonoma Water Agency has piloted and implemented FIRO at Lake Mendocino since 2019 and has increased water stored in that reservoir by nearly 20% while maintaining flood protection. This additional water was immensely helpful in 2021 when the Russian River basin experienced extreme drought conditions.

Among others in support of the bill, the Association of California Water Agencies writes:

SB 599 would increase DWR’s AR forecast capacity which would provide emergency operations with ample lead-time for effective decision-making and allow DWR and water managers across the state to better understand atmospheric rivers and their impacts, more effectively manage water resources, and prepare for natural disasters. These investments will save significant funding over time as the State is better able to manage this limited resource and plan for and avert potential disasters.

Analysis Prepared by: Nikita Koraddi / APPR. / (916) 319-2081

Date of Hearing: July 1, 2025

ASSEMBLY COMMITTEE ON WATER, PARKS, AND WILDLIFE

Diane Papan, Chair

SBPCA Bill Id:SB 599

Author:(Caballero) – As Amended Ver:April 24, 2025

SENATE VOTE: 38-0

SUBJECT: Atmospheric rivers: research: forecasting methods: experimental tools

SUMMARY: Requires the Department of Water Resources (DWR) to research, develop, and implement experimental tools that produce seasonal and subseasonal atmospheric river (AR) forecasts under the Atmospheric Rivers Research and Forecast Improvement Program (Atmospheric Rivers Program). Specifically, this bill:

1. Requires DWR to research, develop, and implement experimental tools that produce seasonal and subseasonal AR forecasts under the Atmospheric Rivers Program.
2. Defines “seasonal forecast” as a forecast that covers approximately one to six months into the future for the purposes of this bill.
3. Defines “subseasonal forecast” as a forecast that covers approximately two to six weeks into the future for purposes of this bill.
4. Makes legislative findings and declarations regarding the positive and negative impacts of ARs and the need for this bill.

EXISTING LAW: Establishes the Atmospheric Rivers Program within DWR and requires DWR, upon appropriation of special funds, to research climate forecasting and the impacts climate change has on ARs. Further, requires DWR to take all actions within its authority to re-operate reservoirs to capture water generated by ARs (Water Code § 347).

FISCAL EFFECT: Unknown. This bill is keyed fiscal.

COMMENTS:

1. Purpose of this bill. According to the author, ARs “are a recently recognized phenomenon that bring narrow bands of water vapor that can transport large quantities of water from tropical regions to California where it falls as rainfall. These ARs have grown in intensity and frequency where they create the potential for flooding, coastal damage and intense winds. [This bill] builds upon [DWR’s] current research operations to advance extended range AR forecasts, to provide emergency operations ample lead-time for effective decision-making. With more accurate data and longer-range certainty, we can better prepare and protect our communities from catastrophic flood and other severe weather events, improve our water management and storage capabilities, and reduce financial devastation caused by flooding.”
2. Background. ARs are long narrow bands of atmospheric water vapor pushed along by strong winds. They transport large amounts of water vapor from the tropics to higher latitudes and strike coastlines globally from California to Europe to Antarctica. They can transport moisture equivalent to more than 25 times the amount of liquid water flowing through the mouth of the Mississippi River.
   
   ARs and their intense precipitation can be both beneficial and hazardous, presenting a special challenge to California’s water managers and flood control planners. According to the Scripps Institution of Oceanography (Scripps) at the University of California, San Diego and the National Oceanic and Atmospheric Administration (NOAA), a small number of ARs supply on average between 40% to 50% of California’s precipitation and snowpack annually. A single AR event can provide up to 30% of annual precipitation in Southern California. Problems occur when AR events last for more than a day or when several occur back-to-back such as occurred in January 2023; several ARs in sequence can cause hazards like flooding, mudslides, or ash flows in areas of recent wildfires. NOAA and Scripps research has shown ARs are implicated in most of the extreme precipitation and flooding events in California. Nearly 80% of levee breaches in the Central Valley have been associated with AR events.
   
   A recent study by researchers at the Center for Western Weather and Water Extremes (CW3E) housed at Scripps examined the February 2017 AR event that contributed to the Oroville crisis and found that “California’s reliance on precipitation from (ARs) is expected to increase as our climate warms.” Similarly, research published in August 2022 (“ARkFuture”) shows how climate change is leading to more extreme AR events. ARkFuture models a 30-day “megastorm” event in which “ARs are the primary storm mode” causing extremely high precipitation and associated runoff that is two to four times higher than historical averages.^[1] Such an event would result in a “megaflood.” Better modeling and forecasting could help to mitigate the risks associated with such an event.
   
   Forecast-Informed Reservoir Operations (FIRO). FIRO is a reservoir operations strategy that, in effect, allows reservoirs to be managed in “real-time” so that operators can keep water in the reservoir if the weather forecast shows no imminent precipitation in a reservoir’s watershed or, alternatively, release water from the reservoir to free up space for additional flow (thereby maintaining flood protection) when the forecast indicates an AR is headed towards a reservoir’s watershed. Enhanced forecasting capability is essential to implementing FIRO. Sonoma Water Agency has piloted and implemented FIRO at Lake Mendocino since 2019 and has been able to increase water stored in that reservoir by nearly 20% while maintaining flood protection. This additional water was immensely helpful in 2021 when the Russian River basin experienced extreme drought conditions.
   
   DWR Atmospheric Rivers Program funding. The Legislature has made previous appropriations to support this research, including $3 million in the 2016-17 Budget, $9.25 million in the 2019-20 Budget, $10 million in the 2021-22 Budget, and $16.75 million in the 2022-23 Budget. The 2024-25 Budget reduced funding by $6.8 million. Previously appropriated funding has allowed DWR to leverage resources and participation from federal and local agencies to deepen the understanding of ARs and implement FIRO at Lake Mendocino. DWR and water agencies are also seeking to implement FIRO in the Feather-Yuba River basin and the Santa Ana River basin.
3. Arguments in support. The Association of California Water Agencies (ACWA) supports this bill and asserts that it will improve weather forecasting to inform decision-making about flood prevention and water management. Further, ACWA maintains that this bill will “increase DWR’s AR forecast capacity which would provide emergency operations with ample lead-time for effective decision-making and allow DWR and water managers across the state to better understand atmospheric rivers and their impacts, more effectively manage water resources, and prepare for natural disasters. These investments will save significant funding over time as the state is better able to manage this limited resource and plan for and avert potential disasters.”
4. Related legislation. AB 30 (Ward), Chapter 134, Statutes of 2023, reconfigures the existing Atmospheric Rivers Program at DWR.

REGISTERED SUPPORT / OPPOSITION:

Support

Association of California Water Agencies
California Groundwater Coalition
County of Fresno
County of Monterey
East Bay Municipal Utility District
Valley Ag Water Coalition

Opposition

None on file

Analysis Prepared by: Pablo Garza / W., P., & W. / (916) 319-2096

1. Xingying Huang and Daniel Swain, “Climate change is increasing the risk of a California megaflood,” Science Advances, 8 (2022): eabq0995, https://www.science.org/doi/10.1126/sciadv.abq0995. ↑

 THIRD READING  

Bill No: SB 599

Author: Caballero (D)

Amended: 4/24/25

Vote: 21

SENATE NATURAL RES. & WATER COMMITTEE: 6-0, 4/22/25

AYES: Limón, Seyarto, Allen, Grove, Laird, Stern

NO VOTE RECORDED: Hurtado

SENATE APPROPRIATIONS COMMITTEE: 6-0, 5/23/25

AYES: Caballero, Seyarto, Cabaldon, Grayson, Richardson, Wahab

NO VOTE RECORDED: Dahle

SUBJECT: Atmospheric rivers: research: forecasting methods: experimental tools

SOURCE: Author

DIGEST:

This bill requires the Department of Water Resources (DWR), when researching, developing, and implementing novel forecasting methods pursuant to a certain atmospheric rivers research program, to also include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts.

ANALYSIS:

Existing law establishes DWR with board jurisdiction over water management including dam safety, drought response and mitigation, water education, flood preparedness, and water supply and storage. (Water Code (Wat. C.) §120 et seq.)

This bill:

1. Defines, for novel forecasting methods researched, developed, and implemented by DWR, requires DWR to include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts.
   
2. Defines “seasonal forecast” as a forecast that covers a period of approximately one to six months into the future.
   
3. Defines “subseasonal forecast” as a forecast that covers a period approximately two to six weeks into the future.
   
4. Makes various findings and declarations about atmospheric rivers, flooding in the state, and benefits of atmospheric river forecasting.

Background

1. An (atmospheric) river runs through it. Atmospheric rivers (ARs) are regions in the atmosphere that stretch as long as thousands of miles and transport relatively elevated concentrations of water. Although many ARs are weak systems that provide a fresh source of water and snow, some of the larger, more powerful ARs can create extreme rainfall and floods capable of causing severe damage to life and property. ARs are approximately 250 – 375 miles wide on average and account for, on average, about 30-50% of annual precipitation in the western states (See https://psl.noaa.gov/arportal/about/; last accessed 4/4/2025).

While ARs are the source of most of California’s water supply, they can also cause most of the state’s flood damage. According to the DWR, about 80% of levee breaches in California’s Central Valley are associated with ARs (See https://water.ca.gov/News/Blog/2019/Feb-19/New-Scale-for-Atmospheric-Rivers-Impact; last accessed 4/4/2025).

California experienced 12 significant ARs between late December 2022 and late March 2023. Additionally, the number of intense ARs is likely to increase as a result of climate change.

1. Forecast-informed reservoir operations (FIRO). FIRO is a flexible water management approach that uses forecasts to help water managers selectively retain or release water from reservoirs. FIRO uses data from watershed monitoring and improved water forecasting to help reservoir managers release water in advance of heavy storms, or to store extra water when the forecast is dry. FIRO helps managers know when and where ARs will occur and how much precipitation they will bring. Managers may opt to keep water in the reservoir if the weather forecast shows no imminent precipitation in the reservoir’s watershed or, alternatively, release water from the reservoir to free up space for additional flow if the forecast indicates an AR is headed towards the watershed. This flexibility helps water managers increase their resilience to droughts and floods.

According to a publication by the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography at UC San Diego (hereafter, CW3E), Better Forecasting of Atmospheric Rivers Mitigates Impact of 2021 Drought, implementing FIRO throughout California could provide more than 150 billion gallons of water annually. CW3E has developed a forecasting model which is able to predict where ARs will make landfall and how much precipitation the ARs will bring up to seven days before the event. According to CW3E’s fact sheet, Atmospheric Rivers Research Powers Forecast Informed Reservoir Operations, the use of FIRO allowed Lake Mendocino to store an additional 20% more water in water year 2020. Additionally, within California, widespread FIRO could save 500,000 acre-feet of water per year.

1. AR Research and FIRO. Knowing when and where ARs will hit, and how much precipitation they will bring, is key to FIRO. According to the CW3E fact sheet, AR Research has improved forecasts by: (1) implementing the AR Reconnaissance (AR Recon) Program; (2) developing a regional weather model, West-WRF, tailored to predicting ARs and extreme precipitation in California; (3) applying forecast skill performance metrics focused on California and FIRO to drive forecast model improvements,; and (4) producing near real-time information supporting California water decisions, including tailored forecast products, models, and observations:
   1. AR Reconnaissance. The AR Recon Program is a targeted field program that was initiated in 2016 to better understand and reduce forecast errors of landfalling ARs by gathering data about the conditions in and around ARs. Led by CW3E, the program utilizes NOAA and U.S. Air Force investments in aircraft-based observations. According to CW3E, the AR Recon Program is now the leading effort within the National Winter Season Operations Plan for airborne weather reconnaissance. Working in coordination with NOAA’s Office of Marine and Aviation Operations and the U.S. Air Force Reserve 53^rd Weather Reconnaissance Squadron, the program conducts data-collecting flights within ARs. It uses dropsondes, which are instruments that collect information related to an AR’s strength, direction, and speed as the instruments drop through the air. The program also uses surface buoys that measure surface pressure and airborne radio occultation, which gathers temperature and moisture data from the sides of an aircraft. On land, weather balloons are released every 3 hours during ARs to capture the evolution of the storm post landfall. Data is collected from the dropsondes flights, surface buoys, and weather balloons and sent to a global database where the data are input into weather prediction models (For additional information, see https://cw3e.ucsd.edu/dec2022-jan2023_arstorymap/).
   2. Subseasonal and seasonal forecasts. A subseasonal forecast is a forecast that covers a period of approximately 2 to 6 weeks into the future. A seasonal forecast is a forecast that covers a period approximately 1 to 6 months into the future. The AR Recon Program has helped provide both types of forecasts and both forecasts can help indicate the life cycle of an AR event.
2. Recent FIRO assessment. A recent report, Final Viability Assessment for Yuba-Feather Forecast Informed Reservoir Operations, evaluated whether FIRO could be implemented at Lake Oroville and New Bullards Bar Reservoir to reduce downstream flood risk without negatively impacting water supplies. The largest FIRO assessment to date, and using “historical forecasts, reservoir storage, and river flow data, scientists found that FIRO, combined with a planned second spillway at New Bullards Bar, could provide additional flood storage capacity at the Yuba-Feather system and reduce downstream peak flows during prolonged storms … .” (https://water.ca.gov/News/News-Releases/2025/Mar-25/Report-Shows-Forecast-Informed-Reservoir-Operations-Can-Increase-Region-Resilience; last accessed 4/14/2025).
   

According to DWR’s website, “FIRO is being implemented successfully at Lake Mendocino in the Russian River watershed in Northern California and will soon be integrated into operations at Prado Dam in the Santa Ana River watershed in Southern California. As the largest and most complex FIRO assessment to date, and the first with the primary goal of reducing flood risk, the Yuba-Feather FIRO Program continues establishing FIRO’s benefit in California as an important part of a comprehensive water management and resilience strategy.”

FISCAL EFFECT: Appropriation: No Fiscal Com.: Yes Local: No

According to the Senate Appropriations Committee, “Ongoing costs of about $5.5 million annually (General Fund) for DWR to expand and update its work on atmospheric rivers (AR) to include experimental tools that produce seasonal (one to six months in the future) and sub-seasonal (two to six weeks into the future) AR forecasts.”

SUPPORT: (Verified 5/23/25)

Association of California Water Agencies
California Farm Bureau Federation
California Groundwater Coalition
California State Association of Counties
County of Fresno
County of Monterey
East Bay Municipal Utility District
Irvine Ranch Water District
Kings River Conservation District
Kings River Water Association
Modesto Irrigation District
Orange County Water District
Santa Clara Valley Water District
Sonoma County Water Agency
Turlock Irrigation District
Upper San Gabriel Valley Municipal Water District
Valley Ag Water Coalition
Western Growers Association

OPPOSITION: (Verified 5/23/25)

None received

ARGUMENTS IN SUPPORT: According to the author, “[a]tmospheric Rivers (AR) are a recently recognized phenomenon that bring narrow bands of water vapor that can transport large quantities of water from tropical regions to California where it falls as rainfall. These ARs have grown in intensity and frequency where they create the potential for flooding, coastal damage and intense winds.”

“SB 599 builds upon the Department of Water Resources’ (DWR) current research operations to advance extended range AR forecasts, to provide emergency operations ample lead-time for effective decision-making. With more accurate data and longer-range certainty, we can better prepare and protect our communities from catastrophic flood and other severe weather events, improve our water

management and storage capabilities and reduce financial devastation caused by flooding.”

Prepared by: Genevieve Wong / N.R. & W. / (916) 651-4116

5/26/25 13:22:30

**** END ****

SENATE COMMITTEE ON APPROPRIATIONS

Senator Anna Caballero, Chair

2025 - 2026 Regular Session

SB 599 (Caballero) - Atmospheric rivers: research: forecasting methods: experimental tools

Bill Summary:

This bill would require the Department of Water Resources (DWR), when researching, developing, and implementing novel forecasting methods pursuant to a certain atmospheric rivers research program, to also include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts.

Fiscal Impact:

• Ongoing costs of about $5.5 million annually (General Fund) for DWR to expand and update its work on atmospheric rivers (AR) to include experimental tools that produce seasonal (one to six months in the future) and sub-seasonal (two to six weeks into the future) AR forecasts.

Background:

ARs are regions in the atmosphere that stretch as long as thousands of miles and transport relatively elevated concentrations of water. Although many ARs are weak systems that provide a fresh source of water and snow, some of the larger, more powerful ARs can create extreme rainfall and floods capable of causing severe damage to life and property. On average, ARs are approximately 250 – 375 miles wide and account for about 30-50% of annual precipitation in the western states. ARs contribute significantly to California’s water supply, but can also cause flood damage. According to the DWR, about 80 percent of levee breaches in California’s Central Valley are associated with ARs.

California experienced 12 significant ARs between late December 2022 and late March 2023. Additionally, the number of intense ARs is likely to increase as a result of climate change.

Forecast-informed reservoir operations (FIRO). FIRO is a flexible water management approach that uses forecasts to help water managers selectively retain or release water from reservoirs. FIRO uses data from watershed monitoring and improved water forecasting to help reservoir managers release water in advance of heavy storms, or to store extra water when the forecast is dry. FIRO helps managers know when and where ARs will occur and how much precipitation they will bring. Managers may opt to keep water in the reservoir if the weather forecast shows no imminent precipitation in the reservoir’s watershed or, alternatively, release water from the reservoir to free up space for additional flow if the forecast indicates an AR is headed towards the watershed. This flexibility helps water managers increase their resilience to droughts and floods.

AR Reconnaissance. The AR Recon Program is a targeted field program that was initiated in 2016 to better understand and reduce forecast errors of landfalling ARs by gathering data about the conditions in and around ARs. Led by CW3E, the program utilizes NOAA and U.S. Air Force investments in aircraft-based observations. According to CW3E, the AR Recon Program is now the leading effort within the National Winter Season Operations Plan for airborne weather reconnaissance. Working in coordination with NOAA’s Office of Marine and Aviation Operations and the U.S. Air Force Reserve 53^rd Weather Reconnaissance Squadron, the program conducts data-collecting flights within ARs. It uses dropsondes, which are instruments that collect information related to an AR’s strength, direction, and speed as the instruments drop through the air. The program also uses surface buoys that measure surface pressure and airborne radio occultation, which gathers temperature and moisture data from the sides of an aircraft. On land, weather balloons are released every 3 hours during ARs to capture the evolution of the storm post landfall. Data is collected from the dropsondes flights, surface buoys, and weather balloons and sent to a global database where the data are input into weather prediction models.

Subseasonal and seasonal forecasts. A subseasonal forecast is a forecast that covers a period of approximately 2 to 6 weeks into the future. A seasonal forecast is a forecast that covers a period approximately 1 to 6 months into the future. The AR Recon Program has helped provide both types of forecasts and both forecasts can help indicate the life cycle of an AR event.

Recent FIRO assessment. A recent report, Final Viability Assessment for Yuba-Feather Forecast Informed Reservoir Operations, evaluated whether FIRO could be implemented at Lake Oroville and New Bullards Bar Reservoir to reduce downstream flood risk without negatively impacting water supplies. The largest FIRO assessment to date, and using “historical forecasts, reservoir storage, and river flow data, scientists found that FIRO, combined with a planned second spillway at New Bullards Bar, could provide additional flood storage capacity at the Yuba-Feather system and reduce downstream peak flows during prolonged storms … .”

According to DWR’s website, “FIRO is being implemented successfully at Lake Mendocino in the Russian River watershed in Northern California and will soon be integrated into operations at Prado Dam in the Santa Ana River watershed in Southern California. As the largest and most complex FIRO assessment to date, and the first with the primary goal of reducing flood risk, the Yuba-Feather FIRO Program continues establishing FIRO’s benefit in California as an important part of a comprehensive water management and resilience strategy.”

Proposed Law:

This bill would require DWR, when researching, developing, and implementing novel forecasting methods pursuant to a certain atmospheric rivers research program, to also include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts. Specifically, it would:

1. For novel forecasting methods researched, developed, and implemented by DWR, require DWR to include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts.
   
2. Define “seasonal forecast” as a forecast that covers a period of approximately one to six months into the future.
   
3. Define “subseasonal forecast” as a forecast that covers a period approximately two to six weeks into the future.
   
4. Make various findings and declarations about atmospheric rivers, flooding in the state, and benefits of atmospheric river forecasting, including that, “Since 2006, more than $4.5 billion in state funding has become available for new flood from bonds and recent budget surpluses. However, investment needs cost estimates range from $50 billion to $115 billion, or approximately $2 billion to $4.5 billion annually for 25 years.”

Related Legislation:

AB 30 (Ward, Chapter 134, Statutes of 2023) renames and reconfigures the Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program as the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AF/FIRO) Program.

SB 277 (Rodriguez, 2023) would have codified the State-Federal Flood Operations Center in DWR and would require DWR and the Office of Emergency Services to report on forecasting and data that would improve flood response. This bill was held on the Suspense File in this committee.

AB 2078 (Flora, 2022) would have renamed and reconfigured the Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program as the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AF/FIRO) Program. This bill was held on the Suspense File in the Assembly Appropriations Committee.

AB 557 (Wood, 2019) would have appropriated $9,250,000 from the General Fund to DWR in the 2019-20 fiscal year to operate the Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program. This bill was held on the Suspense File in the Assembly Appropriations Committee.

Staff Comments:

To assist DWR in implementation of this bill, the author has requested that the 2025-26 Budget include $5.5 million General Fund for the development of extended range AR forecasting.

-- END --

SENATE COMMITTEE ON NATURAL RESOURCES AND WATER

Senator Monique Limón, Chair

2025 - 2026 Regular

Bill No: SB 599 Hearing Date: April 22, 2025

Subject: Atmospheric rivers: research: forecasting methods: experimental tools

SUMMARY

This bill would requires the Department of Water Resources (DWR), when researching, developing, and implementing novel forecasting methods pursuant to a certain atmospheric rivers research program, to also include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts.

BACKGROUND AND EXISTING LAW

An (atmospheric) river runs through it. Atmospheric rivers (ARs) are regions in the atmosphere that stretch as long as thousands of miles and transport relatively elevated concentrations of water. Although many ARs are weak systems that provide a fresh source of water and snow, some of the larger, more powerful ARs can create extreme rainfall and floods capable of causing severe damage to life and property. ARs are approximately 250 – 375 miles wide on average and account for, on average, about 30-50% of annual precipitation in the western states.^[1]

While ARs are the source of most of California’s water supply, they can also cause most of the state’s flood damage. According to the DWR, about 80 percent of levee breaches in California’s Central Valley are associated with ARs.^[2]

California experienced 12 significant ARs between late December 2022 and late March 2023. Additionally, the number of intense ARs is likely to increase as a result of climate change.

Forecast-informed reservoir operations (FIRO). FIRO is a flexible water management approach that uses forecasts to help water managers selectively retain or release water from reservoirs. FIRO uses data from watershed monitoring and improved water forecasting to help reservoir managers release water in advance of heavy storms, or to store extra water when the forecast is dry. FIRO helps managers know when and where ARs will occur and how much precipitation they will bring. Managers may opt to keep water in the reservoir if the weather forecast shows no imminent precipitation in the reservoir’s watershed or, alternatively, release water from the reservoir to free up space for additional flow if the forecast indicates an AR is headed towards the watershed. This flexibility helps water managers increase their resilience to droughts and floods.

According to a publication by the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography at UC San Diego (hereafter, CW3E), Better Forecasting of Atmospheric Rivers Mitigates Impact of 2021 Drought, implementing FIRO throughout California could provide more than 150 billion gallons of water annually. CW3E has developed a forecasting model which is able to predict where ARs will make landfall and how much precipitation the ARs will bring up to seven days before the event. According to CW3E’s fact sheet, Atmospheric Rivers Research Powers Forecast Informed Reservoir Operations, the use of FIRO allowed Lake Mendocino to store an additional 20 percent more water in water year 2020. Additionally, within California, widespread FIRO could save 500,000 acre-feet of water per year.

AR Research and FIRO. Knowing when and where ARs will hit, and how much precipitation they will bring, is key to FIRO. According to the CW3E fact sheet, AR Research has improved forecasts by: (1) implementing the AR Reconnaissance (AR Recon) Program; (2) developing a regional weather model, West-WRF, tailored to predicting ARs and extreme precipitation in California; (3) applying forecast skill performance metrics focused on California and FIRO to drive forecast model improvements,; and (4) producing near real-time information supporting California water decisions, including tailored forecast products, models, and observations.

AR Reconnaissance. The AR Recon Program is a targeted field program that was initiated in 2016 to better understand and reduce forecast errors of landfalling ARs by gathering data about the conditions in and around ARs. Led by CW3E, the program utilizes NOAA and U.S. Air Force investments in aircraft-based observations. According to CW3E, the AR Recon Program is now the leading effort within the National Winter Season Operations Plan for airborne weather reconnaissance. Working in coordination with NOAA’s Office of Marine and Aviation Operations and the U.S. Air Force Reserve 53^rd Weather Reconnaissance Squadron, the program conducts data-collecting flights within ARs. It uses dropsondes, which are instruments that collect information related to an AR’s strength, direction, and speed as the instruments drop through the air. The program also uses surface buoys that measure surface pressure and airborne radio occultation, which gathers temperature and moisture data from the sides of an aircraft. On land, weather balloons are released every 3 hours during ARs to capture the evolution of the storm post landfall. Data is collected from the dropsondes flights, surface buoys, and weather balloons and sent to a global database where the data are input into weather prediction models.^[3]

Subseasonal and seasonal forecasts. A subseasonal forecast is a forecast that covers a period of approximately 2 to 6 weeks into the future. A seasonal forecast is a forecast that covers a period approximately 1 to 6 months into the future. The AR Recon Program has helped provide both types of forecasts and both forecasts can help indicate the life cycle of an AR event.

Recent FIRO assessment. A recent report, Final Viability Assessment for Yuba-Feather Forecast Informed Reservoir Operations, evaluated whether FIRO could be implemented at Lake Oroville and New Bullards Bar Reservoir to reduce downstream flood risk without negatively impacting water supplies. The largest FIRO assessment to date, and using “historical forecasts, reservoir storage, and river flow data, scientists found that FIRO, combined with a planned second spillway at New Bullards Bar, could provide additional flood storage capacity at the Yuba-Feather system and reduce downstream peak flows during prolonged storms … .”^[4]

According to DWR’s website, “FIRO is being implemented successfully at Lake Mendocino in the Russian River watershed in Northern California and will soon be integrated into operations at Prado Dam in the Santa Ana River watershed in Southern California. As the largest and most complex FIRO assessment to date, and the first with the primary goal of reducing flood risk, the Yuba-Feather FIRO Program continues establishing FIRO’s benefit in California as an important part of a comprehensive water management and resilience strategy.”

PROPOSED LAW

This bill would:

1. For novel forecasting methods researched, developed, and implemented by DWR, require DWR to include the use of experimental tools that produce seasonal and subseasonal atmospheric river forecasts.
   
2. Define “seasonal forecast” as a forecast that covers a period of approximately one to six months into the future.
   
3. Define “subseasonal forecast” as a forecast that covers a period approximately two to six weeks into the future.
   
4. Make various findings and declarations about atmospheric rivers, flooding in the state, and benefits of atmospheric river forecasting.

ARGUMENTS IN SUPPORT

According to the author, “[a]tmospheric Rivers (AR) are a recently recognized phenomenon that bring narrow bands of water vapor that can transport large quantities of water from tropical regions to California where it falls as rainfall. These ARs have grown in intensity and frequency where they create the potential for flooding, coastal damage and intense winds.”

“SB 599 builds upon the Department of Water Resources’ (DWR) current research operations to advance extended range AR forecasts, to provide emergency operations ample lead-time for effective decision-making. With more accurate data and longer-range certainty, we can better prepare and protect our communities from catastrophic flood and other severe weather events, improve our water management and storage capabilities and reduce financial devastation caused by flooding.”

Sonoma Water, in support, writes “[t]he AR/FIRO program has already demonstrated success in improving reservoir operations by integrating modern forecasting techniques with reservoir management strategies. SB 599 will build upon this success by enabling DWR to add more forecasting tools to the AR/FIRO toolbox that look at longer lead times. Having better prediction of how many ARs will hit California in an upcoming winter will allow us to know ahead of time if we will experience flooding or drought in the months ahead. In turn, we can then work with our communities to respond to these changing conditions.”

ARGUMENTS IN OPPOSITION

None Received

COMMENTS

Need for bill. Under the existing AR/FIRO Program, DWR currently has the authority to use “experimental tools that produce seasonal and subseasonal atmospheric river forecasts” as those tools could fall within the category of “novel forecasting methods.” However this bill highlights their use by specifically requiring DWR to use these tools to help improve atmospheric river predictions and their impacts on water supply, flooding, post-wildfire debris flows, and environmental conditions. According to information provided by the author’s office, highlighting the use of these tools that provide seasonal and subseasonal forecasts “will (1) extend AR forecasts to provide temperature and precipitation predictions to support hydrologic forecast and water management operations, (2) couple new advances made possible with machine learning and modeling to improve subseasonal and seasonal AR forecasts and validation, (3) provide communications and training for decision-makers and water managers to ensure operational integration of subseasonal or seasonal AR forecast skill, and (4) allow advanced planning to manage and capture excess rainfall during an AR event to prevent flooding.” The usage of more accurate data and long-range certainty will help DWR better prepare for, and avoid, flooding and other severe weather events and improve water management and storage capabilities.

Tying it all together. This bill, by specifically directing DWR to use experimental tools that produce seasonal and subseasonal AR forecasts, would specifically authorize DWR to further collaborate with CW3E and utilize data from CW3E’s AR Recon Program to implement FIRO. Boosting DWR’s implementation of FIRO will help DWR better anticipate AR impacts and aid in the management of reservoirs.

Budget request. To assist DWR in implementation of this bill, the author has requested that the 2025-26 Budget include $5.5 million General Fund for the development of extended range AR forecasting.

Amendments to findings and declarations. Findings and declarations can be an important tool to help demonstrate Legislative intent and provide historical context. Author and committee staff worked together to reorganize and strengthen some of the findings and declarations of this bill. The committee may wish to amend the bill to include these changes. See Amendment 1.

Recent related legislation

AB 30 (Ward, Chapter 134, Statutes of 2023) renames and reconfigures the Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program as the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AF/FIRO) Program.

SB 277 (Rodriguez, 2023) would have codified the State-Federal Flood Operations Center in DWR and would require DWR and the Office of Emergency Services to report on forecasting and data that would improve flood response. This bill was held in the Senate Appropriations Committee on the suspense file.

AB 2078 (Flora, 2022) would have renamed and reconfigured the Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program as the Atmospheric Rivers Research and Forecast Improvement Program: Enabling Climate Adaptation Through Forecast-Informed Reservoir Operations and Hazard Resiliency (AF/FIRO) Program. This bill was held in the Assembly Appropriations Committee on the suspense file.

AB 557 (Wood, 2019) would have appropriated $9,250,000 from the General Fund to DWR in the 2019-20 fiscal year to operate the Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program. This bill was held in the Assembly Appropriations Committee on the suspense file.

SUGGESTED AMENDMENTS

AMENDMENT 1

Amend Section 1 of the bill as follows:

(a) Atmospheric rivers are long narrow bands of concentrated moisture in the atmosphere that form over the Pacific Ocean often originate over tropical oceans and bring sustained, heavy precipitation, especially to the west coast of North America.

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(e) Winter storms from late-December 2022 to mid-January 2023 caused massive flood damage in 40 of California’s 58 counties, resulting in 20 deaths and significant devastation to homes, roads, infrastructure, agriculture, businesses, the economy, and . AcccuWeater estimates the storms likely caused over $30 billion in damages and economic losses, including job and wage losses, crop loss, evacuation costs, airline delays, lost business, and supply-chain interruptions.

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.

.

(j). California spends $1.8 billion annually on flood control operations and maintenance, and $1 billion annually on new investments, but investment needs range from $2 to $4.5 billion, inclusive, annually for the next 25 years.

(k) Since 2006, only more than $4.5 billion in state funding has been allocated to become available for new flood infrastructure from bonds and recent budget surpluses. However, investment needs range from $50 to $115 billion or, put another way, $2 billion to $4.5 billion annually for the next 25 years.

(l) Atmospheric River Reconnaissance (AR Recon), a Research and Operations Partnership with the National Oceanic and Atmospheric Administration’s (NOAA) National Center for Environmental Prediction (NCEP) and led by the Center for Western Weather and Water Extremes, leverages National Oceanic and Atmospheric Administration NOAA and United States Air Force investments in aircraft-based observations to improve predictions for various weather conditions, including atmospheric rivers and their intensity. AR Recon It is now the leading effort within the National Winter Season Operations Plan for airborne weather reconnaissance.

(m) The AR Recon Partnership has provided improved forecasting skill for the Department of Water Resources (DWR) and California water agencies. With the use of AR Recon, statewide Statewide, five-day atmospheric river forecasts in water year 2020 were better than four-day forecasts in 2017.

(n) The Department of Water Resources is Although DWR has already invested in extended forecasts, such as subseasonable (two to six weeks) and seasonal (one to six months) forecasts, it is critical that the Legislature specifically recognize the use of these experimental tools as a key part of the solution to improving predictions about atmospheric river patterns and their impact on our state.

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(q) With minimal investments in the use of tools that help predict seasonal and subseasonal atmospheric river forecasts, California can save billions of dollars in flood damages through advanced warning, effective local planning, and effective emergency management.

(q) (r) With effective seasonal and subseasonal atmospheric river forecasting, California can maximize its efforts to recharge more groundwater and boost our water supply for communities across the state, and protect communities from catastrophic flooding.

SUPPORT

Association of California Water Agencies (ACWA)
California Farm Bureau Federation
California Groundwater Coalition
County of Monterey
East Bay Municipal Utility District
Modesto Irrigation District
Sonoma County Water Agency
Turlock Irrigation District
Upper San Gabriel Valley Municipal Water District
Valley Ag Water Coalition

OPPOSITION

None Received

-- END --

1. See https://psl.noaa.gov/arportal/about/; last accessed 4/4/2025 ↑

2. See https://water.ca.gov/News/Blog/2019/Feb-19/New-Scale-for-Atmospheric-Rivers-Impact; last accessed 4/4/2025 ↑

3. For additional information, see https://cw3e.ucsd.edu/dec2022-jan2023_arstorymap/ ↑

4. https://water.ca.gov/News/News-Releases/2025/Mar-25/Report-Shows-Forecast-Informed-Reservoir-Operations-Can-Increase-Region-Resilience; last accessed 4/14/2025. ↑