California atmospheric river storms: racing the sky rivers before they hit
California atmospheric river storms are no longer obscure science jargon – they are the giant, skyborne “rivers” now shaping winters, water supplies and mega-flood risks across the US West. As a new atmospheric river bears down on Southern California, a small fleet of hurricane-hunter planes and a network of scientists are literally flying into the heart of these storms, trying to buy communities crucial days of warning.
High over the Pacific, US Air Force crews trace huge racetrack patterns across open ocean, dropping sensors into invisible plumes of water vapor thousands of kilometers long. Those measurements are piped straight into forecast models, fine-tuning where and when the next atmospheric river will slam into the coast – and how dangerous it will be.
In this long read, we’ll explore how California atmospheric river storms form, why they cause more than a billion dollars a year in flood damage in the western US, how they can also end droughts, and what it feels like to chase them from the cockpit of a turboprop aircraft in the middle of the night. UC San Diego Today+1
What are atmospheric rivers – and why California depends on them
Sky rivers carrying more water than the Mississippi
An atmospheric river is essentially a long, narrow corridor of extremely moist air – a “river in the sky” that can transport more water than the Mississippi River, but in vapor form. noaa.gov+1
Most of the time, these plumes form over the warm Pacific, often tapping tropical moisture near Hawaii (sometimes called “Pineapple Express” events), then are steered by the jet stream toward the west coast of North America. The Guardian+1
As these moist air streams slam into the coastal mountains of California and the Pacific Northwest, they are forced upward. The air cools, the water condenses, and the result is intense, long-duration rain or heavy mountain snow.
For California:
- A handful of atmospheric rivers each winter can deliver up to 50% of the state’s annual rain and snow in just a few days. noaa.gov+1
- Those same storms are responsible for most of the biggest flood disasters in the region. Research suggests atmospheric rivers account for roughly 84% of flood damage in the western United States, with average losses exceeding $1 billion per year. Nature+1
California atmospheric river storms, in other words, are both lifeline and threat: they refill reservoirs and snowpack – and they also trigger landslides, blow out levees and send rivers over their banks.
Inside AR Recon: the mission to chase California atmospheric river storms
A second “storm season” for hurricane hunters
For pilots like Capt Nate Wordal of the US Air Force 53rd Weather Reconnaissance Squadron, atmospheric rivers are now a second storm season.
From May to November, his unit famously hunts Atlantic hurricanes, punching through eyewalls to collect data that improves tropical cyclone forecasts. Once that season winds down, another begins. From about November to March, many of those same aircraft and crews pivot to the Pacific to hunt California atmospheric river storms.
The mission is called Atmospheric River Reconnaissance, or AR Recon. It’s a partnership between:
- The Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography
- The US National Oceanic and Atmospheric Administration (NOAA)
- The US Air Force Reserve 53rd Weather Reconnaissance Squadron
- Additional partners such as the National Weather Service and the European Centre for Medium-Range Weather Forecasts (ECMWF) cw3e.ucsd.edu+2cw3e.ucsd.edu+2
The goal: collect targeted observations in the northeast Pacific to sharpen forecasts of when and where atmospheric rivers will hit, and how intense they will be. ADS+1
Why they fly from Japan, Hawaii and the US West Coast
Traditionally, AR Recon flights launched from Hawaii and the US West Coast. But as the program expanded, scientists realized they could gain even more lead time by sampling storms earlier in their life cycle – as they were just organizing west of the dateline.
That’s why, in recent seasons, some missions have taken off from Yokota Air Base in Japan, flying out into the western and central Pacific to intercept the “seed” atmospheric rivers that will, days later, become the California atmospheric river storms hitting Los Angeles, San Francisco or the Sierra Nevada.
Because weather systems in the mid-latitudes generally move west to east, the farther upstream you measure a storm, the sooner you can refine the forecast for downstream communities. cw3e.ucsd.edu+2cw3e.ucsd.edu+2
In practice, that means:
- Flights in the western Pacific can improve forecasts 8–10 days before landfall.
- Flights closer to the West Coast still give a valuable 3–5 days of high-confidence warning. ADS+1
To create this upstream “net,” AR Recon now uses a mix of:
- WC-130J Hurricane Hunter aircraft from the Air Force
- Gulfstream-IV jets and other platforms operated by NOAA
- Collaborating aircraft and observing systems, such as drifting ocean buoys, that monitor wave and water conditions along the storms’ paths cw3e.ucsd.edu+1
How you measure a river in the sky
Dropsondes: tiny weather labs tossed from a plane
The basic tool of AR Recon is the dropsonde – a small, cylindrical instrument loaded with sensors.
On a typical flight chasing California atmospheric river storms:
- The crew flies repeated “legs” across the moisture plume, following a mission plan designed by meteorologists on the ground.
- At precisely chosen points along these legs, loadmasters release dropsondes through a tube in the plane’s belly.
- As each sondes falls, it transmits back a continuous profile of:
- Temperature
- Pressure
- Humidity
- Wind speed and direction
This data gives forecasters a 3D X-ray of the storm’s structure, especially the core of intense moisture transport that defines an atmospheric river. cw3e.ucsd.edu+2ADS+2
The flights from Japan and over the central Pacific also deploy drifting buoys – roughly “washing-machine sized” floats that measure waves and sea-surface temperatures, adding vital ocean data to the picture. NAWDIC+1
From cockpit to forecast model in near real time
What makes AR Recon powerful is speed. Data from the dropsondes is processed on the plane and transmitted to global forecast centers in near real time. There, it is assimilated into sophisticated numerical weather prediction models.
Multiple studies have shown that these additional observations significantly reduce errors in the forecast track, timing and intensity of atmospheric rivers hitting the West Coast. ADS+2American Meteorological Society Journals+2
That means:
- Better rainfall and snowfall predictions days in advance
- More accurate estimates of peak river flows and flood risk
- Clearer guidance on whether a particular event will be a weak, beneficial storm or a major, high-impact California atmospheric river storm
For emergency managers and reservoir operators, that difference can shape everything from evacuation orders to how much water to release or store.
What it’s like to fly into California atmospheric river storms
Giant racetracks over the Pacific
From the pilots’ perspective, AR Recon missions feel very different from hurricane flights.
In a hurricane, crews spend hours flying through extreme turbulence, intense bands of rain, lightning and updrafts that can jolt the aircraft around. Some pilots compare it to driving through a carwash in a wind tunnel – loud, chaotic and relentlessly bumpy.
Atmospheric rivers, by contrast, are often smoother at long range. Over the open Pacific, far from land, they can be more like broad highways of moisture. Pilots describe “giant race tracks across the Pacific Ocean,” with long, steady legs through high clouds and diffuse precipitation, punctuated by the regular task of dropping instruments.
But the calm doesn’t always last. As California atmospheric river storms approach the West Coast, they frequently interact with developing low-pressure systems, coastal mountains and colder air masses, morphing into intense storm complexes. That’s when turbulence, strong crosswinds and heavy rain can make landings at coastal airfields much more challenging.
On a typical 8–10 hour mission:
- Two or three pilots rotate duties to manage fatigue.
- Navigators work closely with meteorologists to ensure that the plane slices through the most scientifically valuable parts of the storm while avoiding aircraft hazards like volcanic ash or dense ship traffic.
- Loadmasters are constantly prepping the next dropsonde, checking the chute, and logging instrument IDs.
- Weather officers sit at consoles in the back, monitoring incoming data and communicating with forecast centers and mission scientists on the ground.
Everyone has to stay mentally sharp for the entire flight. If a dropsonde is released in the wrong spot, or data fails to transmit, an entire segment of the storm may go unsampled. In many cases, these crews are the only eyes and instruments inside vast stretches of the Pacific atmosphere. cw3e.ucsd.edu+2ECMWF+2
When atmospheric rivers turned California from drought to deluge
The 2022–2023 parade of storms
California’s recent history shows just how extreme California atmospheric river storms can be.
In the winter of 2022–2023, a relentless parade of atmospheric rivers – nine landfalling events in just three weeks – pummeled the state. American Meteorological Society Journals+1
Impacts included:
- More than 20 deaths and billions of dollars in damage
- Widespread power outages affecting hundreds of thousands of customers
- Levee failures, landslides and sinkholes
- Major road and rail closures as mudslides and flooding cut key routes
- Emergency declarations across dozens of counties Wikipedia+2Guy Carpenter+2
Sensors later showed that parts of California received over a year’s worth of rainfall in just a few weeks. Some mountain locations recorded several meters of snow. Reservoirs that had been worryingly low after years of drought surged toward capacity. Wikipedia+1
These storms offered a stark demonstration of a central paradox: California atmospheric river storms can both end droughts and trigger disasters – sometimes in the same month.
Why clustered storms are especially dangerous
Research from Stanford and other institutions has shown that clusters of atmospheric rivers – multiple storms hitting within short intervals – are far more damaging than isolated events. Soils become saturated, snowpack adds extra meltwater, and each new storm has a smaller capacity for absorption before everything turns to runoff. American Meteorological Society Journals+1
In 2023 and 2024, California saw exactly this pattern, with rapid-fire atmospheric rivers causing:
- Flash flooding in urban areas
- Mudslides in burn scars left by wildfires
- Coastal erosion and high surf along exposed shorelines
- Repeated power outages from downed trees and power lines AP News+2The Guardian+2
AR Recon was in the air for many of those storms, helping forecasters track which atmospheric rivers would stall over small basins, which would slide quickly down the coast, and which would dump their heaviest rainfall over already-saturated watersheds. cw3e.ucsd.edu+1
California atmospheric river storms and climate change
Warmer air means more water in the sky
Basic physics says warmer air can hold more moisture. In a warming climate, that means the potential for stronger atmospheric rivers – and heavier downpours when they make landfall.
Multiple studies now suggest that:
- Atmospheric rivers are projected to become more intense and more frequent as global temperatures rise.
- In climate model simulations, flood damages associated with atmospheric rivers in the western US increase sharply in warmer scenarios – potentially tripling average annual flood damages by around 3°C of warming. Nature+2ScienceDirect+2
In real-world terms, that could mean:
- More multi-billion-dollar winters like 2022–2023
- Higher spillway and levee stresses on aging infrastructure
- Greater landslide risk in steep, fire-scarred terrain
- More emergency declarations and costly recovery efforts
At the same time, California depends heavily on these storms for water. A warmer climate may bring more precipitation as rain instead of snow, changing when and how water is stored in mountain snowpack and released into rivers.
AR scales and new tools to judge risk
Because of their importance, scientists have even developed an Atmospheric River Scale, somewhat analogous to hurricane categories. The scale rates events from AR1 (primarily beneficial) to AR5 (extreme and primarily hazardous), combining both intensity and duration. cw3e.ucsd.edu+1
AR Recon data helps forecasters refine these ratings in real time. If dropsondes show a particularly strong plume of moisture and strong winds in the lower atmosphere, that can bump an event up the scale – prompting more urgent warnings and preparations for a potential AR4 or AR5 California atmospheric river storm.
Forecast lead time: why a few extra days can save lives
From five days to a week or more
Before AR Recon, many landfalling atmospheric rivers were poorly sampled over the Pacific. Forecast models had to infer their structure from sparse satellite and ship data.
By systematically flying into these storms with dropsondes, AR Recon has:
- Reduced key forecast errors in landfall position and timing
- Sharpened estimates of rainfall totals and wind speeds
- Enabled earlier and more confident warnings, often five days before the worst impacts hit ADS+1
By extending flights upstream into the western Pacific, scientists hope to improve forecasts eight to ten days ahead of landfall for some events – a game changer for California atmospheric river storms that can trigger large-scale evacuations. cw3e.ucsd.edu+2cw3e.ucsd.edu+2
A success story: avoiding tragedy through preparation
In one notable case earlier in the AR Recon program, flights over the Pacific sampled an atmospheric river that models initially suggested would be modest. Once the dropsonde data were ingested, forecasts abruptly shifted: the event looked far stronger, with a clear threat of severe flooding and debris flows along parts of the California coast. ADS+1
Armed with this updated information, emergency managers:
- Issued stronger flood and landslide warnings
- Closed vulnerable roads
- Ordered strategic evacuations in high-risk neighborhoods
The storm did indeed cause serious damage – but crucially, there was no loss of life in several of the communities that had been able to prepare. For many scientists, it was a proof-of-concept moment for AR Recon: better upstream data can translate directly into lives saved.
Floods vs. drought: the reservoir balancing act
Why water managers care so much about AR forecasts
No place feels the paradox of California atmospheric river storms more acutely than the state’s reservoirs.
On one hand, large dams and reservoirs must maintain flood control space – empty capacity to absorb incoming storm flows. On the other hand, they must store water for summer irrigation, drinking water and ecosystem needs in an increasingly volatile climate.
The dilemma looks like this:
- If managers release too much water ahead of a predicted big storm, and the storm underdelivers, reservoirs may end the winter too low – worsening drought risk.
- If they don’t release enough, and the storm arrives stronger than expected, the system can become overwhelmed, increasing downstream flood risk and forcing emergency spillway use.
High-confidence forecasts from AR Recon can tip the balance, giving operators enough information to say, “We have strong evidence that a major AR4 event will arrive in three days,” and to make more aggressive but justified pre-releases. cw3e.ucsd.edu+2American Meteorological Society Journals+2
From rule curves to “forecast-informed” operations
California has started experimenting with Forecast-Informed Reservoir Operations (FIRO) – a strategy that explicitly uses advanced forecasts of atmospheric rivers to adjust reservoir rules in real time. cw3e.ucsd.edu+1
In FIRO pilot projects:
- Reservoirs may be allowed to temporarily store more water than traditional rules permit if forecasts show low storm risk.
- Conversely, if AR Recon and model guidance suggest a strong California atmospheric river storm is on the way, operators can make early, controlled releases to create space – rather than waiting for emergency spill.
These experiments have already shown promise at several key reservoirs, helping reduce flood risk while also boosting water supply reliability in recent wet winters. cw3e.ucsd.edu+2Guy Carpenter+2
Life on the ground when atmospheric rivers hit
Mudslides, burn scars and urban flood hotspots
The impacts of California atmospheric river storms are not distributed evenly.
Certain places are especially vulnerable:
- Burn scars from recent wildfires, where vegetation has been stripped away and soils repel water, are prone to dangerous debris flows even in fairly moderate storms.
- Steep coastal and canyon slopes can fail quickly when saturated, sending mud and rocks onto roads and neighborhoods below.
- Low-lying urban neighborhoods with aging drainage systems can see rapid street flooding when intense rainfall overwhelms storm drains.
- River-adjacent communities and farm fields in the Central Valley face levee overtopping or failures when multiple storms pile up runoff. AP News+2Wikipedia+2
Recent events have seen:
- Evacuations across parts of Santa Barbara, Ventura and Los Angeles counties
- Sinkholes opening on city streets
- Temporary closures of major freeways like US-101 and I-5
- Airport shutdowns due to runway flooding and high winds AP News+2Reuters+2
The human toll and hidden stresses
Beyond the visible damage, there is a quieter human cost:
- Families evacuated multiple times over a few winters as new California atmospheric river storms trigger repeat flood and debris flow threats
- Farm workers and outdoor laborers facing lost income when fields are too wet or roads are impassable
- Small coastal towns seeing waves literally bite into protective dunes and bluffs, threatening homes that once seemed safely above the tide line
In interviews after recent storms, many residents describe the mental strain of constantly shifting from drought anxiety (dry wells, empty reservoirs, wildfire smoke) to flood anxiety (sandbags, evacuation routes, insurance claims) as back-to-back atmospheric rivers whip the state between extremes. Council on Foreign Relations+1
It’s precisely that new rhythm of extremes that makes investments in AR Recon, better forecasts and smarter land-use planning so urgent.
The future of California atmospheric river storm forecasting
Expanding the observation network
The AR Recon program continues to evolve. Recent developments include:
- More sophisticated mission planning systems that use ensemble forecasts to identify the most “sensitive” parts of the storm – the locations where new data will most improve model accuracy. American Meteorological Society Journals+1
- Increased international collaboration, with European and Asian partners helping coordinate upstream observations and data sharing across the Pacific basin. NAWDIC+1
- Ongoing workshops at CW3E in La Jolla to bring together pilots, forecasters, modelers and water managers to refine priorities. cw3e.ucsd.edu+1
Scientists are also exploring how machine learning and improved climate models can help anticipate seasons where California atmospheric river storms are likely to be unusually frequent or strong – months in advance – by looking at large-scale patterns such as El Niño and La Niña. cw3e.ucsd.edu+1
From science experiment to everyday protection
When AR Recon began nearly a decade ago, it was essentially a scientific experiment: could targeted observations over the northeast Pacific meaningfully improve landfall forecasts?
Now, with multiple winters of real-world results, it has become part of the operational toolkit for West Coast forecasting and water management. cw3e.ucsd.edu+2ADS+2
As California atmospheric river storms continue to test the resilience of infrastructure and communities, that combination of high-tech flying laboratories, advanced models and on-the-ground preparedness may be one of the state’s best defenses.
The sky rivers will keep coming. The question is how ready we are when they arrive.
Key takeaways about California atmospheric river storms
- Essential but dangerous: California atmospheric river storms provide a huge share of the state’s annual water, but also cause most of its largest flood disasters. Nature+1
- AR Recon matters: By flying directly into these storms, AR Recon improves forecasts days to more than a week ahead, allowing better planning by emergency managers and reservoir operators. cw3e.ucsd.edu+2ADS+2
- Climate change is amplifying risk: Warmer air holds more moisture, making future atmospheric rivers likely to be wetter and more damaging on average. Nature+2ScienceDirect+2
- Water management is evolving: Forecast-informed reservoir operations and updated AR scales are helping California squeeze more benefit and less harm out of each event. cw3e.ucsd.edu+2American Meteorological Society Journals+2
- Communities are on the front line: From burn-scarred hillsides to low-lying neighborhoods, people living under the flight paths of AR Recon are the ones whose lives and livelihoods depend most on its success. Wikipedia+2Guy Carpenter+2
External sources for further reading
- NOAA – “What are atmospheric rivers?” noaa.gov
- NOAA Climate / ENSO Blog – “When rivers reach the sky” Climate.gov
- Scripps Institution of Oceanography / CW3E – Atmospheric River Reconnaissance overview and program pages cw3e.ucsd.edu+2cw3e.ucsd.edu+2
- UC San Diego News – “Atmospheric river storms create $1 billion-a-year flood damage” UC San Diego Today+1
- Nature Scientific Reports – “Climate change contributions to future atmospheric river flood damages” Nature+1
- American Meteorological Society (BAMS) – studies on AR Recon and California’s 2022–2023 atmospheric river sequence American Meteorological Society Journals+2ADS+2
- Wikipedia – “2022–2023 California floods” (summary of impacts and timeline) Wikipedia
- Council on Foreign Relations – “How big a climate threat are atmospheric rivers?” Council on Foreign Relations
- Stanford University News – “Clusters of atmospheric rivers amp up California storm damages” Stanford News
- USDA Climate Hubs – “Atmospheric rivers in the Northwest”
