On December 31, 2022, an atmospheric river dropped about 5.5 inches of rain on downtown San Francisco, one of the wettest days in its history. Streets and freeway ramps flooded across the region, and mudslides closed roads in the hills. It was the first in a string of storms that hit California through mid-January 2023. None of this was unusual in the long view, because the Bay Area has faced storms like these for as long as records exist. The worst of them came in the winter of 1861-62, when weeks of atmospheric rivers caused the Great Flood of 1862. Atmospheric rivers are one of the most important things to understand about living in this region.
How Do Atmospheric Rivers Actually Hit the Bay Area?
An atmospheric river is a long, narrow band of concentrated water vapor moving through the lower atmosphere. Calling it a river is not much of an exaggeration. According to NOAA, an average one carries about as much water vapor as the Mississippi River carries water at its mouth, and the strongest carry many times more. They draw their moisture from warm ocean water in the tropics or subtropics, and the jet stream steers that moisture across the Pacific toward California.
The Bay Area's terrain makes these storms especially potent. When an atmospheric river comes ashore, it runs into the coastal hills and then the Sierra Nevada. The moist air is forced to rise, and rising air cools. Cooler air cannot hold as much vapor, so the moisture falls as rain, sometimes half an inch an hour or more. The hills of Marin and the Santa Cruz Mountains concentrate the rain. A storm that drops 3 inches on downtown San Francisco can drop 6 to 8 inches on the ridges above Woodside or Fairfax over the same hours.
Which Parts of the Bay Are Most at Risk?
Flood risk in the Bay Area is very local, and once you understand the terrain it is fairly predictable. Flat land at the mouths of creeks is the classic danger zone. Heavy rain upstream rushes down into low ground, where creeks and storm drains cannot carry the water away fast enough. Alameda Creek, which drains a large part of the Diablo Range before reaching the bay near Fremont, has a long flood history. So do San Mateo Creek and the smaller creeks that run through Oakland and Berkeley.
The bay shoreline is a second kind of risk. When heavy rain arrives during a high tide, creeks cannot drain into the bay, and water backs up into low neighborhoods. South Bay areas around Alviso, along with East Palo Alto, have flooded this way. In February 1998, San Francisquito Creek overflowed into East Palo Alto and Palo Alto during an El Niño storm.
Hillside neighborhoods face a different danger from landslides and debris flows. The Oakland hills, the ridges of Marin and the Santa Cruz Mountains become prone to them once long rains soak the soil. The most dangerous setup is a big atmospheric river that follows several smaller storms, because the ground is already wet when the heavy rain arrives. The USGS and the state publish landslide hazard maps that are worth checking if you live in one of these areas.
How Much Rain Falls During a Strong Event?
Rain totals from a strong atmospheric river can surprise people who did not grow up in California. The storms are rated on a five-category scale created at UC San Diego's Center for Western Weather and Water Extremes. The scale weighs how much moisture a storm carries and how long it lasts. A Category 3 storm can bring several inches of rain to Bay Area stations over a day or two. The rare Category 4 and 5 storms can bring much more, especially in the coastal mountains. The October 2021 Category 5 storm gave downtown San Francisco about 4 inches in a single day, its wettest October day on record.
How fast the rain falls matters as much as how much falls. Five inches spread over three days gives drainage systems time to carry the water away. Rain that heavy overwhelms them, as storm drains back up and creeks rise faster than warnings can go out. Soaked ground sheds the water instead of absorbing it, which adds to the runoff. Storms that arrive in back-to-back pulses are especially dangerous, because the ground has no time to recover between rounds.
How the Jet Stream Determines Who Gets Hit
Not every atmospheric river hits the Bay Area. The jet stream, the fast river of air high overhead that steers storms across the Pacific, decides where each one comes ashore. When it aims moisture at Central California, the Bay Area gets drenched. When it sits farther north, the rain falls on Oregon and Washington while California stays dry, and when it sits farther south, Southern California takes the hit.
That steering is why California winters vary so much. A winter in which the jet stream keeps aiming at Northern California brings flooding and full reservoirs, while one in which it stays too far north brings drought. Forecasters can now spot many atmospheric rivers about a week ahead, which gives communities real warning. Those forecasts have improved a great deal over the past two decades, thanks largely to research on California's storms.
Climate Change and the Future of Bay Area Flooding
The link between climate change and atmospheric rivers starts with simple physics. For every 1°C of warming, the air can hold about 7 percent more water vapor. As the oceans and air warm, atmospheric rivers are expected to carry more water than they did a few decades ago. A Category 3 storm today may bring heavier rain than a similar storm would have in 1990.
Researchers also expect California's rain to come in fewer, more intense storms with longer dry spells between them. This is sometimes called “weather whiplash,” as the state swings more sharply between drought and flood. That is a real challenge for Bay Area flood control systems built to past standards. Many storm drains, creek channels and detention basins were sized for storms that may no longer represent the worst to come.
The Flip Side: Atmospheric Rivers Fill Our Reservoirs
It would be easy to see atmospheric rivers only as a threat, but they are also how California refills its water supply. Hetch Hetchy Reservoir, which supplies much of San Francisco's water, fills mostly from Sierra snowmelt, and much of that snowpack is built by atmospheric river storms. Del Valle Reservoir in the East Bay, San Luis Reservoir and Lake Shasta all depend on the same storms that cause flooding when they come too fast.
The 2012 to 2016 drought was one of the most severe in California's recorded history. It ended largely because a series of atmospheric rivers in early 2017 piled up snow and rain across the state. The same storms that led to the February 2017 Oroville Dam spillway crisis also brought state water storage back from crisis levels within weeks. Water managers are always balancing between too little rain, which means drought, and too much at once, which means flooding. The ideal winter brings a series of moderate storms spread across the season.
Deep Dive: How Does Rain Turn Into a Flood?
The same amount of rain can be harmless in one storm and destructive in the next. The difference is often the ground. Dry soil soaks up much of the early rain, while saturated soil sends nearly all of it straight into creeks and rivers.
Why Does Wet Ground Matter So Much?
Soil works like a sponge with a limited capacity. Early in the rainy season, the hills can absorb an inch or two of rain with little runoff. After several storms, the spaces in the soil fill with water, and new rain has nowhere to go but downhill.
By our calculation, 4 inches of rain on a single square mile adds up to about 213 acre-feet of water. That is enough to cover about 160 football fields a foot deep. If the soil is already full, most of that reaches the creeks within hours. Cities respond even faster, because pavement and roofs absorb almost nothing. Across a watershed of hundreds of square miles, the volume becomes enormous.
That is why forecasters and emergency managers watch storm sequences as closely as single storms. A moderate atmospheric river arriving after a wet week can cause more flooding than a stronger one that falls on dry ground.
What Happened on the Russian River in 2019?
The flood of February 2019 shows the pattern. An atmospheric river stalled over Sonoma County after a wet winter, dropping heavy rain on ground that was already saturated. According to NASA's Earth Observatory, the Russian River crested at about 45.4 feet in Guerneville, its highest level since 1995.
The National Weather Service reported that Guerneville and Monte Rio were hit hardest, with about 2,000 properties damaged. For a time, floodwater cut off every road into Guerneville, and news reports described the town as an island.
The Russian River is especially prone to this because it drains steep, wet coastal mountains into a narrow valley. Bay Area creeks follow the same rules on a smaller scale. A watershed that has already absorbed several storms can turn the next atmospheric river into a flood within hours.
What to Do Before, During, and After an AR Event
Preparing for an atmospheric river is not complicated, but it needs to happen before the storm is on the way. Start by learning your risk from FEMA's flood maps or your county's flood hazard maps, which show whether your home is in a flood zone. If it is, know your evacuation route and have 72 hours of supplies ready before the storm season starts in November.
Before a big storm, clear gutters and downspouts and move cars away from low spots. Check that sump pumps work, and secure anything in the yard that strong wind could throw. Sign up for your county's emergency alerts, which most Bay Area counties send by text and email. During the storm, stay off flooded roads, because six inches of fast-moving water can knock a person down, and two feet can float most cars. Afterward, stay away from swollen creeks, watch for signs of slides on slopes above you and report downed power lines to PG&E rather than going near them.
Atmospheric rivers will keep coming to the Bay Area. They are the region's defining kind of rainstorm, as much a part of its climate as summer fog or fall heat waves. What changes is how prepared each resident is when the next one arrives. The forecasts are good and the warning time is real, so the only question is whether people use it.
