From late December 2022 to mid-January 2023, a string of storms hammered California, flooding roads, toppling trees and knocking out power to hundreds of thousands of customers. On December 31, 2022, downtown San Francisco recorded about 5.5 inches of rain, one of the wettest days in its history. In February 2017, storms filled Lake Oroville so fast that damage to its spillways forced nearly 200,000 people downstream to evacuate. All of these were atmospheric rivers, and understanding them is one of the most useful pieces of weather knowledge a Bay Area resident can have.
What Is an Atmospheric River?
An atmospheric river is a long, narrow band of concentrated water vapor moving through the sky, much like a river moving water across land. These bands are typically 250 to 375 miles wide and can stretch more than 1,000 miles. According to NOAA, an average atmospheric river carries about as much water vapor as the Mississippi River carries water at its mouth. The strongest storms can carry many times that amount. When that moist air reaches a coastline and is forced to rise, it can release rain or snow at remarkable rates.
The term “atmospheric river” came from researchers at MIT in the 1990s and is now standard in weather forecasting. The storms themselves have always existed, but scientists lacked a precise name for them. Atmospheric rivers supply a large share of California's yearly precipitation, often 30 to 50 percent, and even more in some coastal mountains. A wet California winter usually includes several well-aimed atmospheric rivers. A drought year is often one in which they miss the state or arrive too weak to make up the shortfall.
How They Form
Atmospheric rivers form when winds ahead of a Pacific storm pull large amounts of tropical or subtropical moisture into a narrow stream. Warm ocean water is the key ingredient, because it evaporates readily and loads the air with water vapor. As high-altitude winds steer storms toward the coast, they can tap that moisture and channel it along a concentrated path. The stream of vapor can flow thousands of miles and hold together until it runs into mountains.
When an atmospheric river hits a mountain range, a process called orographic lift takes over. The moist air is forced upward, and as it rises it cools, and cooler air cannot hold as much vapor. The extra moisture condenses and falls as rain or snow, sometimes at very high rates. The Northern California coast ranges and the Sierra Nevada act as huge rain makers when these storms arrive from the right direction. Cazadero, in the coastal hills of Sonoma County, averages more than 60 inches of rain a year, and much of it comes from a handful of atmospheric rivers.
The Pineapple Express
The Pineapple Express is the best-known kind of atmospheric river to reach the Bay Area. Its moisture comes from the warm subtropical Pacific near Hawaii, roughly 15 to 25 degrees north of the equator. A Pineapple Express forms when that moisture is pulled into a storm heading for California. The air arrives carrying tropical water vapor gathered over thousands of miles of warm ocean.
Pineapple Express storms stand out for their warmth as well as their moisture. Because the air comes from the subtropics, the freezing level can rise to 8,000 or 10,000 feet in the Sierra. Precipitation that would normally fall as snow falls as rain instead. Warm rain on an existing snowpack can produce far more runoff than the rain alone would suggest. That combination fed the storms behind the Oroville crisis in 2017. Most arrive between November and March, and they can drop 5 to 10 inches of rain on the Bay Area's coastal mountains in one to three days.
Bay Area Flood Risk
The Bay Area's flood risk from atmospheric rivers is real, and it is concentrated in particular places. Flat land near creek mouths and the bay shoreline is most at risk, because heavy rain upstream arrives just as high bay water keeps creeks from draining. Alameda Creek in Fremont, San Mateo Creek and the small creeks running through Oakland and Berkeley to the bay all have histories of flooding in major storms. The most destructive run of atmospheric rivers in California's recorded history caused the Great Flood of 1862, which turned the Central Valley into a lake.
In San Francisco, the main risk is storm drains that cannot keep up with sudden downpours. Low spots such as the area around 17th and Folsom streets in the Mission have flooded repeatedly in heavy storms. Along the bay shoreline, low areas from the waterfront to the South Bay flats can flood when storm surge and heavy rain arrive together. Hillside neighborhoods face a different danger, because soaked soil on steep slopes can give way in landslides. That is why Marin County and the Oakland hills often see landslide warnings during long wet spells.
The Climate Change Connection
Atmospheric rivers are getting wetter as the climate warms. For every 1°C of warming, the air can hold about 7 percent more water vapor. Warmer oceans also evaporate more readily and load passing storms with more moisture. As a result, the biggest atmospheric rivers, the ones that already push Bay Area flood control systems to their limits, are expected to grow more intense.
At the same time, the dry spells between storms may be getting drier, which would mean sharper swings between drought and flood. California is already prone to those swings, because its Mediterranean climate packs most of the year's rain into a short winter. Climate projections suggest California's future rain will come in fewer, more intense storms with longer dry gaps between them. That is a serious planning challenge for Bay Area infrastructure built around past rainfall patterns.
Deep Dive: How Do Scientists Measure an Atmospheric River?
Scientists measure an atmospheric river by how much water vapor it moves and for how long. The key number is called integrated vapor transport, or IVT. It combines how much moisture the air holds with how fast the wind is carrying it, added up from the ground to the upper atmosphere.
What Does IVT Actually Measure?
Picture a curtain one meter wide stretched from the ground to the top of the atmosphere. IVT is the mass of water vapor flowing through that curtain each second, in kilograms per meter per second. An atmospheric river is usually defined as a long, narrow band where IVT reaches at least 250 kilograms per meter per second.
By our calculation, a strong atmospheric river at 750 kilograms per meter per second moves about 2.7 million kilograms of vapor through each meter of coastline every hour. Over a band 300 miles wide, that is roughly 1.3 trillion kilograms an hour. Only part of it falls as rain, but even a small share is a great deal of water.
How Does the Category Scale Work?
The scale on this page comes from Marty Ralph of UC San Diego's Center for Western Weather and Water Extremes and colleagues. They published it in 2019 in the Bulletin of the American Meteorological Society. It rates an atmospheric river at one location by two things, its peak IVT averaged over three hours and how long IVT stays above 250.
The starting category depends on peak strength. A peak of 250 to 500 starts at Category 1, and 500 to 750 starts at Category 2. Each further step of 250 adds a category, up to Category 5 above 1,250. Duration then adjusts the rating. A storm lasting less than 24 hours drops one category, and one lasting more than 48 hours rises one.
Duration matters because a long storm keeps dropping rain on the same hills. A moderate atmospheric river that stalls for three days can do more damage than a stronger one that passes in a single day. That is why the scale weighs both.
Measuring IVT offshore is hard, because few instruments sit over the open Pacific. Since 2016, the Atmospheric River Reconnaissance program has sent aircraft over the ocean to drop instruments called dropsondes through approaching storms. Those measurements feed forecast models and help sharpen predictions of where the heaviest rain will fall.
Preparation
The good news is that forecast models can usually spot an atmospheric river several days ahead. The National Weather Service does not issue a separate atmospheric river alert. Instead, it warns about the hazards these storms bring through flood watches and warnings, wind advisories and high surf warnings. Major storms are rarely a complete surprise, and residents usually have two or three days to prepare.
Preparing means clearing gutters and drains before the storm, moving cars out of flood-prone spots and checking that sump pumps work. If you live near a creek or in an area with a history of landslides, review your evacuation routes. Keep supplies for a 72-hour power outage, because falling trees and soaked ground often bring down power lines during big storms. If your plans are flexible, it is wise to delay or reroute travel around a major storm.
Understanding the Category Scale
Atmospheric rivers are now rated on a scale from Category 1 to 5, developed by the Center for Western Weather and Water Extremes at UC San Diego. The scale combines how much moisture a storm carries, how fast it moves that moisture and how long it lasts over one place. Category 1 storms are weak and mostly helpful, bringing useful rain with little flood risk. Categories 2 and 3 are moderate to strong storms that fill reservoirs but can also cause local flooding. Categories 4 and 5 are exceptional, mostly hazardous storms that can cause widespread flooding and damage.
The Bay Area sees several weaker atmospheric rivers in a typical winter, while Category 4 and 5 storms are much rarer. The storm of October 24, 2021, was rated Category 5 and gave downtown San Francisco about 4 inches of rain, its wettest October day on record. When forecasters mention a significant atmospheric river, its category is the quickest way to judge how seriously to take it.
