A “Pineapple Express”-type atmospheric river storm once drenched California, prompting massive flooding and a spillway emergency. But it was just one of many atmospheric rivers meandering across the planet at any given time.
Atmospheric rivers transport water vapor, and depending on how high they are in the atmosphere when they hit land, can produce either rain or snow. These narrow corridors of moisture can be hard to predict.
What is an Atmospheric River Storm?
The severe storms that douse California with soaking rain and bury mountain towns in snow are called atmospheric river (AR) storms. They are a natural part of the water cycle, helping to replenish agricultural and urban water supplies. They also build up the snowpack that supplies water during drier seasons. But these “rivers in the sky” can also wreak havoc, triggering landslides and flooding that cause billions of dollars in damage or more.
Scientists distinguish Atmospheric River Storms from other types of storms by the amount of moisture they carry and how long they last. One of the most well-known ARs, known as the Pineapple Express, lines up from near Hawaii to the U.S. West Coast ahead of a cold front each fall, winter or spring. On average, it transports enough water vapor to double the flow of the Amazon River.
The swollen clouds of an Atmospheric River Storm can last for days and dump huge amounts of water in coastal areas. Heavier rainfall occurs inland, where the Atmospheric River Storm crashes into mountains. The wind-driven updrafts wring the moisture out of the air like a sponge, often causing flash floods along rivers and streams.
In addition to flooding, atmospheric river storms can trigger mudslides and landslide hazards, as well as downed power lines. And because the long streams of overhead water carry so much moisture, they can cause erosion and swell river levels. The storms are especially dangerous to mountain communities that have been ravaged by wildfire, which has robbed them of trees and debris that would otherwise absorb moisture and precipitation.
Some researchers are worried that climate change will intensify the impacts of atmospheric rivers. That’s because warmer ocean waters fuel the storms and because they melt the snowpack that protects communities from flooding.
The National Weather Service recently introduced a system for rating AR events, similar to hurricane and tornado ratings. It classifies ARs on a scale of 1 to 5, with 5 being exceptional. A stronger AR can swell with more water and last longer than a weaker one, making it harder to prepare for. A stronger AR could also cause more flooding and mudslides, as well as downed power lines.
How Does an Atmospheric River Storm Work?
The massive storms that have doused California this winter — dumping 30tn gallons of water, burying mountain towns in snow and flooding cities and farmland — are the work of atmospheric rivers. “Rivers in the sky,” as NASA describes them, are narrow corridors of concentrated water vapor that move fast and drop their moisture as drenching rain or mega-snowfall. The science behind them can be complicated, but they’re also incredibly powerful and frequent.
Atmospheric rivers start over warm ocean waters, where evaporation creates high concentrations of moisture. Prevailing winds then blow the moisture swells into jet streams that carry them northward and westward toward land. When they reach the western US, they’re usually as long as 1,500 kilometers (930 miles) and about one-third that wide. That’s about the length of a football field and more than twice as wide as the Mississippi River. That’s why they’re sometimes compared to a fire hose, pointed straight at California.
As they approach the coast, ARs begin to lose their intensity because they’re subjected to stronger winds and cooler temperatures. The moisture in them cools and condenses, forming clouds and precipitation. At their strongest, they can bring a week of pounding rains and raging rivers.
Scientists are working to better understand these giant storms. Marty Ralph, a meteorologist with the Scripps Center for Western Weather and Water Extremes, recently created the first computer model tailored to predict their effect on the west coast. His team has also developed a uniform scale to rank the strength of an atmospheric river based on its size and how much moisture it’s carrying.
But it can be difficult to see them clearly on radar and satellites, especially when they are delivering heavy rain or snow. They are also low in the atmosphere, so they don’t show up on satellite imagery as easily as hurricanes do.
The new scale is a step in the right direction, but more research is needed to fine-tune it and to apply it to specific regions. One challenge is that ARs are more common in recent years than they used to be, partly because of climate change. Hotter air traps more moisture, which makes storm systems bigger and more intense.
What Are the Impacts of an Atmospheric River Storm?
If you live in California, then chances are you’ve heard the term “Pineapple Express” or “Atmospheric River” in the news. These storm systems are a key part of California’s weather patterns, and they can have major impacts on water reservoirs. In fact, DWR Climate Change Program Section Chief Elissa Lynn discussed these rivers in the sky during a recent episode of DWR’s Water Wednesdays.
These giant rivers of water vapor transport significant amounts of moisture from the tropics to mid-latitude locations like Northern California, and they can have intense rainfall rates and strong winds. In addition, they can also produce snow at higher elevations.
While some atmospheric rivers are beneficial, others can cause major problems, including flash floods, mudslides and debris flows. And the risk of these hazards can be compounded when atmospheric rivers stall over parched and burn-scarred landscapes, which are more vulnerable to runoff and erosion.
Scientists have been working to better predict atmospheric rivers to reduce damage. One tool they’ve used is a new rating system. Similar to hurricanes, this scale rates atmospheric rivers according to how much water they transport, their duration and the location where they’re forecast to be present. The ratings also include whether the atmospheric river is primarily beneficial or hazardous, with AR1 being weak and AR5 being exceptional.
The development of this scale is important because atmospheric river events are increasingly common, and the impacts can be disproportionately destructive in different regions. This has led to a growing number of people asking how we should categorize them, and the new rating system is one step in the right direction.
The researchers who created the rating system also hope that it will help to standardize reporting on atmospheric rivers globally, which could be helpful for water managers and other users of weather data. This could lead to improved communications, and ultimately, a more accurate assessment of the risks associated with these rivers in the sky. (Scientific American, March 2019.) — Katerina Gonzales is a science and technology reporter for Scientific American and has reported extensively on extreme weather events.
How Can I Prepare for an Atmospheric River Storm?
Last winter and spring, California was battered by a series of atmospheric river storms dumping staggering amounts of rain and snow. With another one on the way, it’s time to get familiar with these unique weather events.
Atmospheric rivers, or ARs, are long corridors of water vapor that can stretch thousands of miles across the atmosphere. They’re fueled by warm air that travels from the tropics towards the poles, picking up moisture along the way. These storms aren’t as large as traditional hurricanes, but they can still wreak havoc by producing high rainfall rates and powerful winds that drive storm surges. They’re also known to trigger mudslides and cause flooding, especially in coastal areas and low-lying regions.
These water-vapor systems are categorized by how much moisture they transport, measured by something called Integrated Water Vapor Transport (IVT). The more IVT a system has, the stronger it is. They’re also ranked from weak to exceptional based on how long they last and how fast they’re moving.
Despite their impressive size and strength, atmospheric rivers aren’t easy to predict. The storms often appear out of nowhere, forming without an obvious cause and then rapidly changing in intensity as they move over land. And while a growing amount of evidence suggests that climate change is affecting how frequently these storms happen, it’s hard to disentangle the many factors that contribute to them.
The good news is that scientists are getting better at forecasting them. In 2013, they created the first weather model tailored to predicting ARs, and this year they introduced an atmospheric river scale that ranks events from weakest to strongest based on how much water they carry. They also use data from drifting ocean buoys, weather balloons, and even hurricane-hunter planes that drop sensors into the eye of an AR.
All of this research will help meteorologists improve their ability to track and forecast ARs, which can be difficult to see from satellite images due to their low altitudes. And that’s essential, as the most hazardous ARs are those that linger over a region for several days.