For residents of California and other parts of the Western United States, the term “atmospheric river” has become synonymous with trouble. A phrase that used to only exist in the minds and research of meteorologists and climate scientists has now entered the mainstream. It’s not hard to see why.

In late 2022 and early 2023, a series of storms caused by an atmospheric river (AR) brought unprecedented flooding, damage, and harm to a wide swath of California and Nevada after a long period of intense drought. 41 of California’s 58 counties received emergency declarations, and at least 19 people died.
 
What is an atmospheric river, and is atmospheric river flooding something to worry about over the long term?

 

Understanding What Atmospheric Rivers Are

Atmospheric rivers (ARs) are long, slender rivers that run in the sky instead of on the ground, averaging 1,200 miles in length and 250 to 375 miles in width. They transport condensed tropical water vapor toward the poles through the air at a pace matching 7 to 15 times the Mississippi River's typical daily flow.
 

How and Where Atmospheric Rivers Form

Atmospheric rivers form primarily over warm ocean waters, where evaporation occurs rapidly. As warm, moist air rises, it cools and condenses, creating clouds and precipitation. This process is often intensified by prevailing winds that transport the moist air from the tropics toward mid-latitude regions.
 

When the water vapor finally falls, it can deliver much-needed rain. However, it sometimes brings an overwhelming amount of water to a small area in a very short period. This can lead to catastrophic outcomes, including flooding, mudslides, property damage, and even loss of life.
 

Atmospheric River Categories

Some atmospheric rivers (ARs) can be as destructive as hurricanes, which is why a rating system has been established for them, similar to that used for hurricanes. Category 1 ARs are classified as “primarily beneficial” as they can alleviate dry conditions without causing significant damage.

On the other hand, Category 5 ARs are considered “primarily hazardous.” These are capable of causing catastrophic impacts such as intense flooding, severe erosion, landslides, and infrastructure damage, posing significant threats to life and property.

Causes of Atmospheric Rivers

Changes in sea surface temperatures, particularly in the Pacific Ocean, can significantly influence AR patterns. For instance, warmer ocean waters can enhance evaporation, resulting in more moisture available for transport.

Climate change also plays a role, as rising temperatures are expected to increase the intensity and duration of AR events, leading to more extreme weather conditions, including heavier rainfall and flooding in affected areas.

 

The Impact of Atmospheric Rivers

The world’s ecosystem depends on the moisture flow ARs carry from place to place. They are integral to the hydrological dynamics of the U.S. West Coast, enhancing precipitation levels and serving as a pivotal mechanism in alleviating drought conditions. Their absence would significantly diminish rainfall, substantially modifying the region's agricultural framework.

However, it’s the impact of the more damaging side of ARs that’s concerning. Flash floods, flooding, mudslides, property damage, infrastructure loss and degradation, disrupted power grids, and death can all occur with more powerful ARs. There’s a rating system for atmospheric rivers that’s similar to the one used for hurricanes. Category 1 is rated as being “primarily beneficial.” It can alleviate dry conditions without causing significant damage. Category 5 is “primarily hazardous.” It can cause catastrophic impacts, including intense flooding, severe erosion, landslides, and infrastructure damage. It brings significant threats to life and property. At any given time, there are three to seven ARs in the atmosphere. When they move inland and are pushed to higher altitudes by the mountains, the water vapor cools, creating conditions that result in heavy rain or snow. The more moisture in the AR, the greater the potential for damaging floods.
A massive AR unleashing heavy rains on the Western U.S. and Canada could leave millions of people displaced and cost over $1 trillion in damage. 

 

Why Atmospheric Rivers Increase the Need for Flood Insurance

When atmospheric river systems make landfall, they can release heavy amounts of rainwater in just a few hours or days. This rapid moisture delivery often results in intense rainfall that can exceed several inches in hours. The sheer volume of water falling in such a condensed period can create rapid runoff, meaning that the ground and drainage systems may not be able to absorb or manage the influx of water quickly enough.

Urban areas, which often feature impermeable surfaces like concrete and asphalt, are particularly affected by this rapid runoff, since there are fewer natural pathways for the water to soak into the ground. Consequently, drainage systems can become overwhelmed, leading to surface flooding.

In rural areas, even though there may be more open land for absorption, the intense rainfall can lead to swift runoff into rivers and streams, causing them to swell rapidly and overflow their banks. This combination of heavy precipitation and rapid runoff can result in significant flooding events.

Unfortunately, many regions that experience atmospheric rivers aren’t traditionally associated with flooding, leading homeowners to underestimate their risk. This misconception can leave property owners vulnerable to unexpected water damage. Standard homeowner's insurance policies often exclude flood-related damage, making it crucial to secure specialized flood insurance.

 

How Aon Edge Can Help

While there’s no guarantee that someone’s property will or won’t be affected by flooding,  flood insurance can help with peace of mind and protection should the unthinkable occur, especially in those parts of the country that see overlapping natural disasters.
 
Aon Edge is committed to providing flood insurance that works for you. Partnering with Aon Edge simplifies the path to delivering private flood insurance to your clientele. We've refined the process of obtaining flood insurance by offering real-time online quotes, an efficient underwriting approach, and instant policy issuance.

 
Reach out today to determine which flood insurance products suit you and your customers.

This information is provided for general informational purposes only and is not intended to provide individualized advice. All descriptions, summaries or highlights of coverage are for general informational purposes only and do not amend, alter or modify the actual terms or conditions of any insurance policy. Coverage is governed only by the terms and conditions of the relevant policy.