The West Coast has seen its fair share of atmospheric river activity over the past few months, and their destructive power has left several dead and many homes destroyed.
Do these climate events occur on the East Coast as well, and how are they impacting home insurance rates?
Atmospheric Rivers
An atmospheric river (AR) is a long, narrow “river” of condensed water vapor in the atmosphere that moves with the weather. Strong rivers carry between 7.5 and 25 times the flow of water that passes the mouth of the Mississippi. They can be 300 miles wide, one mile deep, and more than 1,000 miles long and are the largest rivers of fresh water on Earth.
Atmospheric rivers form at the equator, where the sun heats the earth most directly. These warmer temperatures cause water to evaporate and rise into the atmosphere. Circulation pulls the water vapor to other regions, much like a conveyor belt in an Amazon warehouse. Typically, they flow in the lowest part of the atmosphere, approximately ½ mile to 1 mile above sea level. Once they reach land, they are pushed upwards, where the vapor condenses and falls as either rain or snow.
Always flowing somewhere on Earth, ARs don’t stay in one place like rivers on the ground. Often, 90% of the water vapor moving towards the poles is concentrated in 4-5 atmospheric rivers around the globe, covering less than 10% of the circumference of the planet.
Most ARs are small and fast-moving, releasing large amounts of water or snow over short periods. They are most often beneficial storms in the West, as they fill reservoirs and irrigation canals for longer-term use. Meteorologists estimate ARs provide over half of the mean annual runoff on the coasts of most non-African continents.
Rating Atmospheric Rivers
Like Hurricanes and tornadoes, ARs are rated according to their intensity and propensity for damage. The rankings use Integrated Water Vapor Transport (IVT) to determine an AR's strength, which shows the amount of water vapor in the system, the wind moving it around, and the duration of the storm.
Category 1 ARs are primarily beneficial, last a short time, and produce modest amounts of rain. They have an IVT of 250-500.
Category 2 ARs are considered primarily beneficial, but also hazardous. They last up to 72 hours, come with little wind, and replenish rivers, reservoirs, and streams.
Category 3 ARs are strong, and equally beneficial and hazardous. They can push ground rivers to near flood stage, and last up to 72+ hours. They have an IVT between 500-1250.
Category 4 ARs are extreme, mostly hazardous, but also beneficial. They drop more rain, push ground rivers closer to flooding, last longer than 72 hours, and have an IVT of 750-1250 or greater.
Category 5 ARs are primarily hazardous. They bring as much rain as the strongest Atlantic storms and cause mudslides, loss of power, and flooding. They can be compared to EF 4 and 5 tornadoes and Category 3, 4, and 5 hurricanes.
The duration of an AR is the main ranking qualification. The storm intensity of category 1 and 3 ARs may be the same, but the 3 will last longer.
East Coast Atmospheric Rivers
Most of the early study of ARs has focused on the western United States, but they can have a significant impact on precipitation in the central and eastern US during cool and warm seasons. The primary difference between the west and east coast ARs is their direction. Western ARs move primarily west to east, while those east of the Rockies move from south to north, running parallel to the Appalachian Mountains.
Between 30-50% of annual rainfall over the southeast US can be attributed to AR activity. Since 2020, patterns of moisture transport in ARS across the Eastern US have increased the frequency and intensity of extreme rainfall events.
Because the atmosphere on the east coast has more moisture sources and holds much more moisture on average than the West Coast, the ARs are not as “impressive.” The rainfall from the rivers tends to be distributed across a wider territory, and it is harder to predict where it will occur. So, according to Kelly Mahoney, a research meteorologist at NOAA, “we typically don’t say that an atmospheric river caused” this event.
Studies of global warming, says Mahoney, show that there will be an uptick in the intensity of ARs around the world. Warmer air can hold more water, “so they will probably be more frequent and the potential for their impacts to be more severe.” Data shows that for every degree Celsius the atmosphere’s temperature rises, it can hold 7% more water vapor.
Home Insurance Impacts
FEMA is adjusting premiums to better account for flood risk that increases along with AR intensity and likelihood. In other words, if your home is located in a flood-prone area, you can expect to pay more to protect it.
Water seepage protection may increase your premiums as well, while surveys of insurance providers have not ruled out the possibility that this coverage will be removed and included only as an endorsement on many HO5 policies.
How to Protect Your Home from Atmospheric Rivers
Make sure your home or prospective home is not in a floodplain. Risk Factor is a free, user-friendly tool that can tell you about your flood risk as related to ARs and other climate-related risks.
Flood-proof your home. Depending on where you live, you may want to elevate your home above the projected flood elevation. In some cases, your local government may provide financial assistance. Check the FEMA website for details.
If you have family heirlooms, a safe, or other important personal belongings you’d prefer not to lose, move them to the upper floors of your home.
Have a disaster kit ready to go.
Have an evacuation plan in place.
Buy flood insurance.
In conclusion, even if your local weather report isn’t predicting an Atmospheric River, storms will be carrying more water than ever, and rain events are going to be heavier than in past decades. It is always a good idea to make sure your home is protected from water, no matter the source.
Sources: ALM PropertyCasualty360, USGS, AGU Publications, Policy Genius