Atmospheric Rivers (ARs) are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport water vapor outside the tropics. ARs affect mid-latitudes around the world and are commonly associated with heavy snow and heavy rain on the West Coast of North America.

ARs can deliver significant amounts of precipitation and are responsible for a majority of extreme rainfall and snowfall events. They contribute up to 30–50% of California’s water supply and are responsible for a major portion of the Sierra and Cascade snowpack.

A few important things to know about ARs are:

• Not all storms that impact the West Coast are ARs
• The criteria for an AR means that there is a significant amount of moisture and wind expected for a period of ~24 hours
• ARs can be ranked on a scale from 1-5, but the ranking is not the same as that for hurricanes

Let’s get into more detail about ARs and what that means for skiing and riding in North America and around the world. 

ARs, Snowfall, and Snow Levels

ARs typically deliver feet of snow rather than inches of snow and can make for an epic powder day. They can last several hours or even a couple of days, bringing continuous heavy snow. Some ARs have produced over 6 feet of snow within 48 hours!

The downside is that ARs are typically warmer storms, which means snow can be dense and wet, contributing to “Sierra Cement,” “Cascade Concrete,” and high snow levels.

Check out the forecast of a major AR bringing several feet of snow through the Sierra and Intermountain West on January 13 - 17, 2023.

ARs contribute to a major portion of the snowpack along the West Coast of North America. This has important implications for skiing but also for hydrologic conditions. A large snowpack can lead to full reservoirs once the snow melts, which has important water usage implications during the summer and fall. 

• Much of California’s agriculture economy relies on a healthy Sierra snowpack
• If ARs are not frequent, this can lead to drought conditions
• Drought conditions can then lead to heightened wildfire conditions

ARs bring warm moist air from the subtropics, which means ARs often bring rising snow levels. Snow levels are determined by the orientation of the AR and whether there is cold air ahead of the AR.

• Warm air will be on the south and east sides of the AR, increasing snow levels
• Cold air will be on the north and west sides of the AR, decreasing snow levels
• If the AR is oriented from southwest to northeast, warm air is transported and snow levels will rise
• If the AR is orientated west to east (zonal), snow levels are more likely to stay constant 
• If there is cold air ahead of the AR, this can limit snow level rise

See the Meteorological Dynamics of an AR for more information on snow levels.

Forecasting ARs

OpenSnow forecasters can see an AR in the forecast up to 7 days in advance. A weather pattern that favors ARs can even be forecasted up to several weeks in advance! 

Below is an example of the GFS model’s forecast on the left on October 26th, 2023 and what occurred 7 days later on November 1st, 2023.

Not all AR forecasts will be this good. Sometimes a forecast comes together a few days before the AR affects an area.

Whether it is a 7-day forecast or a several-week pattern favorable for ARs, the exact timing, intensity, and location of an AR is only made 1 - 4 days in advance.

While a forecaster might know a week or more in advance that an AR will impact the West Coast of the US, the small changes in the intensity and track of an AR will have large impacts on which areas see rain, snow, or nothing. 

Meteorological Dynamics of an AR

An AR is part of a larger low-pressure system that contains a warm front and a cold front. The AR in a storm system is located to the southeast of the low-pressure center in the northern hemisphere. The AR can be various lengths but the length is always longer than the width!

Below is a diagram of an AR as it relates to the larger low-pressure center (L), fronts, and area of highest moisture highlighted. 

The heaviest precipitation will be at the nose of the AR where it makes landfall in regions of high terrain. Other regions with heavy precipitation will be ahead of the warm front and along the cold front. An AR brings warm moist air that often increases snow levels. 

• Snow levels will be higher on the east side of the cold front and north side of the warm front
• Snow levels will be lower on the west side of the cold front

Ranking of ARs

ARs have been recently ranked from 1 - 5 to highlight and warn of potential effects on human infrastructure and the environment. 

• ARs are ranked based on the intensity of integrated water vapor transport and the duration of water vapor transport over a given area
• ARs are NOT ranked based on wind speed

Below is a look at the rank of AR conditions observed from October 31st through November 6th, 2023 along coastal (left) and interior areas (right). 

Here is how the rank of an AR is determined:

Where ARs Originate

Most ARs originate in the tropics or subtropics where warm ocean temperatures in combination with a warm atmosphere can hold large amounts of moisture. Once storm systems develop in the mid-latitudes, they can transport nearby subtropical or additional mid-latitude moisture.

ARs can also originate in the mid-latitudes when several storm systems aggregate together, creating enough moisture for an AR to develop.

As storms in the northern hemisphere spin counterclockwise, they pick up subtropical moisture and transport it around the storm system. Once storms access enough subtropical moisture and become strong enough, an AR develops. 

Below is a look at the transport of moisture from the subtropics to a storm system and AR impacting Alaska and British Columbia on November 8th, 2023. 

ARs in North America

ARs extend along the West Coast of North America from Alaska to the Baja Peninsula of Mexico. If an AR is strong enough, it can reach interior areas of the western US as well. 

Strong Nor’easters that form along the East Coast can have a subtropical moisture connection and create ARs in some scenarios.

ARs are not just a North American weather phenomenon, they occur around the world!

ARs Around the World

Any location in the mid-latitudes, which is roughly between 35 and 65 degrees north and south will experience ARs. This includes Europe, Japan, South America, New Zealand, and many more locations. ARs are also known to impact polar regions in exceptional cases, including Antarctica!

ARs contribute to a major portion of snowpack in South America and New Zealand, much like the West Coast of North America.

Below is a look at an AR affecting South America from August 18 - 24, 2023. 

ARs and ENSO

I recently published an article on El Niño’s Effect on Atmospheric Rivers where I went over how and why El Niño can increase the frequency of ARs across the West Coast, specifically California and the Southwest. 

La Niña (the negative phase of ENSO) impacts ARs as well. La Niña can shift the AR frequency to the Pacific Northwest, British Columbia, and Alaska. 

Additionally, ENSO via both El Niño and La Niña can affect ARs all around the world in the mid-latitudes. 

ARs can be big powder producers or bring warm air that raises snow levels bringing heavy rain. They make a season or blank a season, especially along the West Coast of North America.

OpenSnow forecasts can give you several weeks of warning for an active AR pattern when you plan your next powder adventure. Stay posted on your favorite Daily Snow for when the next AR is coming your way.

Zach Butler