What Is an Atmospheric River? The AR 1 to AR 5 Scale Explained

An atmospheric river is a long, narrow band of concentrated water vapor that flows through the lower atmosphere, usually from the tropics or subtropics toward the middle latitudes. When one runs into hills or mountains, the air rises, cools and drops its moisture as heavy rain or snow. Scientists rate these storms on a five-step scale, from AR 1 (weak and mostly beneficial) to AR 5 (exceptional and mostly hazardous).

On the US West Coast, atmospheric rivers bring a large share of the year’s rain and mountain snow, so the same storm type can end a drought or cause a flood.

What does an atmospheric river look like?

On satellite images of water vapor, an atmospheric river shows up as a thin streak of moist air stretching across the ocean, often hundreds of miles wide and more than a thousand miles long. Most of the moisture travels in the lowest few miles of the atmosphere, carried by strong winds ahead of a cold front.

The best-known example is the “Pineapple Express,” a plume that reaches California and the Pacific Northwest from the waters near Hawaii. Atmospheric rivers also reach Western Europe, South America, the southeastern US and other coasts around the world.

How does an atmospheric river make rain and snow?

Water vapor is invisible gas until it condenses. The river itself carries moisture; the rain and snow come when the air is forced upward. That happens in a few ways:

• Mountains: air pushed up the windward side of a range such as the Sierra Nevada or the Cascades cools quickly and wrings out water. This is why the heaviest totals fall on the slopes facing the ocean.
• Fronts: warm, moist air gliding over colder air along a front also rises and produces steady rain.
• Stalling: when the band stays in one place for a day or more, rain keeps falling on the same watersheds, and rivers and soils fill up.

Snow levels during these storms are often high because the air is warm. Rain can then fall on existing snowpack, melt it and add even more water to streams. The process is different from lake-effect snow, which piles up downwind of the Great Lakes, where cold air picks up moisture from open water over a short distance.

What is the atmospheric river scale?

The scale was developed by researchers at the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography, UC San Diego, and published in 2019. It works a bit like the categories used for hurricanes. It rates each event by two things:

• Strength: how much water vapor the river moves, measured as integrated water vapor transport (IVT). IVT combines how moist the air column is with how fast the wind moves it.
• Duration: how long a place stays under atmospheric river conditions.

An event that lasts 48 hours or more moves up one category. One that lasts less than 24 hours moves down one category. A strong but brief storm can therefore rate lower than a moderate storm that sits over the same area for two days.

What do the five categories mean?

1. AR 1, weak: primarily beneficial. Light to moderate rain that helps soils and reservoirs.
2. AR 2, moderate: mostly beneficial, with some hazards such as minor flooding.
3. AR 3, strong: a balance of beneficial and hazardous effects. Reservoirs fill, but local flooding becomes more likely.
4. AR 4, extreme: mostly hazardous. Flooding, mudslides and damaging winds are more common.
5. AR 5, exceptional: primarily hazardous. These events can cause widespread, serious flooding.

The CW3E team publishes forecasts and current ratings on the Center for Western Weather and Water Extremes website.

Why are atmospheric rivers helpful and dangerous?

In the western US, much of the water supply comes from winter storms. A handful of atmospheric rivers in one season can decide whether a year ends in drought or with full reservoirs. Weaker events soak in slowly and build snowpack that melts in spring.

The danger grows with intensity and time. Back-to-back storms saturate the ground, so later rain runs straight off into creeks. Steep slopes, and especially hillsides burned by recent wildfires, can slide as mud and debris flows. Strong winds that come with these storms can bring down trees and power lines, and heavy mountain snow can close passes. Road crews then lean on plows and treatments that work like road salt lowering the freezing point of water.

How do forecasters predict atmospheric rivers?

Weather satellites track the moisture plume over the ocean several days before landfall, and computer models estimate how much water vapor it carries and where it will come ashore. Over the Pacific, NOAA and partner agencies also fly reconnaissance aircraft that drop instruments into the storms to measure moisture and wind directly. Those measurements feed the forecast models.

Forecasts for the exact landing spot remain uncertain a few days out, because a shift of a hundred miles can move the heaviest rain from one watershed to another. Local warnings come from the National Weather Service, which issues flood watches and warnings for specific areas.

What should people do when a strong atmospheric river is forecast?

A rating of AR 3 or higher is a reason to pay attention to local alerts. Practical steps follow mainstream emergency guidance:

• Follow National Weather Service watches and warnings and any evacuation orders from local officials.
• Clear gutters and storm drains near your home before the rain starts.
• Never drive or walk through flooded roads; a small depth of moving water can sweep a car away.
• If you live below a burn scar or a steep slope, know your evacuation route in advance.
• Prepare for power outages with flashlights, charged phones and stored water.

Quick answers

Is an atmospheric river the same as a hurricane?

No. A hurricane is a rotating tropical storm with its own wind field. An atmospheric river is a stream of moisture, usually tied to a mid-latitude storm system and its cold front.

Why is it called the Pineapple Express?

The name refers to atmospheric rivers whose moisture comes from the tropical Pacific near Hawaii, a region known for growing pineapples.

Can an AR 1 storm cause damage?

It can in places that are already saturated or burned, but most weak events bring useful rain with few hazards.

Do atmospheric rivers happen outside the West Coast?

Yes. They reach many coasts worldwide, including Western Europe, Chile and the southeastern United States.