What Is an Atmospheric River? The AR Scale From AR1 to AR5

Direct answer: an atmospheric river is a long, narrow band of water vapour in the atmosphere — NOAA calls them rivers in the sky — that carries moisture from the tropics towards the mid-latitudes. When one reaches a coast and is pushed up over hills and mountains, that vapour falls as rain or snow. NOAA says the average one carries about as much water as the mouth of the Mississippi River, and the strongest up to 15 times that. Most are weak and beneficial; the strongest, especially the ones that stall, cause floods and landslides.

The AR scale: AR1 to AR5

Like the hurricane scale, atmospheric rivers are ranked 1 to 5. The scale was introduced in 2019 by the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography, and it is now used in forecasts along the US West Coast:

• AR1 — weak. Primarily beneficial: useful rain and snow.
• AR2 — moderate, AR3 — strong and AR4 — extreme. A mix of beneficial and hazardous effects, with more of the hazard as the rank rises.
• AR5 — exceptional. Primarily hazardous.

The rank comes from two things measured at one place: how much water vapour the river moves at its peak, and how long it lasts. Duration matters as much as strength. An atmospheric river that sits over the same place for more than 48 hours is moved up one rank; one that passes in less than 24 hours is moved down one. So a moderate river that stalls for three days can do more damage than a strong one that sweeps through overnight.

Why they matter: water supply and floods

Atmospheric rivers are a large part of the West Coast’s water. NOAA points to the storms of 10–22 December 2010, a series of atmospheric rivers that brought 11 to 25 inches of rain to parts of California and gave the Sierra Nevada 75 percent of its annual snow by the first full day of winter. The same kind of storm, stronger or slower, is what floods rivers, triggers mudslides on steep and burned ground, and closes roads.

The best known is the Pineapple Express, a strong atmospheric river that carries moisture from the tropics near Hawaii to the US West Coast.

Where they hit

They reach the west coasts of continents in the mid-latitudes: California, Oregon, Washington and British Columbia most famously, and Europe’s Atlantic coast too, where researchers have applied the same AR scale. The wet season on the US West Coast runs from autumn through winter, which is when “atmospheric river” starts appearing in forecasts.

What to do when one is forecast

1. Watch for the warnings, not the name. “Atmospheric river” describes the weather; the action is in the flood watches, flood warnings and high wind warnings your national weather service issues. NOAA says its forecasters can issue warnings for the heavy rain they bring as many as five to seven days in advance.
2. Know your flood and landslide risk. Steep slopes and recent wildfire burn scars are especially at risk. Our guides to landslide warning signs and flash floods versus river floods explain the difference in timing.
3. Never drive into flood water. The flood safety guide has the rest.

How this site shows them

An atmospheric river is not an event of its own here; what it causes is. Flood and wind warnings the National Weather Service rates severe or extreme appear in our US weather alerts by state, the Copernicus river forecasts in our river flood forecast show which rivers are expected to rise, and heavy-rain events worldwide are on the heavy rain page. This site does not issue warnings; national weather services do.

Frequently asked questions

What is an atmospheric river?

A long, narrow band of water vapour in the atmosphere that carries moisture from the tropics towards the mid-latitudes. When it reaches a coast and rises over mountains, the vapour falls as heavy rain or snow.

What do AR1 to AR5 mean?

The atmospheric river scale: AR1 weak, AR2 moderate, AR3 strong, AR4 extreme and AR5 exceptional. AR1 is primarily beneficial and AR5 primarily hazardous. An AR lasting over 48 hours moves up one rank; under 24 hours, down one.

Are atmospheric rivers bad?

Most are not. Most are weak and bring rain and snow that refill reservoirs and snowpack. The strongest, especially ones that stall over one area, cause floods and landslides.

What is the Pineapple Express?

A strong atmospheric river that carries moisture from the tropics near Hawaii to the US West Coast.

Sources

• NOAA - What are atmospheric rivers?
• CW3E - New forecast tools for the Atmospheric River Scale
• CW3E - European West Coast atmospheric rivers: a scale to characterize strength and impacts
• Zhang et al. (2024), The Cryosphere 18, 5239 - describing the Ralph et al. 2019 AR Scale

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