What is an ‘atmospheric river’? The science behind the term flooding forecasts

There has been a lot of talk about the term “atmospheric river” in the last month or so. Some feel the term is now being overused or even misused. So what is an atmospheric river?

The term first appeared in modern scientific literature in the early 1990s and generated quite a debate among atmospheric scientists and climatologists about the term’s meaning. Since then, a definition of the term was established in the Glossary of Meteorology. Read on for the full definition below.

In short, an atmospheric river is a stream of moisture from tropical or subtropical sources that is transported into mid-latitude storm systems, producing copious amounts of rainfall with relatively high snow levels. This feature is actually quite common around the globe and contributes significantly to the bulk of mid-latitude annual rainfall. Here is a water vapor (moisture) satellite image showing a typical atmospheric river.

Atmospheric river impacts

Some of the wettest atmospheric rivers over the years have produced as much as 40 inches of rain in just a few days in the western Washington mountains. These heavy rainfall amounts often result in significant and widespread flooding of our rivers and streams. If soils are already saturated, this heavy rainfall can also lead to landslides.

The warm air flow also means freezing levels rise – well above 6,000 feet. The rain falling into a mountain snowpack is usually absorbed like a sponge. U.S. Corps of Engineers studies have found that only up to 15% of mountain snow melts into river runoff. Heavy rainfall is by far the biggest contributor to western Washington river flooding.

Examples of significant atmospheric river events

One atmospheric river and warm heavy rainfall event occurred in early November 2006. No mountain snowpack was in place, so the heavy amounts of rain in the mountains of up to 40 inches in just a few days ran directly into the rivers. Flooding was massive and widespread. Many rivers reached near record levels during that event.

Another similar event was just a year later in early December 2007. The hardest hit area was in southwest Washington. Up to 38 inches of rain fell in the Willapa Hills in approximately 24 hours, resulting in what the U.S. Geological Survey described as a 500-year flood on the Chehalis River. I-5 and many area roadways and neighborhoods were under a foot of water. The freeway was closed for days before the floodwaters receded.

The Pineapple Express

There is a subtle difference between an atmospheric river and what some refer to as the “Pineapple Express.” That term was coined by a National Weather Service forecaster in Seattle back in the 1980s, when he noted a moisture stream on Eastern Pacific weather satellite imagery that originated near the Hawaiian Islands and extended into the Pacific Northwest. He used the term “Pineapple Express” in a state forecast discussion, and area media picked it up and used it to help explain an incoming heavy rainfall event.

Pineapple Express events are a type of atmospheric river that is easily recognizable on satellite imagery. Here is an example from a February 2006 event that resulted in extensive major flooding from western Washington into western Oregon.

So far this fall

This year has seen one unique atmospheric river event. The plume of sub-tropical moisture extended from near Taiwan all the way across the North Pacific into western Washington. The Seattle-Tacoma International Airport (SEA) received just more than 1.5 inches of rain from this event. Fortunately, thanks to the dry year, rivers were running well below average, and no significant widespread flooding occurred.

It is important to note that not all heavy rainfall events are atmospheric rivers. The nearly one inch of rain that fell at SEA around Halloween was such an event. This satellite image shows a typical Pacific frontal system that did not have a subtropical moisture source.

No. 1 disaster

Washington averages about one presidentially declared weather-related disaster per year, going back to 1950. Flooding is number one on the list. About half of all these declared disasters involve significant flooding, including the major flooding in Whatcom County in November 2021.

November is usually the wettest month of the year and often experiences the most flood events, largely due to the fact that the mountains do not yet have an established snowpack to help absorb much of the heavy rainfall.

The flood season – getting ready

The western Washington flood season usually extends from late October into mid-March. Flooding impacts more than just homes and businesses adjacent to rivers and streams. Many motorists drive or commute through low-lying areas prone to flooding, plus heavy rainfall often results in standing water on roadways, creating hazardous driving conditions. Storm drains can become overwhelmed, particularly if fallen leaves clog the drains.

Getting prepared in advance of any hazardous weather or earthquakes is important. To help better prepare before, during, and after a flooding event, helpful tips are available at ready.gov/floods.

The terms “atmospheric river” and “Pineapple Express” become quite popular this time of year. Take a moment to learn if any incoming Pacific weather system has a source of sub-tropical moisture embedded in the system before the storm gets either of those labels.

Oh, and get ready to hear the terms “bomb cyclone” and “polar vortex” as the calendar heads into winter. These terms also make for good headlines.

The definition as it appears in the Glossary of Meteorology

Atmospheric river – A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources. Atmospheric rivers frequently lead to heavy precipitation where they are forced upward — for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapor transport in the mid-latitudes primarily occurs in atmospheric rivers and is concentrated in the lower troposphere. Atmospheric rivers are the largest “rivers” of fresh water on Earth, transporting on average more than double the flow of the Amazon River.

Ted Buehner is the KIRO Newsradio meteorologist. Follow him on X and Bluesky. Read more of his stories here.