Atmospheric Rivers: Streams in the Sky

Californians, on a normal year, typically celebrate New Year’s Eve with parties in backyards, beaches, or rooftop bars. Not this year. It rained, a lot. In late 2022 into early 2023, a long channel of warm moist air called an atmospheric river, flowed up from the tropics into the mid-latitudes (30-60N). It brought rain and gloomy weather to much of the west coast for weeks on end. Each day these rivers carry up to the equivalent of 20x the flow of water as the Mississippi River. This flow drops normally 30-50% of California’s annual precipitation.

December 27 Satellite Photo of the Atmospheric River over the Eastern Pacific and California.
Source: NOAA

What is this?

Atmospheric Rivers (ARs) can quite literally be pictured as rivers in the sky. These ARs have a thickness of only 300-400 miles yet a length of thousands of miles. Sometimes called the “Pineapple Express” in reference to their start near Hawaii, extremely humid, or air highly saturated with water vapor heads straight towards North America. The trajectory of this moist air flows West to East, like many other weather patterns in the world. It’s essentially a ribbon of exceptionally moist air that flows from the tropics to the mid-latitudes.

The life cycle of an AR. It starts at the equator and shuffles its way outward as the moisture-rich air gets farther north and south from the equator. ARs get pushed by winds coming across the pacific and hitch rides toward North America. Source: NOAA

Atmospheric Rivers aren’t new however, these atmospheric rivers have been recorded extensively for the last 10 years. Scripps Oceanography at the University of California San Diego is leading the research on these across the globe. Below is a map of recording rivers and their intensity. They usually occur in the autumn/winter months and are VERY difficult to predict. We can use infrared satellites to look at water vapor in the atmosphere, yet it is difficult to know what to look for. They are tough to plan for because they flow low in the atmosphere, along with ordinary clouds away from where satellites can easily see and recognize them. We know when they are happening, but have a difficult time telling when they will happen next.

In winter, ARs come all along the west coast of North America. Their varying intensity and paths make them almost impossible to accurately predict with weather models.
Source: C2W2 @ Scripps Oceanography Institute

How it impacts surfing

When these rivers make landfall, they continue east. The wet air rises, condenses, and precipitates like none other when it hits the mountains in eastern California. The precipitation falls as rain in the lower altitudes and snows up in the higher mountains. It is great news for the reservoirs and ski resorts, yet it leads to floods and mudslides closer to the coastal areas.

Atmospheric River in California. Source: Paul Duginski

For example, Santa Barbara had 168% of its rainfall for January 2023. San Diego’s Brown Field Municipal Airport is at 187%. Up north, San Fransico has 192% rainfall compared to a 30-year normal. More CA cities can be seen here. This is dangerous for ocean folks because the rain runoff mixes with societal pollution.

2023 California Precipitation Levels. Source: California Water Watch

Well, can you surf them?

The Department of Environmental Health suggests waiting 72 hours after “normal” rainfall before surfing. Atmospheric Rivers skew the “Safe Wait Time” with their prolonged rains. Sewage, industrial runoff, and other toxins are mixed constantly into the ocean as the precipitation pours. The wait time should be based on the quality report from the local authorities.

However, the waves during these ARs can be as large as 35+ feet. On January 4-5 in 2008, there was an AR that gifted California about a foot of rain in the lowlands, 6-10 feet of snow in the mountains, and 32-foot waves. The height of an Olympic high-dive. These are no joke.

January 4-5, 2008 Atmospheric River. Source: F. Marty Ralph (UCSD)

While the waves of these storms might be truly gnarly after the chaos, there is a risk. Increased toxins in the water can make you gravely sick. To avoid this, pay attention to the color of the water. If there are beautiful waves, yet the water is muddy, grimy, and brown. Remember there are always going to be more waves. Don’t get sick being stoaked, surf safely.

Published by Danny Schmiegel

Great Lakes surfer and Rocky Mountain skier. Atmospheric and Oceanic Sciences - CU Boulder