What Is an Atmospheric River, or Pineapple Express?
Read about this oddly-named, but common weather occurrence that can pack quite a punch!
Quick Reference
- What it is: An atmospheric river that runs from Hawaii to the U.S. West Coast.
- Size: Thousands of miles long, ~100 miles wide.
- Water content: A single atmospheric river can carry more water than the Amazon River.
- Frequency: 6 to 10 atmospheric rivers are active globally at any moment.
- Impact: Heavy rain, blizzards, flooding, mudslides on the U.S. West Coast.
“Atmospheric river” headlines have become a winter staple on the U.S. West Coast. The popular nickname for one variety is the Pineapple Express. The name sounds tropical and friendly. The storms are neither. Here is what a Pineapple Express actually is, why it forms, and what it does when it lands.
The Storm That Named Itself
In early December 2014, a powerful storm pummeled the U.S. West Coast over two weeks. Heavy rain, blizzards in the Sierra Nevada, high winds, flooding, and mudslides across California. A rare tornado even touched down in Los Angeles, a city that almost never sees thunderstorms. The storm was a double-edged sword. It caused highway closures, property damage, and widespread outages, but it also broke a three-year drought across much of the region. Reporters reached for an explanation. The term that lit up the screens was Pineapple Express.
What an Atmospheric River Actually Is
Pineapple Express is slang for one specific type of atmospheric river. An atmospheric river is a moving band of concentrated water vapor flowing through the troposphere. The Pineapple Express variety runs in a long arc from the warm waters near Hawaii to the U.S. West Coast.
Atmospheric rivers are huge. They are typically several thousand miles long and around 100 miles wide. They are made of water vapor (water in gas form, not liquid), but the amount they transport is staggering. A single strong atmospheric river can carry more water than the Amazon River, Earth’s largest liquid river. The name fits.
According to the NOAA Physical Sciences Laboratory, between 6 and 10 atmospheric rivers are active across Earth at any given moment. They cover roughly 10 percent of the planet’s surface but move up to 90 percent of all water vapor transport between the tropics and the mid-latitudes. Most pass without much trouble. A few land hard.
Why the Pineapple Express Is Different
What turns a routine atmospheric river into a destructive Pineapple Express is the collision. Warm moist tropical air streaming up from Hawaii meets the cooler, drier continental air mass over the West Coast in winter. The temperature contrast forces rapid condensation. Rapid condensation releases huge amounts of latent heat. That energy drives the storm. The result is high winds, heavy rain at low elevations, deep snow in the mountains, and flooding in the valleys.
How Often Does the Pineapple Express Hit?
Strong, multi-day events on the scale of the 2014 storm hit the West Coast once or twice a decade. Moderate atmospheric river events that produce a day or two of heavy rain are common, with several per winter typically reaching California, Oregon, or Washington. The 1996 New Year’s storm, the 2017 Oroville Dam event, and the January 2023 series of nine successive rivers that hit California are all on the record books.
Atmospheric River Categories
In 2019, NOAA and Scripps Institution of Oceanography introduced a five-category scale for atmospheric rivers, similar to the Saffir-Simpson scale for hurricanes:
| Category | Strength | Impact |
|---|---|---|
| AR Cat 1 | Weak | Primarily beneficial. Drought relief. |
| AR Cat 2 | Moderate | Mostly beneficial. Some hazards. |
| AR Cat 3 | Strong | Balanced benefit and hazard. Localized flooding. |
| AR Cat 4 | Extreme | Mostly hazardous. Widespread flooding, slides. |
| AR Cat 5 | Exceptional | Primarily hazardous. Major property damage and life threat. |
Frequently Asked Questions
What is the Pineapple Express?
A slang name for an atmospheric river that runs from the warm waters near Hawaii to the U.S. West Coast. It carries massive amounts of warm, moist tropical air and produces heavy rain, mountain snow, and flooding.
What is an atmospheric river?
A long, narrow band of concentrated water vapor flowing through the lower atmosphere. Typically a few thousand miles long and around 100 miles wide. A strong one can carry more water than the Amazon River. NOAA estimates 6 to 10 are active globally at any moment.
How often does a Pineapple Express hit the U.S. West Coast?
Major multi-day events the scale of December 2014 or January 2023 hit once or twice a decade. Moderate atmospheric river events are more common, with several per winter typically reaching California, Oregon, or Washington.
Are atmospheric rivers getting stronger?
Research suggests yes. A warmer atmosphere holds more water vapor (about 7% more per degree Celsius), and the strongest atmospheric rivers have been intensifying over the past four decades. Scripps and NOAA continue to track the trend.
Are atmospheric rivers always bad?
No. Lower-category atmospheric rivers (Cat 1 and Cat 2) are primarily beneficial. They deliver most of California’s annual rainfall and refill the Sierra Nevada snowpack that feeds Western U.S. agriculture. Only Cat 3 and above tilt toward hazard.
How do I prepare for a Pineapple Express in my region?
Clear gutters and storm drains before December. Stock a 72-hour kit. Plan an evacuation route if you live near burn-scarred hillsides or in a known flood zone. Sign up for county emergency alerts. Monitor the Farmers’ Almanac long-range forecast for early signals.
Jaime McLeod
Jaime McLeod is a longtime journalist who has written for a wide variety of newspapers, magazines, and websites, including MTV.com. She enjoys the outdoors, growing and eating organic food, and is interested in all aspects of natural wellness.
- Jaime McLeod
- Jaime McLeod
- Jaime McLeod
- Jaime McLeod
