# What an atmospheric river is, and what the Pineapple Express means Published November 10, 2026, 12:01 AM ET By Ryan Hall, Y'all Written by our automated news desk from National Weather Service data. An atmospheric river is a long, narrow stream of water vapor in the sky. The average one carries about as much water as the Mississippi River at its mouth, and the strongest carry up to 15 times that. Most are weak and often bring beneficial rain or snow, while some cause floods and mudslides. The Pineapple Express is a well-known strong atmospheric river that can carry moisture from the tropics near Hawaii to the U.S. West Coast. ## Key points - **Atmospheric river:** a long, narrow region of the atmosphere that carries most of the water vapor outside the tropics. - **How much water:** the average one carries about as much water vapor as the average flow at the mouth of the Mississippi River. Exceptionally strong ones carry up to 15 times that. - **Helpful or harmful:** most are weak and bring rain or snow the water supply depends on. Some cause extreme rain, floods and mudslides. - **Pineapple Express:** a strong atmospheric river that can carry moisture from the tropics near Hawaii to the U.S. West Coast. An atmospheric river is a long, narrow stretch of the sky packed with water vapor, moving along like a river overhead. When one reaches land, it often drops that moisture as rain or snow. Most are weak and often helpful, but the strongest can cause floods and mudslides, particularly in the western United States. This description comes from NOAA, and an atmospheric river is a weather pattern, not an alert. The alerts that come with one are the National Weather Service's heavy rain and flooding warnings. ## What an atmospheric river is Picture a ribbon of moist air stretching across the sky. It is long and narrow, and it moves a huge amount of water vapor from one place to another. Outside the tropics, these ribbons carry most of the water vapor that travels around the planet. Water vapor is invisible, so you can't see the river itself. You see what it does when it reaches land: rain or snow. That makes atmospheric rivers a key part of the global water cycle. They are closely tied to both water supply and flood risk, particularly in the western United States. ## How much water one carries They vary a lot in size and strength. Scientists compare them by taking the mass of the water vapor and dividing by the density of liquid water. That turns an invisible gas into a number you can set next to a real river. By that measure, the average atmospheric river carries about as much water vapor as the average flow of water at the mouth of the Mississippi River. The exceptionally strong ones can carry up to 15 times that amount. That is why two atmospheric rivers can feel nothing alike when they arrive. ## Why they can help or hurt Most atmospheric rivers are weak. They often bring beneficial rain or snow that is crucial to the water supply, which matters a great deal in the West. The dangerous ones carry the most water vapor and have the strongest winds. They can cause extreme rain and floods, often by stalling over watersheds that are already vulnerable to flooding. They can disrupt travel, cause mudslides and do catastrophic damage to life and property. A stretch of storms from December 10 to 22, 2010, shows how big a series can get. Atmospheric rivers fueled storms from western Washington to southern California. Some areas got 11 to 25 inches of rain, and the Sierra Nevada got 75 percent of its annual snow by December 22. ## What the Pineapple Express is The Pineapple Express is a well-known atmospheric river. It is a strong one that can bring moisture from the tropics near Hawaii to the U.S. West Coast. The name refers to its connection to tropical moisture near Hawaii. Not every Pineapple Express is a disaster. It is a strong atmospheric river, and what it does depends on how much water it carries and how long it sits over one spot. ## What to do - **Use the lead time.** NOAA research helps National Weather Service forecasters warn of potential heavy rain and flooding from atmospheric rivers as many as five to seven days ahead. If one is headed your way, that is the window to plan your travel and get ready. - **Take the warnings seriously.** A heavy rain or flooding warning is the real alert. Check [every active warning](/warnings) when a river of moisture is aimed at your area. - **Expect travel trouble.** The strongest ones can disrupt travel and cause mudslides, so build in extra time or change plans for the worst of it. - **Watch for stalling.** The worst flooding often comes when an atmospheric river parks over a flood-prone watershed instead of moving on. - **Get the alerts sent to you.** You can [sign up for free Y'all Alerts](/alerts) so a warning reaches you wherever you are. For where heavy rain is most likely to cause flooding right now, check our live [flash flood outlook](/excessive-rainfall). More on: [Weather explainers](https://ryanhallyall.com/news/topic/weather-explainers) ## Sources - [NOAA, "What are atmospheric rivers?"](https://www.noaa.gov/stories/what-are-atmospheric-rivers), NOAA --- Canonical: https://ryanhallyall.com/news/what-an-atmospheric-river-is From Ryan Hall, Y'all (https://ryanhallyall.com). Always follow official warnings from the National Weather Service.