Most winters, Bay Area residents hear the phrase “Pineapple Express” on the news and picture something vaguely tropical, with warm rain and dramatic flooding. That instinct is closer to the truth than it sounds. A Pineapple Express is an atmospheric river that draws its moisture from the tropical Pacific near Hawaii and carries it to California. It often arrives in amounts that strain every creek, reservoir and storm drain in its path. It is one of the most important weather patterns in the western United States, and the Bay Area sits squarely in its usual landing zone.
What Is a Pineapple Express?
Technically, a Pineapple Express is one kind of atmospheric river, a long, narrow band of concentrated water vapor in the sky. The Pineapple Express is the kind whose moisture comes from the tropical Pacific near Hawaii, roughly 15 to 25 degrees north of the equator. Not every atmospheric river that reaches California fits that description. Many draw their moisture from cooler waters farther north and arrive with different temperatures and rain totals.
In practice, a Pineapple Express carries an unusually large amount of water vapor, because warm tropical air can hold far more moisture than cooler air. When the jet stream steers that air toward California, it is forced to rise over the coastal ranges and the Sierra Nevada. As it rises and cools, it can release rain at rates that overwhelm drainage systems built for ordinary storms.
Why Is It Called That?
The name is informal, and many scientists simply call these storms tropical atmospheric rivers. “Pineapple Express” has stuck because it says something true. The moisture comes from near Hawaii, famous for its pineapples, and it arrives in a fast, steady stream. Water vapor can take several days to cross the Pacific. Once the stream is aimed at the coast, though, the rain can keep coming for a day or more.
The term has been in use for decades. Forecasters and reporters needed a quick way to describe warm, wet winter storms that behave differently from typical cold fronts. It works well for the public even though scientists prefer more precise language. When a forecaster says a Pineapple Express is on the way, it means a large plume of warm tropical moisture is headed for the coast.
The Path from Hawaii to California
A Pineapple Express does not travel in a straight line. It curves across the Pacific along the path of the jet stream. In winter, the jet stream can dip south over the Pacific and lock into a pattern that pulls subtropical air northeast toward California. The jet then acts like a conveyor belt, gathering tropical moisture into a narrow corridor that can stretch more than 1,000 miles.
In a typical event, a low-pressure system off the California coast pulls moist air up from the tropics, and the jet stream focuses it into that narrow band. The whole system then moves onshore and runs into the terrain. The coastal ranges catch the first heavy rain and the Central Valley floor gets moderate amounts. The western slopes of the Sierra Nevada get the heaviest totals, because that is where the moist air is forced up the most.
Historical Events: When Pineapple Express Storms Changed California
The New Year's storm of 1997 is one of the most studied Pineapple Express events in California history. It brought days of extreme rain to Northern California and dumped warm rain on a deep Sierra snowpack. The runoff pushed the Feather River and other rivers past their levees, and levee breaks flooded towns near Marysville and forced large evacuations. The event showed what happens when warm rain falls on months of built-up snow, because the melting snow adds its water to the rain at the same time.
February 2017 brought a similar story to Northern California. A series of atmospheric rivers that winter, several with tropical moisture, helped end the drought of 2012 to 2016 and refilled reservoirs across the state. At Oroville Dam, the tallest dam in the United States, the flood of water damaged the main spillway. Erosion below the emergency spillway then forced about 188,000 people downstream to evacuate. The storms of late December 2022 and early January 2023 were another reminder that warm, wet storms can be hard to contain.
Why These Storms Are Warmer Than Regular Winter Storms
The warmth of a Pineapple Express is one of its defining features, and it matters a great deal for flood risk. Because the moisture comes from the tropics, the air is warm, and the freezing level sits much higher than in a typical cold winter storm. In an ordinary Northern California storm, the freezing level in the Sierra might sit at 5,000 to 7,000 feet. In a Pineapple Express, it can rise to 9,000 to 11,000 feet or higher.
That difference changes how fast water reaches the rivers. Snow piles up in the snowpack and melts slowly over weeks or months, while rain runs off right away. When the freezing level rises to 10,000 feet, most of the Sierra's watershed sits below it. Much of the precipitation that would normally build the snowpack falls as rain and flows straight into rivers and reservoirs. Any snow already on the ground below that level can melt as well and add to the flood.
The Blessing and the Curse: Water Supply vs. Flooding
California has a mixed relationship with these storms, which can refill reservoirs drained by drought in a matter of weeks. Wet storms like these are a big part of how the state gets its water each wet season. That includes San Francisco's supply, the East Bay's water district and the farms of the Central Valley.
The trouble comes when a Pineapple Express arrives after a wet stretch, with soils already soaked and reservoirs near capacity. Water managers then face hard choices about their reservoirs. Releasing water before the storm lowers the flood risk, but that water may not be replaced if the rest of the winter turns dry. Holding it raises the risk of an uncontrolled overflow, so these storms force fast decisions with high stakes.
Deep Dive: What Sets the Snow Level in a Warm Storm?
The snow level is the height below which falling snow turns mostly to rain. It sits below the freezing level, because snowflakes take time to melt as they fall into warmer air. In a Pineapple Express, both levels climb high into the Sierra, and that difference decides how much of the storm runs straight into the rivers.
Why Does the Snow Level Sit Below the Freezing Level?
Snow does not turn to rain the moment it crosses the freezing level. A flake first develops a film of water, then slowly collapses into a raindrop as it keeps falling. The layer where this happens is called the melting layer.
Weather radars show the melting layer clearly. Wet, partly melted flakes reflect radar energy strongly, so they appear as a bright horizontal stripe called the bright band. The American Meteorological Society's glossary says the band is usually centered about 100 meters below the freezing level. Below it, the flakes collapse into smaller, faster-falling raindrops.
NOAA's Hydrometeorology Testbed has used small vertically pointing radars in the Sierra Nevada foothills to track the bright band through storms. Its height gives forecasters a direct reading of the snow level as it rises and falls. In one radar study, the freezing level sat about 1 kilometer above the bottom of the melting layer, so the gap can be large.
How Can Heavy Snow Lower the Snow Level?
Melting takes energy, and the falling snow pulls that energy from the surrounding air. In light precipitation, the effect is small. In a heavy storm, it can cool the air enough to lower the snow level.
By our calculation, precipitation falling at 0.2 inches of water an hour absorbs about 1.7 million joules per square meter each hour as it melts. If that cooling is spread through a layer 1,000 feet thick, the layer cools by roughly 8°F an hour. The air in the melting layer can then settle near 32°F, and snow reaches lower elevations than the freezing level forecast suggested.
That effect is much weaker in a true Pineapple Express. Warm air keeps flowing in from the subtropics and replaces the cooled air, so the freezing level often stays near 9,000 to 11,000 feet. When that happens on top of an existing snowpack, rain can melt the snow below that level as well, a rain-on-snow event that sends even more water downstream.
How to Prepare When One Is Coming
Forecast models can often spot a Pineapple Express taking shape several days before it reaches California. The Center for Western Weather and Water Extremes at UC San Diego publishes atmospheric river forecasts. As the storm approaches, the Weather Service's Bay Area office issues rainfall and flood forecasts, though there is no separate Pineapple Express warning. Instead, watch for flood watches and warnings, wind advisories and high surf warnings, and prepare as you would for any strong atmospheric river.
Before the storm, clear gutters and downspouts and move cars away from spots that flood. Know your evacuation routes and whether you live in a flood zone or a landslide-prone area. Sign up for your county's emergency alerts so warnings reach you directly. Keep 72 hours of supplies on hand, because these storms often bring strong winds as well as heavy rain, and wind-related power outages can last several days.
These storms are a permanent part of California's climate. As the oceans and air warm, future Pineapple Express events are expected to carry more moisture than past ones. The forecasts are good, the warning time is real and the preparation is simple. The most important step is to take the forecasts seriously rather than assume they are overblown. The 1997 and 2017 storms were forecast days ahead, yet they still caught many people unprepared.
