Pineapple Express: What Is It?

A mid-winter phenomenon known as the Pineapple Express is on its way to bring heavy rain and mountain snow to the western U.S.

It sounds fruity but this well-known plume of moisture, or atmospheric river, received its name by way of the direction it flows. As the river in the sky streamed from the tropical Hawaiian Islands to the West Coast, it became known as the Pineapple Express.

The Pineapple Express forms during the mid-winter months when the jet stream shifts and allows for tropical moisture to be brought further north. As the water vapor reaches the West Coast, it is forced upward into the atmosphere where it can cool and condense forming rain clouds. This rain can go on for days at a time in some events or they can last for a couple of hours. As the rain reaches the mountains, the moisture within the clouds is forced even higher into the atmosphere allowing for the rain to change to snow. Several feet of snow can accumulate in the mountains in the strongest events.

Although this event usually accounts for plenty of drought-extinguishing rainfall, it can often cause flash flooding, especially in areas that were previously affected by wildfires. As wildfires scorch the soil, the soil has a hard time soaking in moisture and lets the water runoff. This can eventually cause mudslides which, if occurring near a roadway, can block roads for days at a time. 

A recent Pineapple Express event in the early months of 2017 caused historical flooding and snowfall across California. Northern California was hit the hardest when it came to the heavy rain. However, the rain and snow ended one of the Golden State’s worst droughts ever. The annual rainfall is measured in “water years,” which runs from October-September.The 2016-17 water year in California quickly broke the 34-year-old record with 89.7 inches of water, converted from the snow amount, accumulating along the Northern Sierra by mid-April 2017. The Pineapple Express was the facilitator of this record event. 

Also, by mid-April 2017, the snowfall totals thanks to the consistent plume of atmospheric moisture rose to almost 600 inches across Mammoth Mountain, Calif. That’s FIFTY feet of snow! West Coast meteorologists referred to the events as “endless plumes of moisture,” especially since records had been shattered 5 months before the water year technically ended. 

When a Pineapple Express is expected, be sure to prepare early. If you approach a water-covered roadway, “Turn Around, Don’t Drown,” as water levels could be deeper than they appear. Also, keep an emergency kit in your vehicle, especially if you are near a dam and are forced to evacuate. This kit should include fresh water, non-perishable food, blankets, a cell-phone charger, any medication and any important copies of documents you may need. Also be sure to have cash on hand if power goes out and ATMs are unavailable for use.

Image: Flood under the Old Route 49 bridge crossing over the South Yuba River in Nevada City, California, saw local and regional visitors during the atmospheric river event across Northern California on January 9, 2017. Kelly M. Grow/ California Department of Water Resources. 9 January 2017.