What is it, exactly? Pineapple Express: an immense river

Several storm systems pose a risk of heavy rains and strong winds across the West this week and next week. These rains could bring flooding, landslides, and coastal troubles. The West’s stormy pattern is common during the winter months and there is even a term for the parade of storms that sweep through the area: The Pineapple Express. What is it exactly? Let us explain.

The Pineapple Express is not exactly a surface storm, it is a type of atmospheric river. Imagine a river of moisture flowing higher up in the atmosphere. This weather phenomenon is a large, strong, and rather persistent flow of warm and moist air coming from the waters to the northeast of Hawaii coming onto the West.

For it to be considered a “Pineapple Express” the storm must originate from the northeast of Hawaii. Pineapples are a loved commodity in Hawaii, many recipes involve the high-in-Vitamin-C fruit, hence the name “Pineapple Express” coined in the 1960s by TV weathercasters.

These moisture-filled rivers happen mostly during winter months as there is plenty of moisture that plumes from the tropics with “peaks” that extend to the northeast. As the jetstream becomes more undulated, it extends and grabs these tropical moisture peaks transporting it to the east, eventually reaching the West coast. It’s like the storms jump on a train that takes them right onto the West.

One storm after the other arrives in the Pacific Northwest with plenty of moisture to create flash floods, coastal flooding, and erosion. Its effects are experienced over the mountains, where heavy snow usually sets records, but the heavy precipitation can also cause landslides and mudslides as it becomes too heavy for it to sustain. These phenomena are important because it provides a big chunk of the mean annual runoff of the east and west coast.

Atmospheric rivers, such as the Pineapple Express, can be greatly beneficial to drought and could put out wildfires. But all this moisture is also future fuel for future wildfires. Water allows vegetation to grow more plentiful, and when this is not treated, it often becomes fuel for when the fires spark.

Atmospheric Rivers can set up an entryway anywhere from California to Washington. They often happen over several days and usually warmer than normal temperatures are experienced to the southeast of where the storms are entering. This is because there is often a high pressure in place which keeps the southwesterly flow.