What’s a Pineapple Express? Diving into this robust atmospheric river
Atmospheric rivers are responsible for some of the West Coast’s heaviest rainfall events
Atmospheric rivers are big business on the West Coast. These events carry moisture high in the atmosphere directly from the tropics to fuel intense rains across British Columbia.
Every once in a while, we hear about a Pineapple Express headed for the coast. But what’s the difference between a ‘regular’ atmospheric river and a Pineapple Express?
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Diving into a Pineapple Express
An atmospheric river is a narrow band of enhanced moisture aloft that streams from the tropics into the middle latitudes.
Low-pressure systems can guide atmospheric rivers into the West Coast and fuel dayslong bouts of heavy precipitation throughout the region.
Some particularly potent atmospheric rivers can originate from a vast reserve of moisture that lingers near the Hawaiian Islands. These events are commonly dubbed a “Pineapple Express.”
Not every atmospheric river is a Pineapple Express, but all Pineapple Express events are atmospheric rivers.
Atmospheric rivers can trigger flooding rains
Communities under the influence of an atmospheric river can see 100+ mm of rain in short order, carrying the risk for widespread flooding. A sharp rise in freezing levels can cause rapid snowmelt and an avalanche danger across the alpines.
Meteorologists recently started ranking atmospheric river events on an AR scale, which is somewhat similar to the Saffir-Simpson Hurricane Wind Scale.
A robust Pineapple Express event can easily rank AR3 or higher, capable of producing hazardous conditions as a result of flooding rains, snowmelt, and potential avalanches.
What kind of precipitation can a strong event produce? Relentless moisture streaming into southern California courtesy of a Pineapple Express covered the region with drenching rains in February 2024. The weather station in downtown Los Angeles recorded 216 mm of rain in just three days—equivalent to 60 per cent of the city’s average annual rainfall.
Header image created using graphics and imagery from NOAA and Canva.