California weather and the Pineapple Express atmospheric river, explained

California weather and the Pineapple Express atmospheric river, explained

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When extreme weather events occur, strange and often ominous terms appear – “firenado”, “polar vortex”, “thunderstorm”. On Wednesday, however, California declared a state of emergency as the latest in a string of storms hit the West Coast with a surprisingly sunny name: the Pineapple Express.

The storm, which has brought winds in excess of 100 mph and could cause flooding and mudslides, is no beach day, however. So what exactly is a Pineapple Express?

The powerful storm strain gets its name from its origins in the tropical Pacific around Hawaii and the island nation’s affinity for the sweet treat. Pineapple Express storms carry moisture north from the tropics and dump it in high concentrations on the West Coast and Canada.

To make matters worse, this particular storm system is also a rapidly intensifying “bomb cyclone” system (another storm called a threat that refers to the rate at which air pressure drops).

Bomb cyclone enters high-impact atmospheric river in California

Driven by a powerful southerly jet stream, which is strongest in winter, according to the American Meteorological Association, the Pineapple Express is sometimes compared to a “conveyor belt” of moisture. It could bring up to 5 inches of rain a day, the National Oceanic and Atmospheric Administration says.

Californians prepared for another massive winter storm on Jan. 4 by putting down sandbags and staying indoors. (Video: Julie Yoon/The Washington Post)

Pineapple Express storms are a particularly well-known type of “atmospheric river,” considered a fundamental feature of the earth’s water cycle. They can be useful — supplying fresh water and even mitigating droughts or putting out wildfires — but they can also hit the West Coast and Canada with dangerous amounts of snow and rain. Scientists have warned that they could worsen amid climate change.

These rivers in the sky can stretch thousands of miles long and are often only a few hundred miles wide. The largest freshwater “rivers” in the world, they can hold more than twice the volume of the Amazon.

They also occur elsewhere – in the United Kingdom and the Iberian Peninsula, for example, which receive moisture from the Caribbean. In February, Brisbane, Australia, received 80 percent of its typical annual rainfall in three days from an atmospheric river.

Similar to hurricanes, atmospheric rivers are ranked from 1 to 5. The scale—which goes from “mostly beneficial” to “mostly dangerous”—corresponds to how much moisture they carry and how long they last in a given area. The rating system was not developed until 2019.

On Wednesday, the Center for Western Weather and Water Extremes predicted the next Pineapple Express would reach a Category 3, a “strong” event, in the San Francisco Bay area. In the coming days, it could rise to 4, but the organization warns that it is difficult to make predictions too far ahead.

While California is known for its long dry spells, the Golden State is no stranger to such weather events. The researchers found that from 1979 to 2019, atmospheric rivers of varying intensity hit the West Coast an average of 24 times a year. In October 2021, one brought some relief to California after a record dry spell.

And atmospheric rivers have hit the region again in recent weeks. In December, such events dumped 11.6 inches of rain on San Francisco, The Washington Post reported.

Scientists have predicted that such extreme weather — dry-to-wet precipitation events — could increase by 25 to 100 percent in California by the end of the century. And as the planet warms, atmospheric rivers may become wider, longer and more intense, studies have suggested.

Mike Branom and Kasha Patel contributed to this report.

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