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The Pineapple Express Has Arrived In California

The Pineapple Express.  It’s not a train, but it is making its first, in what could be, a series of stops in California.  And it’s going to make huge impacts with varying types of weather.  We’ll get to that in a second.  But first, what exactly is The Pineapple Express?

It’s essentially an atmospheric river, a narrow region in the atmosphere, that transports moisture from the tropics northward.  The moisture builds up in the tropical Pacific, near Hawaii, where coincidentally, pineapples grow.  Prevailing winds carry warm bands of tropical water vapor, to form a “river” of moisture.  This river flows northeast, and makes its way onshore in California, and the Pacific Northwest.

The typical Pineapple Express setup is illustrated below.  High pressure in the western U.S. and low pressure in the eastern Pacific, helps funnel the tropical Pacific moisture towards the west coast.  This is accompanied by a strong polar jet-stream.


Looking at today’s water vapor imagery, we can easily see how this plume of moisture is working northeast from the tropical Pacific.

The blue colors indicate large amounts of water vapor over California, as well as the northwestern U.S.  The Pineapple Express is certainly alive and well.

Atmospheric rivers that contain large amounts of water vapor can bring heavy rainfall, which can lead to flooding.  This can have a negative impact on travel, and cause property damage.  Parts of California will receive between three and six inches of rain by Friday afternoon.  Flooding will be a distinct possibility, and flood alerts have been issued for areas in California and Nevada.

When the moisture reaches the mountains, temperatures are often cold enough to support heavy snowfall.  This again looks to be the case in the Sierra Nevada.  Twelve to twenty-four inches of snow are possible by Friday afternoon.  Winter weather alerts, including Winter Storm Warnings have been issued.

These conditions can certainly make life and travel difficult for many residents.  But the heavy rain and snow is also a good thing.  California residents count on the rain to replenish the reservoirs in the northern portion of the state.  These reservoirs are a critical part of California’s water supply.  And many hotels, restaurants, and resorts count on the snow to accommodate skiers, who often flock to the mountains after big snow events.  The Pineapple Express typically begins in November, and lasts through the winter.  So this may be just the first of several heavy rain and snow events in California in the coming months.

For WeatherNation:  Meteorologist Matt Monroe

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