What is the Atmospheric River?

What makes the Pineapple Express a big deal for California
Published: Jan. 28, 2021 at 8:58 AM CST
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California - The normally fair weather of the Golden State has turned grey with buckets of rain and snow. The reason? An “Atmospheric River” of moisture has been unleashed on the west coast this week. This Atmospheric River is caused by a low pressure system and a high pressure system steering a ribbon of moisture from the tropics near Hawaii into the west coast. Since the origin is near Hawaii and the nature of the moisture flow is quick (7.5-15x faster than the flow at the mouth of the Mississippi River), this is called the Pineapple Express. The Pineapple Express is a narrow “river” of concentrated water vapor that is transported from the tropics into the west coast. On average Atmospheric Rivers are 250-375 miles in width and account for 30-50% of the west coast’s annual rainfall in just a few atmospheric river events.

A low pressure system off the northern west coast and a ridge of high pressure in the central...
A low pressure system off the northern west coast and a ridge of high pressure in the central US steers tropical moisture from Hawaii into the southern west coast, making The Pineapple Express(KBTX)

The Pineapple Express accounts for most of the region’s climatological rainfall. Reports of 11 inches of rain have already poured in from this Atmospheric River event. This rain is much needed in the drought filled state, but it also creates a hazard of “debris flows”. Debris flows are essentially fast-moving mudslides that are caused by sudden rainfall on recently burned, dry ground. The ground is unable to absorb the newly introduced amount of rain, and it causes threatening landslides. Debris flows are fast-moving and can cause damage to life and property because little to no warning is available for the time and place they may occur.

Chalk Cliffs, CO in 2009
Chalk Cliffs, CO in 2009(US Geological Survey)

This atmospheric river also brings moisture into the Sierra Nevada mountain range, allows for an intense amount of snowfall. 2 feet of snow have already been reported and some forecasted localized 3-5 feet of snow possible with this system. This is beneficial to water supply and ski slopes but does pose a threat to travel.

The Atmospheric River that affected SoCal this week was classified as a category 3 which means it is equally beneficial as it is destructive.

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