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What is an atmospheric river?

We explain what an atmospheric river is, how it's different from a bomb cyclone and how it's related to a Pineapple Express.

SEATTLE — What's an atmospheric river? And how is it different from a bomb cyclone? And how is it related to a Pineapple Express? 

These questions are top of mind as California was pummeled with a series of atmospheric rivers causing widespread flooding and evacuations. 

An atmospheric river will start to impact western Washington beginning on Thursday, according to the National Weather Service

What is an atmospheric river? 

According to the National Oceanic and Atmospheric Administration (NOAA), atmospheric rivers are "relatively long, narrow regions in the atmosphere, like rivers in the sky -- that transport most of the water vapor outside of the tropics." 

Atmospheric rivers carry roughly the amount of water vapor equivalent to the average flow of water at the mouth of the Mississippi River. They can cause extreme rainfall and flooding, disrupt travel, induce mudslides and cause catastrophic damage to life and property, according to the NOAA. 

How are a bomb cyclone and an atmospheric river different?

While an atmospheric river is a long, narrow region of water vapor, a bomb cyclone is a term used when a storm system strengthens rapidly over a certain period of time. 

Both weather events can result in heavy amounts of snow or rainfall.

A bomb cyclone, otherwise known as bombogenesis, is a term used by meteorologists when a cyclone rapidly intensifies or strengthens over the course of a day, according to the NOAA. A storm can be considered a bomb cyclone if pressure drops by a certain amount within a 24-hour period, which is measured by millibars. 

The number of millibars pressure needs to fall by in order for a storm to be considered "bombogenesis" changes based on latitude. At 60 degrees latitude, a drop of at least 24 millibars over 24 hours classifies. At the latitude of New York City, the required drop is about 17.8 millibars over 24 hours, according to the NOAA. 

Storms that undergo bombogenesis are among the most "violent weather systems," according to Accuweather.

What is a Pineapple Express?

A Pineapple Express is a strong atmospheric river that builds up in the tropical pacific, according to the NOAA. 

Moisture that builds up around Hawaii can impact the West Coast of the U.S. and Canada with heavy rainfall and snow. Winds cross over "warm bands of tropical water vapor" to form the "river" that travels across the Pacific Ocean as part of something called the global conveyor belt, according to the NOAA.

The global conveyor belt is a "constantly moving system" in the ocean driven by changes in temperature and salinity. 

When the "river" of water vapor reaches the West Coast, the Pineapple Express can dump as much as five inches of rain in one day.

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