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'Atmospheric river' brings heavy rainfall to western US

Normally, weak systems bring beneficial rain or snow to the western U.S., but larger and stronger ones can cause catastrophic damage due to heavy rainfall or snow.

SACRAMENTO, Calif. — The West Coast is seeing heavy rain, strong winds, flooding and mudslides as a result of a strong atmospheric river. 

In Sacramento, Calif., after 200 days of no rainfall, the area received more than 5 inches of rain in a single day. According to sister-station KXTV-TV, it is the second-wettest day ever recorded in the area. 

For context, KXTV says between Oct. 1, 2020, and Sept. 30, 2021, the area recorded only 6.61 inches of rain. 

Most of the western U.S. is currently suffering from a bad drought, with 46 percent of California, 27 percent of Oregon and 25 percent of Washington under exceptional drought, the most severe category of drought conditions, according to sister-station KREM.

The extremely wet weather is caused by two weather phenomena. According to NOAA, bombogenesis, also known as a "bomb cyclone," occurs when a low-pressure midlatitude cyclone rapidly intensifies, dropping at least 24 millibars over 24 hours.

Accompanying this storm is an atmospheric river, which is a flowing column of condensed water vapor in the atmosphere responsible for producing significant levels of rain and snow, predominantly in the Western United States, according to NOAA

When atmospheric rivers move inland and sweep over mountains, the water vapor rises and cools to create either rain or snow. 

While many atmospheric rivers are weak systems that provide "beneficial" rain or snow to the area, some are larger and more powerful. 

These can create "extreme" rainfall and flooding that can cause mudslides and "catastrophic" damage.

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