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Weather Blog: California hit by another atmospheric river event

By Meteorologist Rob Duns,


Yet another push of heavy rain and snow moved over California today, the result of yet another atmospheric river event impacting the west coast.

Atmospheric river is the term used to describe the weather setup that’s been bringing the state paralyzingly deep snowfall and dangerous flash flooding. It revolves around a band of water vapor from the Pacific Ocean that directs rain and snow to move toward the U.S. west coast, with several battering the state since late 2022.

Though different from other natural disasters like hurricanes and tornadoes, atmospheric river events do share something similar in that they also have a category scale to convey to the public how big of a deal they’re expected to be. This is similar to the weather classification scales you’ve learned about in the past.

When we are in hurricane season, for instance, it is easy to judge the difference between the impacts of a Category 1 and Category 5 storm. Likewise, the extent of damage observed in the aftermath of an EF1 tornado varies greatly from that of an EF5 tornado.

However, unlike the hurricane and tornado scales, which make their judgement greatly around the wind, the category scale used to classify atmospheric rivers takes into consideration how long the event is expected to last and how much water vapor is involved.

Shorter events with less water vapor bring less impacts, while longer-lived atmospheric rivers that involve more water vapor bring significantly greater impacts.

You can get an idea of how atmospheric rivers break down in the graphic below.

Something interesting you’ll note when examining the categories is that the word “beneficial” appears more than once. That’s because although atmospheric rivers make the news because of the destruction they sometimes bring, they can also be helpful to the state by providing it with water. In fact, a Category 1 atmospheric river event is described as being mostly beneficial!

Too much of a good thing is when things get dicey, as Category 4 and Category 5 atmospheric river events can last for several days and involve so much water vapor that too much snow or rain falls in too short of a period of time. That’s when things get hazardous and for those caught in the middle of one, downright dangerous.

The most recent atmospheric river event impacting California today is considered a Category 4 event. Though it will clear out by Thursday, the volume of water vapor involved will unleash yet another major rain and snow event for parts of the hard hit state.

Learn more about the atmospheric river classification scale from the United States Geological Survey here.

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