Intensifying nor'easter is bringing heavy snow, winds and coastal flooding to the Northeast

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Millions of people in California were under flood alerts Tuesday as an atmospheric river makes its way over a region already drenched by rain.

An atmospheric river is a plume of moisture that helps carry saturated air from the tropics to higher latitudes, delivering unrelenting rain or snow. Typically 250 to 375 miles wide, atmospheric rivers can stretch more than a thousand miles long, the National Oceanic and Atmospheric Administration says.

Atmospheric rivers aren’t a new phenomenon — in the western US, they account for 30% to 50% of annual precipitation, according to the National Oceanic and Atmospheric Administration. However, the eastern half of the US also experiences atmospheric rivers, with moisture pulled from the Gulf of Mexico.

While atmospheric rivers are an incredibly important source of rainfall, they can also bring flash flooding, mudslides and landslides, sometimes killing people and destroying property.

Impact of climate change: As the world warms, the atmosphere can hold more moisture – which will lead to rainier atmospheric river events.

“It’s expected that as the air temperatures increase, the air can hold more water vapor, and therefore any storms that are comprised of water vapor will have more of it,” said Jason Cordeira, associate professor of meteorology at Plymouth State University.

Atmospheric rivers will be “significantly longer and wider than the ones we observe today, leading to more frequent atmospheric river conditions in affected areas,” a NASA-led study found. The frequency of the most intense atmospheric rivers will likely double, the study found.