The Golden State is about to get drenched.

A major storm system, called a mid-latitude cyclone — air and clouds rotating around a region of low atmospheric pressure in this part of the world — is helping carry a potent stream of moisture into California. Areas of low pressure invite air to pour into a region, often bringing clouds and rain, and this specific stream of moisture is called an “atmospheric river.”

Atmospheric rivers are formidable bands of moisture that often deluge California with rain and snow in the winter, sometimes to damaging degrees. Spinning mid-latitude cyclones often drive these long bands of moisture, as they pull the atmospheric river behind the storm. This latest high-altitude river will bring deluges to parts of already-soaked California on Wednesday and Thursday.

“It is forecast to impact much of California, bringing widespread heavy and excessive rainfall,” Allison Santorelli, a meteorologist with the National Weather Service, told Mashable.

Coastal ranges will receive lots of rain and wind, she said. Interior mountains will see snow and gusty winds. Landslides are likely, especially on burn scars from recent huge fires. Northern California will see the most rain, but Southern California will experience plentiful precipitation, too. Crucially, this adds up to dangerous driving conditions. “A strong system will bring heavy rainfall Wednesday – Thursday,” the National Weather Service’s Sacramento Office tweeted. “Expect areas of urban flooding, & rises streams, creeks, & rivers. Stay alert if living near streams and creeks, follow evacuation orders, and be especially cautious driving at night.”

Check your local National Weather Service office for the most relevant local updates. Much of Northern California, for example, is under a “Flood Watch.”


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Overall, atmospheric rivers are integral to California’s water supply. Lower-intensity storms supply the Golden State with bounties of water, filling the state’s colossal reservoirs and nourishing the region’s famously productive farms. These storms supply the state with some 30 to 50 percent of its annual water. But potent, high intensity atmospheric rivers often mean too much water in too short of time. This translates to flooding, especially when the ground is already soaked.

That’s why meteorologists expect significant floods. “This storm could be more hazardous than beneficial in some locations,” Santorelli said.

In the bigger picture, storms generally have boosted odds of dropping extreme rains in a warmer climate. That’s because when air temperature is warmer the atmosphere can naturally hold more water vapor (heat makes water molecules evaporate into water vapor), meaning there’s more water in the air, particularly in many humid or rainy regions. Consequently, this boosts the odds of potent storms like thunderstorms, mid-latitude cyclones, atmospheric rivers, or hurricanes deluging places with more water.

“Once you have more moisture in the air, you have a larger bucket you can empty.”

“Once you have more moisture in the air, you have a larger bucket you can empty,” Andreas Prein, a scientist who researches weather extremes at the National Center for Atmospheric Research, previously told Mashable. As research shows, this can result in pummeling downpours. “You can release more water in a shorter amount of time — there’s very little doubt about that,” Prein said.

Atmospheric rivers, specifically, can pack a damaging punch. Scientists have found that the largest of these winter phenomena cause billion-dollar flooding disasters. And they’re getting worse. “They are becoming more intense with climate change,” Tom Corringham, a postdoctoral research economist at the Scripps Institution of Oceanography, told Mashable. Critically, just small increases in an atmospheric river’s intensity drive big increases in damages. “As we see more superstorms, we’re going to see really big impacts on the economy,” Corringham said.

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California’s latest atmospheric river will likely be followed by more such storms.

“It should be noted that it could be the first in a series of atmospheric rivers that we’re looking at over the next week,” Santorelli said.

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