California’s 10th Atmospheric River This Winter Causes Levee Breach and Thousands to Evacuate. Here’s What to Know About the Storms 

Thousands of Californians were forced to evacuate Saturday after the Pajaro River levee in the state’s central coastal region broke due to flooding caused by the tenth atmospheric river to hit the state this winter.

More than 8,500 people were under evacuation orders and warnings this weekend in the flooded agricultural region, joining the thousands of other California residents whose counties have been overwhelmed by severe weather this winter.

“My heart hurts tonight for the residents of Pajaro. We were hoping to avoid and prevent this situation, but the worst case scenario has arrived with the Pajaro River overtopping and levee breaching at about midnight,” said Luis Alejo, chair of the Monterey County Board of Supervisors, in a tweet.

The flooding that led to the 100-foot-wide levee breach was part of yet another atmospheric river in California this season—storms that carry huge amounts of precipitation, released dramatically as they make landfall.

Atmospheric rivers are particularly prevalent in the west coast and more powerful than most others. The robust precipitation the storms have brought this year has helped alleviate some of the state’s serious drought concerns, but also brought deadly snowstorms and overwhelming flooding throughout the state.

Officials are now exploring ways to harness the potential of atmospheric rivers to combat drought and help provide enough water for Californians, while also preparing to keep Californians safe from the increasingly dangerous storms.

Here’s what to know:

What is an atmospheric river?

Atmospheric rivers are long, narrow channels that run through the atmosphere transporting water vapor. Occurring all around the globe and spanning 250 to 375 miles wide on average, they shift and change impacted by other weather phenomena. Once atmospheric rivers make landfall, they often release the water vapor in the form of rain or snow.

Particularly strong atmospheric rivers—which are common in the western United States—carry large amounts of water vapor, fueled by gusty winds that can induce hazardous rainfall and flooding, often stalling above watersheds that are vulnerable to flooding. Powerful atmospheric rivers can carry condensed water vapor that equals 7.5 – 15 times the amount of water flowing at the mouth of the Mississippi River. Atmospheric rivers can create mudslides, cause serious damage to property and infrastructure, threaten air travel and even be fatal.

The atmospheric river at play this week is named Pineapple Express. The powerful and recurring Pineapple Express has taken the same route many times before, first building momentum when wind crosses over Hawaii’s tropical warm watershed and traveling across the Pacific Ocean, eventually reaching the west coast. In previous years, the Pineapple Express dumped up to five inches of rain on California per day, according to the National Oceanic and Atmospheric Association. About 30% – 50% of precipitation that falls in California each year comes from atmospheric rivers.

California’s challenges

State reservoirs that had been concerningly low are now filled above average for the first time this year due to increased rainfall and snow. California water managers are exploring methods to preserve storm water as a way to overcome the drought that’s plagued the state for the past three years.

On Friday, Governor Gavin Newsom signed an executive order aiming to make it easier for farmers and water agencies to refill aquifers with floodwater. Groundwater provides almost half the state’s water supply annually, but has been overdrawn for years now.

“California is seeing extreme rain and snow, so we’re making it simple to redirect water to recharge groundwater basins,” Newsom said. “This order helps us take advantage of expected intense storms and increases state support for local stormwater capture efforts.”

Nearly a third of the state’s water supply is derived from the Sierra Nevada mountain range. Snow levels in the range are currently more than double what they typically are in the spring when they’re at their peak, but the most recent atmospheric river has melted swaths of the lower snowpack.

The snowpack can usually absorb rain with its high elevation, reaching 14,000 feet, but snow on its lower peaks may begin to melt which could fuel more flooding.

Elsewhere in Southern California, at least four people have died this month as a massive snowstorm pummeled mountainous San Bernardino county, trapping people in remote homes and blocking roads in layers of snow, up to 10 feet high. Rescue efforts are still underway in the region, and it’s expected that the death toll may climb.

Newsom declared emergencies in 34 counties over the past several weeks and is expecting more federal assistance. Forecasters are already seeing signs of more atmospheric rivers off California’s coast encroaching in the coming weeks.