California braces for more ‘brutal’ flooding and mudslides as experts warn it won’t quench historic drought

By Rachel Ramirez, CNN

Parts of drought-plagued California are facing an onslaught of powerful storms to start the new year, bringing flooding rainfall and even mud and debris flows to the state.

The latest in the series of storms are expected to reach the coast Wednesday morning, and while the entire state will see impacts by the end of Thursday, Northern California and the Bay Area are likely to see the worst of the weather.

A so-called “bomb cyclone” over the Pacific Ocean — named because of how rapidly it intensifies over a short period of time — will sling a series of fronts at the West Coast. These fronts are being super-fueled with tropical moisture from a potent atmospheric river that stretches west to Hawaii.

While the prolonged wet conditions will provide some relief to the drought conditions, the rain has proved too much too fast.

According to the National Weather Service, the storm could trigger more widespread flooding, roads washing out, hillside collapsing, fallen trees, major power outages, “immediate disruption to commerce, and the worst of all, likely loss of human life.”

“This is truly a brutal system that we are looking at and needs to be taken seriously,” the NWS Bay Area office added.

The storms are called “atmospheric rivers” because they are essentially a conveyor belt of concentrated moisture in the atmosphere emerging from the warm waters of the Pacific Ocean. A similar storm unleashed rains, deadly floods, debris flows and hurricane-force winds, particularly in Northern California including the Bay Area, over the weekend.

It’s all happening against the backdrop of a yearslong, climate change-fueled megadrought that has drained the state’s reservoirs and triggered water shortages. These storms usher in much-needed rainfall and snow to the state. But Daniel Swain, climate scientist at the University of California in Los Angeles, said it is not enough to erase the decadeslong deficit that the unrelenting drought has built up.

“This is really going to help a lot with the short-term drought in Northern California, perhaps even erase short-term drought conditions, but it’s going to take a lot more to completely obviate the longer term, multi-year drought impacts,” Swain said, emphasizing that Wednesday’s atmospheric event will be a “high-impact storm.”

This dramatic swing in periods of drought and high precipitation, or weather whiplash, can occur more often and become more intense under a rapidly warming climate. And scientists say the chances of these sudden transitions happening in California will become much higher, if humans continue to pump out planet-warming gases.

‘Two sides of the same coin’

Climate researchers have said it’s a lack of precipitation, higher temperatures, and an increase in evaporative demand — also known as the “thirst of the atmosphere” — that has pushed the West’s drought into historic territory.

As wells run dry and reservoirs drain, Julie Kalansky, a climate scientist at Scripps Institution of Oceanography in San Diego, said these storms are desperately needed more than ever to alleviate the drought, despite the hazards they bring in some areas.

“They’re two sides of the same coin: they can be extremely beneficial because they bring so much of California’s water supply to the state or they can also be drought busters,” Kalansky told CNN. “But when the duration becomes too long, they become too strong, they come back-to-back, and the landscape doesn’t get an opportunity to absorb all the rain, it can lead to this flooding.”

This winter is already showing some signs of respite for a state that’s still almost entirely in drought conditions. Large reservoirs in Northern California including lakes Shasta and Oroville, are slowly being replenished. Meanwhile, smaller reservoirs like Folsom Dam saw an increase of roughly 40 feet of water in three days.

Swain said the storms will have largely improved dry conditions in Northern California in the short term. But in the long term, he said climate change has already made its mark and that it would take a lot more than one exceptionally wet year — it will take consecutive wet years and cooler conditions to bust this drought.

“In a warming climate, the severity of droughts in places like the Southwest and California are being driven by increasing evaporative demand,” he said. “Essentially, the atmosphere is requiring more water as temperatures rise, so you’d actually need more precipitation than you used to have to balance that out — and we’re not necessarily seeing more precipitation than we used to.”

Persistent drought with periods of excessive rain

An average atmospheric river carries more than 20 times the water the Mississippi River does, but as vapor. California is prone to floods from these storms as they come ripping off the Pacific Ocean, and major floods from them have happened before — but climate change is raising the stakes with millions of people likely to be impacted.

Rainfall in parts of California exceeded 8 inches over the weekend as the last storm moved across the state. Oakland saw its wettest day on record on December 31 when 4.75 inches of rain fell, and San Francisco marked its second-wettest day with 5.46 inches — nearly half of its typical December rain.

A 2022 study authored by Swain found that climate change has already doubled the chances of a disastrous megaflood happening in California in the next four decades — a storm unlike anything anyone alive today has ever experienced.

And while the recent series of storms isn’t the “big one” yet, the study paints a picture of what the state could face as the planet warms.

“We haven’t seen the mega floods, but we have definitely seen hints of increasingly extreme precipitation even in the middle of what has otherwise been a period characterized by a pretty severe and persistent drought,” Swain said.

Yet despite this wet start to the year, it’s worth noting that last year was relatively much wetter around this time — and the state was still mired in drought for the remainder of 2022.

“The face of droughts is changing,” Swain said. “It’s easier and easier to get into a drought — even following a really wet winter — because we just have that growing evaporative demand and hotter summers.”

“Multi-year droughts are going to look different than they used to,” he said.

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CNN’s Brandon Miller contributed to this report.