California has been hit by what the National Weather Service calls a “seemingly endless parade” of powerful storm systems that began late December last year and are still ongoing. Called atmospheric rivers, these are long, narrow streams of very moist air that can cross oceans and bring large amounts of rain or snow when they land. These storms bring much of the western precipitation, but they also cause most of the region’s flooding, with associated economic losses of up to $1 billion annually.
A series of storms this winter have killed more than a dozen people in California and sent tens of thousands more under evacuation orders and surveillance. Rain on December 31, 2022 reached 5.5 inches in downtown San Francisco and south he flooded all six lanes of his Highway 101 in the city of San Francisco. On January 8, heavy rains and 70 mph winds cut power to more than 345,000 people in the state capital, Sacramento.
With more atmospheric rivers predicted in the next few days, flash floods are threatening across California, and recent wildfires have caused devastating mud and debris flows that have left 21 burn scars across the state. It may occur. Governor Gavin Newsom said he declared a state of emergency on January 4th, and the White House he declared a presidential state of emergency in California on January 8th.
To learn more about why these storms are hitting California and their potential dangers and benefits, visit Scientific American We spoke with Caterina Gonzalez, an extreme weather expert who studied atmospheric rivers as a graduate student at Stanford University and is now a postdoctoral fellow at the University of Minnesota.
[An edited transcript of the interview follows.]
Weather forecasters didn’t expect the California winter to be this wet. Why?
We often try to use El Niño and La Niña (large weather patterns in the Pacific) as surrogates for prediction. Simply put, El Niño is wet and La Niña is dry. This was his third year of La Niña and expectations were high for his first two years when the winter was not very wet.
Why did this year’s predictions fall short?
Simple stories are not always true. Northern California is at the apex of the dry-wet pattern. It’s kind of crap. Either method is fine. El Niño and La Niña can tip the scales wet or dry, depending on what’s happening along the coast.
What else is causing these powerful storms in California, and are they rare?
There is all sorts of chaos going on in the atmosphere, but it can stay in a certain configuration for a while, causing the storm truck to get stuck. A storm track is a jet stream-like wind current that carries a river of moisture from west to east. La Niña was able to further tweak the storm track towards Northern California and now favors the landfall of atmospheric rivers on this part of the coast.
Atmospheric rivers line up, very strong and very wet. This convergence is the core of the problem. It’s rare that a storm comes like this and it’s super juicy.
Mountains usually squeeze water out of atmospheric rivers. But much of the moisture from last week’s California storms reached Minnesota, dropping a foot and a half of snow. There is too much moisture in the system. It’s unusual that there are so many.
What is the impact of this atmospheric river parade?
The first storm saturates the soil and soaks it up like a sponge, holding as much water as it can and draining any more. Rivers, streams, and reservoirs also fill up with water, which can cause flooding.
The silver lining is snow cover. It’s January, so it’s cold, and most of the mountains are covered with snow. Fresh snow underfoot. If this were his March, it would be warmer and rain on top of the snow. The snow melts and the water reserves decrease, causing flooding.
California has been hit by a drought for three years. Did you finish?
We are still in drought conditions. Snow cover and reservoirs are good, so surface storage is fine. However, the aquifer is still depleted. Groundwater needs to be recharged, which takes a long time. We cannot rely on atmospheric rivers to save us. California has extremely wet and dry areas. That is our present reality and our future. we have to prepare.
How is climate change affecting atmospheric rivers? And how can California prepare?
Atmospheric rivers are becoming more and more violent due to climate change because they hold more water. For a long time, I thought we should put more effort into science to improve forecasting. But we need to adapt to the climate change that has already begun. You can’t predict how to get out of a more massive atmospheric river.
Huge investments need to be made in green infrastructure, such as floodplains, parks and rain gardens, that use nature to absorb runoff. Our infrastructure was built for his 20th century climate that no longer exists. More intense days are coming and these storms are just a preview.
