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Winter storms in California will become more intense as climate change accelerates, study finds

The American River Parkway in Sacramento, Calif., Tuesday, Jan. 10, 2023.
Andrew Nixon
The American River Parkway in Sacramento, Calif., Tuesday, Jan. 10, 2023.

An estimated 32 trillion gallons of water — in the form of rain and snow — came down on California in a series of nine back-to-back atmospheric rivers between late December and mid-January.

To put this in perspective, that amount is just shy of the quantity of water held within Lake Tahoe, one of the deepest lakes in North America. The lake has, on average, about 37 trillion gallons of water.

These storms were destructive and deadly, claiming the lives of at least 20 people, and the estimated cost is likely to end up being in the billions. And new research is revealing these storms will likely become larger and drop even more rain than what we have experienced so far this winter.

Dr. Ruby Leung, an atmospheric scientist at the Pacific Northwest National Laboratory in Washington state, joined CapRadio’s Vicki Gonzalez to discuss what this means for California’s future.

This interview has been edited for length and clarity.

This research that you recently conducted suggests that powerful winter storms impacting California are only becoming wetter and larger. Can you put into context how much larger and wetter future storms could be compared to the series of very powerful storms that we just experienced? 

We use computer models to project the future. Our simulations go to [a] very high resolution so that we can look at storms particularly. We count the storms and we look at how they look, how big they are. And that's how we can summarize how the storms will be changing in the future.

We're particularly looking at the western United States where most of our storms are coming in the winter; most of our precipitation comes during December, January, February. And so what we found is that overall, per storm on average, we see that the precipitation amount will be increasing by about 30%.

This increase in the precipitation amount comes from two factors. Number one, the storms will become bigger. We find that the storm area will be increasing by about 22%. The storms are getting bigger, [but] they are also producing more intense precipitation. We see that the precipitation will increase by 9% in terms of intensity.

What we found in this study is that the precipitation increase [will not be] uniform within [future] storms. Usually we see a storm produce more precipitation near the center, and this is where we’ll see a much larger increase of precipitation in the future: Near the center of the storm, we [found] that precipitation will increase by about 19 to 20%. This is what we call "sharpening.”

Where does California stand, in terms of the intensity of these storms? 

When we look at the western United States, we look separately in three different regions: the Pacific Northwest, including Washington state and Oregon, and then we look at California and also the southwestern United States. What we see is that California is the place where you often get more winter precipitation than other states. And then looking at the future, the sharpening is even more [intense] in California compared to other states, meaning that the precipitation produced by the storms will be even more concentrated towards their centers.

Why is that? 

Partly the reason is that California has very interesting geography. So [the Sierra Nevada] mountains are really squeezing out a lot of water vapor from the atmosphere … And the Sierra Nevada is very narrow, it causes very sharp rising air and therefore, storms become more sharp.

We often talk about global warming in terms of temperature, but what is important to know is that with warmer temperatures, you also get more moisture in the atmosphere, just like when you go to the tropics. In the future, we expect more moisture and more precipitation in a more concentrated area because of the narrow mountain [range].

As opposed to historically relying on a snowpack, correct? 

Historically, we rely on the snowpack to give us water supply not only in the wintertime, but when the snow melts in the spring and in the summer. In the future [we are expecting] precipitation to increase and [become] more concentrated near the center. Because of warmer temperatures, precipitation will be more like rain rather than [snow] accumulating in the mountain snowpack. We would probably be getting less snowpack that would produce the water that we need in the summertime.

I would imagine that leads to water managers having conversations about how to adjust to this new normal. 

Exactly, yes, this is like a double whammy. During the winter time when you have a lot of precipitation, [that precipitation] becomes more intense, causing flooding because it is not falling as snowfall. Because [precipitation is] falling more as rain rather than snow, we don't get enough snowpack to provide water in the summer. So this is really difficult for water managers. How do you manage flooding in the wintertime and then not enough water in the summertime?

You also have research into the conditions that create a healthy snowpack in northern California. What are the ideal factors? 

The ideal factor for creating a good snowpack is of course we need to have precipitation, right? But also the temperature is important because if the precipitation comes in with warmer temperatures, then it would be rainfall, and the water would go into the river more immediately. So you need both factors: precipitation and temperature.

Are there other factors contributing to these more intense forecasted storms that aren’t rising to the top of the conversation, that you think needs to be discussed more? 

Well, first of all, I have to emphasize that just global warming is one of the biggest factors, because global warming affects everyone everywhere … There are also other factors, for example, land use change. When we have more urban areas, that can also cause changes like the temperature, more heat.

Looking across the country, are you seeing increased rain or severity of storms in other parts of the United States? 

Definitely. In fact, we have also looked at data not only projecting the future, but looking at data from the past to see whether storms might have changed. One of our studies showed that over the central United States … storms don't come in the wintertime like the storms that we have in the western United States. Their storms are mostly in the warm season, like in the spring and in the summer.

What we found is that based on data in the last few decades, the storms have already been changing over that part of the country, where the warm season or summer storms have been lasting longer and producing more extreme precipitation as well.

What can we do at an individual level? What advice do you have when it comes to being best prepared for what you are forecasting? 

Changing storms will affect our life, but having the knowledge that the storms will change can help us better prepare.

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