California flood risk rises as snow melts

California flood risk: Side-by-side orbital views of green and brown landscape. Large blue swaths in right panel.
Tulare Lake, long dry, began to reemerge in March 2023 as flood water spread across farm fields. This image shows how the snowmelt has created flooding over the course of a month. The California flood risk is rising as the epic snowfall is melting. Image via NASA Earth Observatory.

Help! EarthSky needs your support to continue. Our yearly crowd-funding campaign is going on now. Donate here.

By Chad Hecht, University of California, San Diego

California flood risk rises

To get a sense of the enormous amount of water due to atmospheric rivers dumped on the Western U.S. this year and the magnitude of the flood risk ahead, take a look at California’s Central Valley, where about 1/4 of the nation’s food is grown.

This region was once home to the largest freshwater lake west of the Rockies. But the rivers that fed Tulare Lake were dammed and diverted long ago, leaving it nearly dry by 1920. Farmers have been growing food on the fertile lake bed for decades.

This year, however, Tulare Lake is remerging. Runoff and snowmelt from the Sierra Nevada have overwhelmed waterways and flooded farms and orchards. After similar storms in 1983, the lake covered more than 100 square miles (260 square km). And scientists say this year’s precipitation is looking a lot like 1983. Communities there and across the west are preparing for flooding and mudslide disasters as record snow begins to melt.

We asked Chad Hecht, a meteorologist at the Center for Western Weather and Water Extremes at the University of California San Diego’s Scripps Institution of Oceanography, how 2023’s storms compare to past extremes and what to expect in the future.

How extreme were this year’s atmospheric rivers?

California average is about 44 atmospheric rivers a year. But, typically, only about six of them are strong storms that contribute to most of the annual precipitation total and cause the kind of flooding we’ve seen this year.

This year, in a three-week window from about December 27, 2022, to January 17, 2023, we saw nine atmospheric rivers make landfall. Five of them fell into the category of strong or greater magnitude. That’s how active it’s been, and that was only the beginning.

In all, the state experienced 31 atmospheric rivers through the end of March: one extreme, six strong, 13 moderate and 11 weak. And other storms in between gave the Southern Sierra one of its wettest Marches on record.

Map of U.S. West Coast with many multicolored arrows from Pacific Ocean to coastline.
This chart shows where atmospheric rivers hit during the 1st half of the 2023 water year, which started October 1, 2022. The arrows show where the storms were strongest, but their impact reached far wider. Image via Center For Western Weather and Water Extremes, Scripps Institution of Oceanography.

Water throughout the west

These storms don’t just affect California. Their precipitation has pushed the snow-water equivalent levels well above average across much of the west. That includes Oregon, Nevada, Utah, Idaho and the mountains of western Colorado, Arizona and New Mexico.

In terms of records, the big numbers this year were in California’s Southern Sierra Nevada. The region has had 11 moderate atmospheric rivers – double the average of 5.5 – and additional four strong ones.

Overall, California has about double its normal snowpack. Some locations have experienced more than double the number of strong atmospheric rivers they typically see. The result is that Northern Sierra snow water content is 197% of normal. The central region is 238% of normal, and the Southern Sierra is 296% of normal.

Map of western U.S. divided into many sections each with a number in it.
Snow water equivalent is a measure of the water in snowpack. Many basins across the west were well over 200% of average in 2023. Image via NRCS/ USDA.

Mountain snow creates California flood risk

There is a lot of snow in the Sierra Nevada. And it is going to come off the mountains at some point. It’s possible we are going to be looking at snowmelt into late June or July in California, and that’s far into summer here.

Flooding is certainly a possibility. The closest year for comparison in terms of the amount of snow would be 1983, when the average statewide snow water content was 60.3 inches (153 cm) in May. That was a rough year, with flooding and mudslides in several parts of the west and extensive crop damage.

This year, portions of the Southern Sierra Nevada have passed 1983’s levels. And Tulare Lake is filling up again for the first time in decades. Tulare Lake is an indication of just how extreme this year has been, and the risk is rising as the snow melts.

Weather whiplash from extreme drought to record snow

California and some other parts of the west are known for weather whiplash. We frequently go from too dry to too wet.

2019 was another above-average year in terms of precipitation in California. But after that we saw three straight years of drought. We went from 13 strong or greater magnitude atmospheric rivers in 2017 to just three in 2020 and 2021 combined.

California relies on these storms for about half its water supply. But if the west gets too many atmospheric rivers back to back, that starts to have harmful impacts, like the heavy snowpack that collapsed roofs in the mountains this year, and flash flooding and landslides. These successive storms are typically referred to as atmospheric river families and can result in exacerbated hydrologic impacts by quickly saturating soils and not allowing rivers and streams to recede back to base flow between storms.

Map of western U.S. with yellow patches in north, green and blue patches nearly covering southern section.
The onslaught of powerful atmospheric rivers pushed precipitation to well above average across large parts of the west in 2023, following 3 years of severe drought. Image via Center for Western Weather and Water Extremes, Scripps Institution of Oceanography.

Are atmospheric rivers becoming more intense?

There’s been a lot of research on the impact of temperature because of how reliant California is on these storms for its water supply.

Atmospheric rivers are long, narrow corridors of water vapor in the sky that typically start in the tropics as water evaporates and is pulled poleward by atmospheric circulations. They carry a lot of moisture. On average, their water vapor transport is more than twice the flow of the Amazon River. When they reach land, mountains force the air to rise, which wrings out some of that moisture.

In a warming climate, the warmer air can hold more moisture. That can increase the capacity of atmospheric rivers, with more water vapor resulting in stronger storms.

Research by some of my colleagues at Scripps Institution of Oceanography also suggests that California will see fewer storms that aren’t atmospheric rivers. But the state will likely see more intense atmospheric rivers as temperatures rise. California will be even more reliant on these atmospheric rivers for its snow, which will result in drier dries and wetter wets.

Animation with large brightly colored streams flowing from mid-Pacific to North American west coast.
An example of an atmospheric river approaching the West Coast. Image via Space Science and Engineering Center/ University of Wisconsin-Madison.

Forecasting the future

So, we’re likely to see this whiplash continue, but to a more extreme level, with longer periods of dry weather when we’re not getting these storms. But when we do get these storms, they have the potential to be more extreme and then result in more flooding.

In the more immediate future, we’re likely headed into an El Niño this year, with warm tropical Pacific waters that shift weather patterns around the world. Typically, El Niño conditions are associated with more atmospheric river activity, especially in Central and Southern California.

So, we may see another wet year like this again in 2024.

Chad Hecht, Research and Operations Meteorologist, Center for Western Weather and Water Extremes, University of California, San Diego.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: The California flood risk is rising as the epic snowpack is melting in the warmer spring weather. Learn more about the flooding risk here.

Read more: Atmospheric rivers now have intensity rankings

Read more: California superbloom carpets landscape in color

The Conversation

April 19, 2023

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

EarthSky Voices

View All