Chad Hecht, University of California, San Diego
To get a sense of the enormous amount of water 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 a quarter 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, 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 with 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 averages about 44 atmospheric rivers a year, but typically, only about six of them are strong storms that contribute 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 Dec. 27, 2022, to Jan. 17, 2023, we saw nine atmospheric rivers make landfall, five of them categorized as 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.
These storms don’t just affect California. Their precipitation has pushed the snow-water equivalent levels well above average across much of the West, including in 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 an additional four strong ones.
Overall, California has about double its normal snowpack, and some locations have experienced more than double the number of strong atmospheric rivers it typically sees. 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.
What risks does all that snow in the mountains create?
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 for 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 in May. That was a rough year, with flooding and mudslides in several parts of the West and extensive crop damage.