To get a sense of the sheer volume of water discharged from atmospheric rivers in 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 has grown.
This region was once home to the largest freshwater lake west of the Rockies. But the rivers that fed Lake Tulare were dammed and diverted long ago, making it almost dry in 1920. Farmers have been growing food on the lake’s fertile soil for decades.
This year, however Tulare Lake is coming back together. Runoff and snowmelt from the Sierra Nevada flooded waterways and inundated farms and orchards. After similar storms in 1983, the lake was covered over 100 square milesand scientists say this year’s precipitation is much like that of 1983. Communities there and across the West are preparing for flooding and landslides as pick up snow starts 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 the storms of 2023 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 six of those are strong storms that contribute the bulk of total annual precipitation and cause the kind of flooding we’ve seen this year.
This year, in a three-week period from approximately December 27, 2022 to January 17, 2023, we saw nine atmospheric rivers come ashore, five of which classified as strong or greater size. That’s how active it has been, and that was just the beginning.
In total, the state experienced 31 atmospheric rivers at the end of March: one extreme, six strong, 13 moderate, and 11 weak. And other storms in between gave the southern Sierra one of the wettest marches on record.
These storms aren’t just affecting California. Their precipitation has pushed up snow-water equivalent levels well above the average in much of the Westalso 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 another four strong ones.
In general, California has approx double his normal snowpack, and some locations have experienced more than double the strong atmospheric rivers it normally sees. As a result, the snow water content in the Northern Sierra 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 entail?
There is lots of snow in the Sierra Nevada, and it will come out of the mountains at some point. It’s possible that in California we’re looking at snowmelt until the end of June or July, which is well into the summer here.
Flooding is certainly a possibility. The closest year for comparison in terms of the amount of snow would be 1983, when the statewide average snow water content was 60.3 inches in May. That was one tough yearof floods and mudslides in different parts of the West and extensive crop damage.
This year, parts of the southern Sierra Nevada have surpassed 1983 levels, and Tulare Lake fills up again for the for the first time in decades. Tulare Lake is indicative of how extreme this year has been, and the risk increases as the snow melts.
The transition from extreme drought in 2022 to record snow was swift. Is that normal?
California and some other parts of the West are known for whiplash. We often 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 consecutive years of drought. We went from 13 strong or larger atmospheric rivers in 2017 to just three in 2020 and 2021 combined.
California trusts for these storms about half of the water supplybut if the West gets too many atmospheric rivers in a row, it starts having damaging effects, like the heavy snowpack that collapsed roofs in the mountains this year, and flash flood And landslides. These successive storms are usually referred to as atmospheric river families and can result in exacerbated hydrological effects by quickly saturating the soil and preventing rivers and streams from receding to the base stream between storms.
Are atmospheric rivers intensifying with a warming climate?
Much research has been done on the impact of temperature due to California’s dependence on these storms for its water supply.
Atmospheric rivers are long, narrow corridors of water vapor in the air that usually start in the tropics as water evaporates and is drawn towards the poles by atmospheric circulations. They transport a lot of moisture – their water vapor transport is on average more than twice the flow of the Amazon River. When they reach land, mountains force the air to risethat 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 are not atmospheric rivers. But the state will probably see more intense atmospheric rivers as temperatures rise. California will depend even more on these atmospheric rivers for its snow, resulting in drier dry and wetter rains.
So we’ll probably see this whiplash continue, but at a more extreme level, with extended periods of dry weather when we don’t get these storms. But if we get these storms, they could be more extreme and result in more flooding.
In the near future, we probably are heading for an El Nino this year, with warm tropical waters in the Pacific Ocean changing 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 in 2024.