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Utah climate scientist weighs in on California Megaflood

Atmospheric rivers are sinews of moisture from the tropics. The one pictured here appeared over the Northern Pacific on Jan. 3.
NOAA
Atmospheric rivers are sinews of moisture from the tropics. The one pictured here appeared over the Northern Pacific.

California is in a historic drought but recently the state has made headlines for quite the opposite reason: megafloods. About once every 200 years the state has experienced a month-long rainstorm so extreme that it devastates the landscape and leaves indelible geological marks. Research published last week shows that with every degree of temperature increase in the Earth’s atmosphere, the likelihood of a megaflood-causing storm nearly doubles. This has implications for states along the West Coast, and even states as far away as Utah.

Climate scientist and recent Utah State University doctoral graduate, Jacob Stuivenvolt Allen, said this type of storm, sometimes called the ARkStorm, is caused by atmospheric rivers, corridors of fast-moving air that carry water from the tropics to the North. He said that atmospheric rivers are constantly forming as a natural part of the weather system, but as climate change continues, they can carry too much of a good thing.

“There's a point at which atmospheric rivers are beneficial, we're getting a lot of water, and in a time of a drought that’s really useful. But then at some point, if the atmospheric river is too strong, it becomes really hazardous. So this is what we've seen with a lot of the flash flooding and the mudslides. Especially after a drought year, or a fire, if an atmospheric river comes in, it can be really detrimental to the landscape and to the folks in those communities,” Stuivenvolt Allen said.

He said that an ARkStorm occurs when the right weather ingredients mix to create the conditions for a stable corridor of atmospheric rivers. So instead of one to two days of rain California might typically experience from one transient atmospheric river, during an ARkStorm it’s pummeled with a month of successive rainstorm rivers. Although scientists still aren’t sure exactly what conditions are needed to cause an ARkStorm, they do know that climate change increases the risk for a California megaflood in three ways.

“Climate change is expected to increase the likelihood of a certain type of El Niño that is exactly associated with more of these ARkStorm events. Climate change can impact the amount of moisture that air can hold. So if you take a room full of air, and you increase the temperature by a degree Celsius it can hold almost 7% more moisture. We will move away from having more primarily beneficial ARS-- those kinds of lower intensity ARS that are good for bringing water resources-- and will move towards more hazardous atmospheric rivers. And then the third mechanism is rain on snow. Because in a warmer atmosphere it is more likely that we will get rain at lower elevations. And if that rain is falling on existing snowpack, that's a really big flooding hazard,” said Stuivenvolt Allen.

According to Stuivenvolt Allen, California, Oregon and Washington are at the greatest risk during an ARkstorm event, but 30-days of back-to-back atmospheric rivers would also threaten the Intermountain West.

“When California gets a few strong ARS we often get them, we get remnants in Utah, they call them ‘inland penetrating’. So you know, the best snow days, the best days for me to go ski are often when a big atmospheric river has come through. What's going to happen if we have 30 days of successive precipitation in the Intermountain West? I think the focus is deliberately on California, for good reason, but an atmospheric river doesn't stop at the Sierra Nevadas. It keeps moving. And especially if we have a few weeks of consistent atmospheric river type precipitation, that could have adverse impacts or large impacts for much of the Intermountain West as well. The impacts would not stop at the Sierra Nevadas,” Stuivenvolt Allen said.

In addition to increasing the frequency of ARkStorms, computer modeling shows that climate change will significantly increase precipitation rates during these storms, making their effects even more damaging.