Flooding caused by rain falling on snowpack could more than double by the end of this century in the West and Canada, according to a study by University of Colorado-Boulder and the National Center for Atmospheric Research.

The greatest flood risk increases are projected for the Sierra Nevada, the Colorado River headwaters and the Canadian Rocky Mountains, the study said. Lower elevations in coastal regions of California, Oregon, Washington and maritime British Columbia could see decreases in rain-on-snow flood risk.

The findings were published in the journal Nature Climate Change.

Rain-on-snow flooding varies widely but can overwhelm downstream rivers and reservoirs.

In 2017, California’s Oroville Dam nearly failed, leading to the evacuation of 188,000 people and $1 billion in infrastructure damages.

“Rain-on-snow events can be intense and dangerous in mountainous areas, but they are still relatively poorly understood,” said Keith Musselman, lead author of the study and a research associate at CU Boulder’s Institute of Arctic and Alpine Research. “We can infer a little bit from streamflow, but we want to get better measurements and model more of the variables involved.”

To study rain-on-snow events, the researchers turned to a state-of-the-art weather modeling dataset developed at NCAR.

Known as CONUS 1, the dataset contains weather simulations across the U.S. in the current climate and a warmer future based on projected climate trends.

The data, which took NCAR’s Yellowstone supercomputer more than a year to compile – offer unprecedented detail and resolution.

“This high-res dataset allows us to resolve mountains in granular fashion and examine the factors that combine to melt the snowpack when a warm storm comes in and hits cold mountains like the Sierra,” Musselman said.

The authors found that in a warmer climate, less frequent snow cover at lower elevations would decrease the risk for rain-on-snow floods. But at higher elevations, where winter snow will accumulate despite climate warming, such as the High Sierra and Rocky Mountains, rain-on-snow events could become more frequent and intense.

The rain and melt produced during rain-on-snow events is projected to increase for a majority of western North American river basins as rain rather than snow affects more mountain watersheds, increasing the corresponding flood risk by as much as 200 percent in localized areas and potentially straining existing flood control infrastructure.

The researchers hope that continued investment in snowpack monitoring networks and efforts such as NASA’s Airborne Snow Observatory will provide additional ground information, allowing hydrologists and climate scientists to verify their models against observations and better inform flood risk assessment now and in the future.

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