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In June 2013, Keith Musselman was living in the Canadian Rockies when the nearby Bow River flooded. “We were in a valley, so we were stuck for about five days,” Musselman told Ars. “The community was devastated.”
The flood was one of the costliest and most devastating natural disasters in Canada’s history, with five people killed, more than 100,000 evacuated, and extreme property damage. Heavy rainfall falling on late snow in the mountains had overwhelmed rivers and reservoirs, and Musselman, a hydrologist, realized that this kind of rain-on-snow flooding wasn’t properly understood.
“Forecasters have a good handle on what happens when rain falls,” he says. “But when that rain falls in mountains where there’s deep snow, we don’t have a good handle on what the flood volume will be.”
Musselman and a group of colleagues used historical data to sketch out what the rain-on-snow flood risk looks like across western North America—and how that risk is likely to change in a warming world. Their paper, published in Nature Climate Change this week, finds that some areas will see reduced risk thanks to declining snowfall. But at higher elevations, there’ll still be snow, and the increasing amount of rain falling on that snow will have implications for flood risks.
Hexbyte – Tech News – Ars Technica | The perfect storm
At drier and higher elevations, rainfall on snow isn’t so much of a problem, either because there’s not enough rain or not enough snow for it to matter. But when the conditions are just right, heavy rainfall can combine with snowmelt to produce floods that are more catastrophic than the rainfall would cause.
When Musselman moved to work at the National Center for Atmospheric Research (NCAR), his colleagues there had also identified this as something they wanted to study—and “they had a great dataset,” Musselman says. “We used that to understand how rare these large flood events are.” Work has been done on this before, but this team was able to use more fine-grained data to build up a detailed picture.
They built a simulated world of weather systems that would produce rain-on-snow floods in the same places and with the same frequency as the 13 years of historical data they used. Once they were sure that the simulation matched the historical patterns of snowpack, snowmelt, and other factors, they used it to build up a picture of how those kinds of events might change in the future.
The model showed shifting patterns: areas that currently have a high risk of flooding will see a decrease, as precipitation will occur more frequently as rain than snow. But at higher elevations, where once there would have been snow falling on a snowpack, rising temperatures will now cause rain to fall on that snow. The Sierra Nevada, Canadian Rockies, and Colorado River headwaters saw the highest increases in risk, with floods twice as likely in some cases.
Philip Mote, who studies climate-related snowpack c