The term, coined in a new study published in Science Advances, describes spring heat waves that can nearly double the rate of snowpack melt. Snow typically melts at about 9 millimeters of water per day during the March-to-July snowmelt season. But on snow-eater heat wave days, that rate jumps to about 17 millimeters per day, according to the study.
"There was basically a doubling of the rate of snowmelt during those days, compared to other days," said Dan McEvoy, a researcher with the Desert Research Institute in Reno and a co-author of the study.
Not every warm spring day qualifies. Researchers defined these events by sustained temperatures at least 6 degrees Celsius above normal for three or more days — and, critically, staying above freezing through the night. That overnight warmth keeps the snowpack from refreezing, so instead of melting only at the surface, the entire snowpack "ripens" and becomes primed to melt fast.
These events are also becoming more frequent and arriving earlier in the year, according to the study, led by Lawrence Berkeley National Laboratory, in collaboration with the Desert Research Institute and several other institutions.
Since 1850, the study found the area covered by these heat waves has grown by roughly 102,000 square kilometers per century, and they're now arriving about a month earlier in the year than they did in the mid-1800s. Since the 1950s, the Mountain West has typically seen three to five of these heat waves each year.
Researchers pointed to the 1983 Memorial Day heat wave — which triggered flooding along Utah's Wasatch Front and forced Salt Lake City to build emergency canals — as a real-world example.
McEvoy said the rapid melt these events cause can be difficult for water managers to plan around, since reservoirs are typically built to handle snow coming down gradually.
"Is there room in the reservoir? They're used to it more slowly coming down the system," McEvoy said. "And that rapid melt has a much higher potential to cause flooding if there's enough snow left."
That rapid melt isn't good news even in a dry year. A snow-eater heat wave doesn't add water to the system — it just makes existing snowpack melt faster and earlier, leaving less available later in the summer when demand is highest. The study found that some of the largest snow-eater melt events on record occurred during major Western snow droughts, including California's 2013 to 2015 drought.
This story was produced by the Mountain West News Bureau, a collaboration between Boise State Public Radio, Wyoming Public Media, Nevada Public Radio, KUNR in Nevada, KUNC in Northern Colorado, KANW in New Mexico, Colorado Public Radio and KJZZ in Arizona as well as NPR, with support from affiliate newsrooms across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting and Eric and Wendy Schmidt.
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