"Snow eaters" get a threshold: three to five days above freezing, day and night

A study in Science Advances has, for the first time, put numeric conditions on the spring and early-summer heat waves that researchers now call "snow eaters": temperatures that stay above freezing through both day and night for multiple days, typically three to five ^[1][2]. That matters because the same paper finds these events can roughly double the rate at which snow melts, which converts stored water into a flood-timing and reservoir-operations problem ^[3].

The definition is the useful part. A warm afternoon does not do much damage if the snowpack refreezes overnight; the overnight floor above freezing is what keeps melt running around the clock. Matthew LaPlante, a journalist and climate scientist at Utah State University who worked on the study, described waking after a warm night to find the snow level down by inches: "It feels like a monster came and just in the middle of the night, took a bite out of a snowpack" ^[4][5].

The trend numbers are the second reason to care. Since the 1850s, according to the study, the area of the Western United States affected by snow eaters has grown by an average of about 40,000 square miles per century, and the season's first snow eater has arrived about one month earlier per century ^[6][7]. Those rates work out to roughly 400 square miles a year and about three days per decade of earlier onset ^[8][9] - slow in any single season, large across a planning horizon built on the assumption that snow is a reservoir that releases in summer.

For water managers, the mechanism is straightforward and unwelcome. "Water that would otherwise be stored as snow comes out early, and then we have to deal with it as a hazard instead of a resource at our reservoirs and along the rivers and streams," said Ben Hatchett, a study co-author and scientist at Colorado State University's Cooperative Institute for Research in the Atmosphere ^[10].

There is a feedback that the threshold alone does not capture. Noah Molotch, a geography professor at the University of Colorado Boulder who was not involved in the study, said solar radiation is a major and sometimes overlooked driver of snowmelt, and that warming snow crystals lose structure and light-reflecting ability, so the pack absorbs more sunlight and melts faster ^[11][12]. Molotch also noted that much of the West saw record-low snowpack this spring, with Colorado short on precipitation and California receiving plenty of it as rain rather than snow; the common factor across the region was above-average air temperatures ^[13][14]. He linked that snowpack deficit to mountain forest drought stress and, in turn, to wildfire intensity and frequency, including fires from Utah to Spokane, Washington, where hundreds of homes burned ^[15][16].

Two limits are worth holding onto. The study covered only the Western United States, and LaPlante said snow eater heat waves almost certainly occur elsewhere without having been measured there ^[17]. And the label is older than the science: people in the West were talking about "snow-eating" chinook winds by at least the 1880s, and "snow-eater heat wave" only reached headlines in March, during an early heat wave that stripped snowpack in the Colorado Rockies and the Sierra Nevada ^[18][19].

What to watch: whether operational forecast products start flagging multi-day above-freezing-minimum spells as a distinct category, and whether the same threshold reproduces outside the Western US, where nobody has yet run the count ^[1][17].