Three days above zero, even at night. This is one of the conditions used by researchers to recognize “snow eaters”, the heat waves that arrive when there is still snow in the mountains and manage to make it disappear at an impressive speed. During these events, snow loss can be approximately double the seasonal average. And in the western United States they are taking more and more territory and coming earlier and earlier.
The name literally means “snow-eater” and seems like that of a creature with quite an appetite. This time the image works too well. A new study published in Science Advances reconstructed these episodes in the mountains of the American West from 1850 to 2015, trying to understand how long they last, how often they occur and above all what they do to the snowpack.
When even the night stops cooling the snow
The authors define the snow-eater heat waves as periods with exceptionally high temperatures for the season and above freezing, maintained for several consecutive days both during the day and during the night. The minimum threshold used to identify them is three days. In these conditions the snow continues to receive energy without the usual nightly pause and the melting accelerates.
To reconstruct them, the group led by Alan Rhoades used data from 20th Century Reanalysis Version 3the TempestExtremes algorithm, and SNOW-17, an operational model for estimating snowmelt. The estimates were then compared with observations from the SNOTEL mountain stations. A fair amount of calculations to give a scientifically usable name to something that, when viewed from a mountain, is quite simple: it was cold, unseasonably warm weather arrives and the snow goes away much faster.
The identified events generally last three to five days and, since the 1950s, three to five have occurred on average during a melt season. On the days affected by snow eatersthe melting is approximately double compared to normal seasonal values.
There is also a link that closely affects those who manage rivers and basins. Of the 11 major spring flood events fueled primarily by snowmelt analyzed in the study, seven occurred within five days of one of these heat waves. A strong temporal proximity, which the authors consider useful above all for improving the predictions of the most extreme mergers and the management of water resources.
The “snow eaters” always arrive first
The most interesting part of the reconstruction lies in the direction taken by the phenomenon in over a century and a half.
Since 1850, the area affected by snow eaters has increased on average by about 102,000 square kilometers each century. To give an order of magnitude, it is a surface area equal to about a third of Italy. The frequency has also grown, by almost one event per century, while the average duration has shortened slightly, by about a quarter of a day per century.
Then there’s the calendar. The first episode of the season now appears about a month earlier for each century that has passed. It means that conditions capable of rapidly devouring the snowpack are increasingly entering the part of the year in which that snow should still function as a water reserve.
This detail weighs heavily in the American West. The mountain snowpack in fact works like a gigantic natural reservoir: it accumulates water in the cold months and progressively releases it with the spring and summer melting. Anticipating that release means having a lot of water all at once when it can create problems and finding less months later, when it is needed for agriculture, cities and ecosystems. The US National Integrated Drought Information System considers the amount of snow and the timing of melting to be central factors for summer water availability.
In 2026 it happened before the eyes of researchers
The study reconstructs the period up to 2015, so the last decade remains outside the trend analysis. 2026, however, offered a very eloquent example of what the authors are trying to measure.
In March a very strong heat wave swept through much of the Western United States. According to NIDIS, the record heat, together with the lack of precipitation, caused a sudden and very early melting of a snowpack that already started from exceptionally low levels. The same report from the US body defines the March episode as a “snow-eater heat wave”.
The result can be seen in the numbers. On April 1, 2026, Arizona, Colorado, Idaho, Nevada, New Mexico, Oregon, Utah and Wyoming recorded the lowest values ever recorded for that date of SWE, the “snow water equivalent”, i.e. the quantity of water contained in the snow, since the beginning of SNOTEL measurements in the 1980s. California had the second lowest value. Across the West, 64% of SNOTEL monitoring points and manual readings had reached or equaled an all-time low.
The snow had also reached its seasonal maximum 21 to 34 days earlier than normal, depending on the state. In Colorado the peak arrived on March 9, almost a month early; in numerous Utah stations the snow had already completely disappeared between 30 and 50 days before the usual dates.
The problem continues when the mountain turns brown again. Little snow that disappears early gives soils and vegetation more time to dry and can therefore bring forward the fire season. NIDIS specifies that the relationship is not automatic: favorable local conditions and a trigger are still needed. However, the persistent heat of 2026 had already accelerated the drying of the landscape and increased concerns about summer fires.
The study covers mountains in the western United States, so those numbers can’t be directly transferred to the Alps or other mountain ranges. However, the method developed by the researchers can be applied elsewhere, precisely to understand where and how much these events affect the melting of the snow.
The snow eaters they have thus found a definition precise enough to be able to be recognized and, in the future, better predicted. Three or five days may seem like a small thing. For a mountain that stores some of its summer water as snow, they can be pretty hungry.
