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Global warming is affecting ecosystems worldwide, with mountainous regions particularly vulnerable. At mid-altitudes, changes in winter precipitation will have immediate effects on lake functioning. Lake Tovel (1178 m a.s.l.), a deep (39 m) aquifer-fed lake in the Brenta Dolomites (Italy), showed unusually high water temperatures in 2017. As evidenced by a principal component analysis, monthly data from 2000 to 2016, including the hot European year 2003 and the locally warmest year 2015, did not show comparably high water temperatures. Volume-weighted mean water temperature from June to September 2017 was 1.3-2.1 degrees C higher than long-term temperatures (mean 2000-2016: 6.6-8.8 degrees C). Warmer air temperatures in December 2016 and February and March 2017 led to a shift in winter precipitation from snow to mostly rain. The consequent lack of a spring snowmelt event resulted in whole-lake warming and increased stability of the water column, as evidenced by volume-weighted mean water temperature and Schmidt stability. Stable isotopes were used as tracers for the origin of lake water. The yearly mean delta O-18 of lake water in 2017 was more enriched (-10.9 parts per thousand) with respect to the long-term mean (2009-2016: -11.9 parts per thousand), supporting a pluvial instead of a nival origin in 2017. The temporary warming of Lake Tovel is an early sign of future changes in mountain hydrology, and we advocate for increasing attention to lake catchments considering their impact on many ecosystem services.