High-elevation snowpack loss during the 2021 Pacific Northwest heat dome amplified by successive spring heatwaves

A heatwave in June 2021 exposed Pacific Northwest (PNW) snowpack to record temperatures, allowing us to probe seasonal snowpack response to short-term heat extremes. Using high-resolution contiguous snowpack and temperature datasets (daily 1 km 2 SNODAS, 4 km 2 PRISM), we examined daily snowmelt in cooler, higher-elevation zones during this event, contrasted with the prior 18 years (2004–2021). We found that multiple early season (spring) heatwaves, concluding with the 2021 heat dome itself, resulted in dramatic early season melt including the most persistent fraction of PNW snowpack. Using longer-term station records (1940–2021), we show that springtime +5 °C daily anomalies were historically rare but since the mid-1990s have doubled in frequency and/or intensity, now potentially affecting typically cool La Niña periods (2021). Collectively, these results indicate that successive heat extremes drive rapid snowmelt, and these extremes may increasingly threaten previously resilient fractions of seasonal snowpack.