| Summary: | Cold-season storms are responsible for generating most of the snow that accumulates in mountainous watersheds across the western United States, but with overwhelming evidence of warming temperature trends, this seasonal snowpack is at risk for melt, The vast majority of snow trend studies utilize undifferentiated air temperature records – these studies do not segregate days with precipitation from days without precipitation. This aggregation may create a potential bias and dampen or amplify temperature signals relevant for snowpack monitoring. This study investigates the hydrologic significance of winter storm day metrics on precipitation regimes across the western United States. In this work, daily SNOTEL meteorological data were used to: 1) investigate the temporal evolution of storm temperature, accumulation and number of storm days over the last 30 years (WY1984–2016), 2) characterize phase differences within these variables by largescale modes of climate variability – the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Pacific North American pattern (PNA), the Arctic Oscillation (AO), and 3) characterize the influence of atmospheric rivers on storm metrics in Oregon. Monthly storm day temperatures are significantly different from undifferentiated monthly temperatures, with the average difference varying by location and time of the year. Statistically significant trends for snow-accumulating storm days show declines in fall storm days and widespread warming across the west. Snow-depleting storm days show spring cooling in eastern Oregon, Idaho, Montana, Wyoming and Utah, with declines in daily snowmelt in both the Cascades of the Pacific Northwest and the Basin region. Across the west, positive phases of the ENSO and the PNA patterns were associated with fewer, warmer storm days with lower daily accumulation amounts. In the coastal and intermountain west regions, positive AO phases were associated with colder storm days and lower daily accumulation amounts. Storm day metrics ...
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