Changes to rainfall, snowfall, and runoff events during the autumn-winter transition in the Rocky Mountains of North America

In cold conditions, early winter precipitation occurs as snowfall and contributes to the accumulating seasonal snowpack. In a warming climate, precipitation may occur as rainfall in mountainous areas. Detecting changes during seasonal transitions is difficult because these may encompass changes in timing, magnitude and phase, which may not be consistent between years. In this study, a sampling window from September to December is used to assess trends in magnitude, frequency and duration (of rainfall, snowfall and runoff events) in 127 climate stations and 128 watersheds with more than 30 years of record across the Rocky Mountains of North America. Rainfall events have increasing frequencies and durations, and magnitude trends are predominantly increasing at mid-latitude and mid-elevation. Snowfall events have the largest numbers of significant trends, with both increasing and decreasing trends in each of magnitude, frequency and duration. Snowfall events have decreasing frequencies and durations in the northern low-elevation sites and increasing frequencies and durations at the southern high-elevation sites. Trends in runoff events are less common than for rainfall and snowfall but are greater than expected by chance, and show similar frequency and duration patterns to snowfall trends. Snowfall and runoff events are decreasing in frequency and duration north of Wyoming and increasing to the south. Snowfall magnitude is generally decreasing to the north and increasing to the south, with runoff magnitude trends showing the reverse.