The Reynolds Creek Experimental Watershed (RCEW; 240 km^2 area) in southwestern Idaho, USA was designated in 1961 as an outdoor laboratory to monitor hydro climatology and grazing practices in semiarid agricultural rangelands. Growing season streamflow in the RCEW is strongly influenced by water stored in large snow drifts that persist until the late summer. However, a large proportion of the catchment exists within the elevational rain to snow transition zone, signaling that small shifts in precipitation phase from snow to rain may alter the streamflow regimes that downstream farmers and ranchers have depended on for more than a century. This work will quantify both seasonal and elevational changes in precipitation phase over the last 40 years. Beginning in the early 1980s, automated meteorological stations were steadily installed across the catchment elevation gradient (1099 – 2240 m asl), enabling detailed monitoring of when and where precipitation fell as snow. This spatial analysis is culled from an hourly, 10-meter resolution gridded dataset of air temperature, relative humidity, wind speed and direction, incoming solar radiation, and precipitation derived from station measurements for the time period 1 October 1983, to 30 September 2023. Calculated hourly wet-bulb temperature was used to parse falling precipitation as either snow, rain, or mixed snow and rain across the watershed using a thresholding approach. Here, we present the workflow used to derive precipitation phase, the associated trends in the spatial dataset, the relationships between those trends and measured streamflow, and finally the ways in which these trends may relate to other semiarid catchments around the globe.