Overview of ParallelClim-WestUS-Daily – high-resolution observed and counterfactual daily climate data for the western United States, 1951–2025

Abstract. Extreme climate events and trends commonly elicit curiosity as to the role of anthropogenic climate change. In the western United States (US), severe droughts and heatwaves over the first quarter of the 21st century have strained water-management, agricultural, and ecological systems while also putting humans at risk from wildfires and hydrological hazards. Understanding whether and how climate change has promoted or intensified these processes requires estimating anthropogenic effects on a wide range of climate variables, including background mean conditions, higher-frequency variances, and how these changes have been distributed intra-annually and spatially. Here we introduce ParallelClim-WestUS-Daily, a new dataset of observed daily, high-resolution (~4 km) gridded climate data for the western US from 1951–2025 as well as a parallel, counterfactual realization of the observed sequence of climate but excluding anthropogenic changes in background mean climate or daily variances. Our dataset includes eight surface climate variables commonly used in hydrological, vegetation, and wildfire modeling: daily precipitation total, daily maximum and minimum temperature, and daily means of vapor pressure, wind velocity, downwelling shortwave and longwave radiation, and surface pressure. Anthropogenic climate change is estimated as the weighted mean change among 24 climate models relative to the second half of the 19th century according to the CMIP6 Historical experiment through 2014, extended with SSP2-4.5 for 2015–2025. Model weightings are based on agreement with global surface temperature trends, limiting the influence of models with likely unrealistic climate sensitivities to greenhouse-gas forcing. The ParallelClim-WestUS-Daily dataset will allow new opportunities to explore effects of anthropogenic climate change on western US hydroclimate and complex hydroclimatic impacts such as soil moisture, snowpack, streamflow, vegetation productivity, and wildfire.