As part of an ongoing study of the regional climate and hydrology of the southwestern United States, in this paper we investigate the systematic biases of two versions of the PSU/NCAR mesoscale model (MM4). These are a standard version and one that includes a more detailed treatment of radiative transfer, surface physics, and soil hydrology. We simulated the period 1–30 January 1979, in which nine Pacific storms moved across the western United States. Results from both model versions are compared to the large scale analysis used to provide initial and lateral boundary conditions. Both models show a lower tropospheric cold bias of 1–3 K near the surface over land and an upper tropospheric warm bias of less than 1 K, which suggest high model stability and reduced vertical mixing. The model atmospheres are wetter than that of the analysis, particularly in the lower troposphere and over the ocean. The wind magnitude bias is positive near the surface (∼1.5–3 m s−1), negative in the upper troposphere (... Abstract As part of an ongoing study of the regional climate and hydrology of the southwestern United States, in this paper we investigate the systematic biases of two versions of the PSU/NCAR mesoscale model (MM4). These are a standard version and one that includes a more detailed treatment of radiative transfer, surface physics, and soil hydrology. We simulated the period 1–30 January 1979, in which nine Pacific storms moved across the western United States. Results from both model versions are compared to the large scale analysis used to provide initial and lateral boundary conditions. Both models show a lower tropospheric cold bias of 1–3 K near the surface over land and an upper tropospheric warm bias of less than 1 K, which suggest high model stability and reduced vertical mixing. The model atmospheres are wetter than that of the analysis, particularly in the lower troposphere and over the ocean. The wind magnitude bias is positive near the surface (∼1.5–3 m s−1), negative in the upper troposphere (...