Interaction of a warm-season frontal system with the coastal mountains of the western United States. Part I: Prefrontal onshore push, coastal ridging and alongshore southerlies

Observational analyses and numerical simulations are used to investigate the interaction between a warmseason frontal system/trough and the complex terrain of the western United States. Prior to frontal landfall, synoptically driven offshore flow was associated with the northward development of a thermal trough, and a coastal pressure ridge and associated southerly winds moved northward along the coasts of northern California, Oregon, Washington, and southwestern British Columbia. This coastal pressure ridge, coupled with the subsequent weakening of the offshore flow, resulted in the onshore push of cool marine air approximately 24 h before frontal passage. The onshore surge of cool marine air into warm, dry continental air associated with the thermal trough produced a mesofront that moved over the coastal and Cascade Mountains. Diagnosis of a realistic simulation of the event shows that both solar heating and adiabatic warming from downslope flow over the Rockies and the Cascade Mountains are important in producing the inland thermal trough. Model experiments without surface fluxes and orography indicate that surface fluxes are important in the formation of the prefrontal onshore push and that damming on the coastal mountains helps produce the mesoscale coastal ridge and alongshore southerlies.