Frontal interaction with the Sierra Nevada as a precursor to intense frontogenesis over the Great Basin

On 25 March 2006 an intense cold front developed rapidly over the Great Basin producing one of the strongest frontal passages in Salt Lake City over the past 25 years. Prior to the frontal development, a cyclone, upper-level cyclonic potential vorticity anomaly, and attendant occluded front made landfall along the northern California coast. High-density surface observations provided by the MesoWest cooperative networks and numerical simulations by the WRF model suggest that this front slows and weakens as it moves across northern California and impinges on the Sierra Nevada and Cascade Mountain ranges. Meanwhile, a new surface cold front forms and intensifies over the Great Basin as the upper-level cyclonic potential vorticity anomaly moves downstream of the Sierra Nevada. The hourly temperature fall accompanying this new front increases from <2.5°C to 12°C in about 10 h as the front progresses across Nevada. This study examines the antecedent conditions responsible for this rapid frontogenesis, including the apparent discontinuous evolution of surface fronts across the Sierra and a comparison with idealized studies of frontal interaction with mountain barriers.