Discrete Frontal Propagation over the Sierra/Cascade Mountains and Intermountain West

On 25 Mar 2006, a complex frontal system moved across the Sierra–Cascade Mountains and intensified rapidly over the Intermountain West where it produced one of the strongest cold-frontal passages observed in Salt Lake City, UT during the past 25 years. Observational analyses and numerical simulations by the Weather Research and Forecast (WRF) model illustrate that the frontal system propagated discretely across the Sierra–Cascade Mountains and western Nevada. This discrete propagation occurs in a synoptic environment that features a mobile upper-level cyclonic potential vorticity (PV) anomaly that is coupled initially with a landfalling Pacific cyclone and attendant occluded front. The eastward migration of the upper-level cyclonic PV anomaly ultimately encourages the development of a new surface-based cold front ahead of the landfalling occlusion as troughing, confluence and convergence downstream of the Sierra Nevada intensify pre-existing baroclinity over Nevada. Trajectories show that the new cold front represents a boundary between potentially warm air originating over the desert southwest, some of which has been deflected around the south end of the high Sierra, and potentially cool air that has traversed the Sierra near and north of Lake Tahoe, some of which has been deflected around the north end of the high Sierra. Although diabatic processes contribute to the frontal sharpening, they are not needed for the discrete propagation or rapid cold frontal development. Forecasters should be vigilant for discrete frontal propagation in similar synoptic situations and recognize that moist convection or differential surface heating can contribute to but are not necessary for rapid Intermountain frontogenesis.