Climatology of Strong Cold Fronts over the Western United States

Strong cold fronts, which may be accompanied by high winds and dramatic temperature falls, move across Salt Lake City, UT (SLC) and the adjoining Intermountain West several times a year. A 25-yr (1979–2004) climatology of strong cold fronts, identified based on a temperature fall of 7°C or greater in a 2–3 h period, a concurrent pressure rise of 3 hPa or greater, and the presence of a large-scale temperature gradient of at least 6°C/(500 km), illustrates that these extreme events are most common in the spring (March–May) and in the late afternoon and early evening (1500–1900 LST). The number of strong cold fronts exhibits a strong continental signature with very few events (< 10) along the Pacific coast and more than 200 events at stations east of the Continental Divide. The event frequency increases eastward from the Cascade Mountains/Sierra Nevada to a local maximum at SLC, suggesting that many strong cold fronts develop or intensify over the Intermountain West.

A composite of the 25 strongest events at SLC (based on the magnitude of the temperature fall) reveals that confluent deformation acting on a broad baroclinic zone over central Nevada commonly initiates Intermountain frontogenesis. The confluent deformation develops in southwesterly large-scale flow with flow deflection around the Sierra Nevada appearing to enhance the confluence. Quasi-stationary development and intensification of a southwest–northeast oriented cold front then occurs as a mobile upper-level trough approaches from the west. The front becomes mobile as cold advection and ascent associated with the upper-level trough overtake the low-level front. Cloud and precipitation observations suggest that differential diabatic heating contributes to the frontal collapse in many events.