Climatology of Strong Intermountain Cold Fronts

Abstract

Motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern Utah, this paper presents a 25-yr climatology of strong cold frontal passages over the Intermountain West and adjoining western United States. Using conventional surface observations and the North American Regional Reanalysis, strong cold frontal passages are 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 700-hPa temperature gradient of at least 6°C (500 km)−1. The number of strong cold frontal passages exhibits a strong continental signature with very few events (<10) along the Pacific coast and more than 200 events east of the Continental Divide. The number of events increases dramatically from the Cascade Mountains and Sierra Nevada to northern Utah, indicating that the Intermountain West is a frequent cold front breeding ground. A composite of the 25 strongest events at Salt Lake City (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 and appears to be enhanced by flow deflection around the Sierra Nevada. Quasi-stationary development and intensification of the 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 rapid frontal intensification in many events.

Wind regimes and aeolian transport in the Great Basin, U.S.A.

“…In spite of this, a generalized picture of Great Basin winds and associated aeolian deposits appears to fit recent meteorological studies of how frontal systems behave in the Great Basin (Shafer and Steenburgh, 2008).…”

“…While much of the region has low precipitation and abundant potential aeolian material (much of it derived from pluvial lakes of the last glacial maximum), its diverse geographic terrains and locally extreme topographic gradients exert tremendous influence on surface winds (Stewart et al, 2002;Cairns and Corey, 2003;Shafer and Steenburgh, 2008). Previous work documenting aeolian transport and areas of dune formation in the western U.S. includes the Mojave desert (Muhs et al, 1995;Tchakerian and Lancaster, 2002;Lancaster and Tchakerian, 2003), lower Colorado River basin (Muhs et al, 1995), northern Mexico (Lancaster et al, 1987), Wyoming (Alhlbrandt et al, 1983;Gaylord and Dawson, 1987;Mayer and Mahan, 2004) and New Mexico (Wells et al, 1990).…”

Observational and Synoptic Analyses of the Winter Precipitation Regime Change over Utah

“…The linear trend in RMS[y9(500 mb)] (Fig. 14b) is a decreasing one over an elongated region from southern Alaska toward the Pacific Northwest, a region coincident with the common route taken by winter synoptic disturbances (e.g., Shafer and Steenburgh 2008); this is accompanied by increasing transient activity over southern California. Combined with the wavelike circulation anomalies, these synoptic conditions are reflective of a climate change pattern that signifies the poleward (equatorward) shift of storm tracks in the North Atlantic (North Pacific) as was depicted by Bengtsson et al (2006).…”

“…Winter precipitation events in Utah (and much of the Intermountain West) result from specific weather patterns. The two most common weather patterns include the following: Pacific fronts associated with approaching troughs (Hobbs et al 1980;Colle et al 1999;Shafer and Steenburgh 2008) and zonal/anticyclonic patterns that interact with the mountains that are supplied by moisture via the subtropical jet (e.g., Colle and Mass 1995). These weather patterns are recognizable through synoptic analysis of the wind and geopotential height fields.…”

Coherence between the Great Salt Lake Level and the Pacific Quasi-Decadal Oscillation

“…Precipitation in the intermountain region is mainly produced by transient synoptic activity such as cyclone waves and frontal passages interacting with orography (Ely et al 1994;Harnack et al 1998;Shafer and Steenburgh 2008). Thus, the transient component of water vapor flux divergence ($ Á Q9 D ) was extracted to examine the role of transient synoptic activity on the hydrological cycle, following Eq.…”