Extreme Snowfall Variations and Cold-Air Damming in the Front Range Heavy Snowstorm of 17-19 March 2003

This study examines the dynamics and thermodynamics associated with the climatologically extraordinary 17. 19 March 2003 snowstorm that impacted the Colorado Front Range and some surrounding areas. In particular, several anomalously high local snowfall gradients are examined to improve our understanding of precipitation distributions associated with upslope flow, blocking, and cold-air damming. This unusual event was established as a result of a deep, slowly propagating closed low. The continental-scale closed low was characterized by persistent strong upslope (or easterly) flow that transported large amounts of moisture into the Colorado Rocky Mountain barrier. Warm easterly inflow well above freezing was blocked by the terrain, and this soon created a barrier jet-like feature over the urban corridor. Simultaneously, low-level cold advection from the north occurred over the east side of the barrier due to larger-scale processes, a situation not associated with a classic barrier jet. Nevertheless, subsequently the warm inflow was lifted over the cold dome. A fourth important process, diabatic cooling from melting hydrometeors, contributed strongly to the cold dome and generated a nearly isothermal low-level temperature profile. Both cooling mechanisms (advection and diabatic) were significant. Resulting temperatures just east of the foothills were at or very close to the freezing point as the heavy snow event occurred. Precipitation type and snowfall density were major contributors to the heterogeneity of the precipitation distribution. The roles of cold-air damming and barrier jet-like features as the driving forces in both the dynamics and thermodynamics of the storm along the urban corridor are described in detail using radar, surface, and high-resolution nested model data from the Pennsylvania State University. National Center for Atmospheric Research fifth-generation Mesoscale Model. The dammed cold air, centered over and just east of the foothills, was sloped laterally upwards towards the high terrain to the west, and topped by a stable, strongly sheared layer. This dynamic setup produced heavy precipitation away from the steep terrain gradient. Heavy precipitation rates played a major role in the development of a persistent rain/snow boundary well east on the plains. The northerly flow within the cold air also generated some local downslope flow, leading to areas of drastically reduced snowfall even when compared to that of lower elevations to the east.