An Examination of the Evolution of Occlusions in the Central United States

Over the past two decades, there has been an increased interest in the occlusion of mid-latitude cyclones and the impact on heavy snowfall. In particular, research has focused upon the trough of warm air aloft (TROWAL), the frontogenesis that occurs within the TROWAL, and its relationship to heavy snowfall. Schultz et al. (1998) showed that evolution of fronts during cyclogenesis was dependent upon the large-scale flow. When the PV anomaly was moving into an area with large-scale diffluence, the cold front was stronger and the warm front was weaker. However, when the PV anomaly moved toward an area with large-scale confluence, the warm front was stronger and the cold front was weaker. As a result, two conceptual models have developed in the last ten years. One conceptual model by Moore et al. (2005) indicated that the focus of ascent within the occlusion is associated with the frontal circulation that developed along the warm front to the northwest of the cyclone. A second by Han et al. (2007) indicated that the focus for ascent within the TROWAL was associated with the frontal circulation that developed along the cold front northwest of the cyclone. Two cases studies are examined to compare the differing evolution of precipitation bands prior to and during the occlusion of cyclones in the central United States. The first cyclone, 27-28 November 2005, examines a snow and ice storm across the Dakotas and Nebraska. This storm produce in excess of 30 cm of snow, 2 cm of ice accumulation, and wind gusts over 30 m s-1. A positive potential vorticity (PV) anomaly moved northeastward from the Colorado Rockies into the Great Lakes with a jet streak located upstream of the PV anomaly. The large-scale flow ahead of the across the Great Lakes was diffluent. Heavy snow and freezing rain initially developed along a warm front north of the cyclone center. As the cyclone occluded, the stability increased above the warm frontal surface and there was a corresponding decrease in the ascent and precipitation rates. A new band of precipitation formed along a mid-level equivalent potential temperature (θe) gradient in an area where convective instability existed. As this band of precipitation organized, a thermal gradient developed and frontogenesis increased. This band of precipitation became dominant once the cyclone occluded and only light precipitation remained in the vicinity of the warm front. The resultant band is similar to that described by Han et al. (2007). The second cyclone, 10-12 December 2010, examines a snowstorm that extended from South Dakota to Wisconsin producing up to 60 cm of snow along with wind gusts over 25 m s-1. A positive PV anomaly moved from the northern Rockies into eastern Iowa. The large-scale flow was confluent over the Great Lakes as the PV anomaly approached and two jet streaks were observed. The first was located downstream of the PV anomaly over the northern Great Lakes and the second was located upstream of the PV anomaly over the Rockies. Similar to the first storm, precipitation developed along a warm front east of the cyclone center. However, as the cyclone occluded, this warm-frontal band expanded westward behind the cyclone center as strong frontogenesis remained focused on the warm front. The warm-frontal band remained the dominant precipitation band within the occlusion from eastern South Dakota to Wisconsin. While no instability was observed above the frontal surface within the TROWAL, the weak symmetric stability combined with strong frontogenesis and large-scale forcing for ascent resulted in a heavy band of snow extending upstream of the cyclone and within the occlusion. While a lighter band of precipitation did develop along the cold front, the band remained south of the surface cyclone and did not extend behind (upstream) of the cyclone. The evolution is similar to that described by Moore et al. (2005).

By determining whether the flow downstream of a PV anomaly is confluent or diffluent can help forecasters to ascertain whether heavy snowfall will remain along the mid-level warm front or reform along the mid-level cold front. This can improve communication to the public and other users as to when and where heavy snowfall will occur with a winter storm, and how heavy snow will evolve during a storm's evolution.