A tale of two damaging convective storms during the record-breaking Norwegian summer of 2014

Two cases of severe thunderstorms that impacted the Oslo area have been investigated. The thunderstorms occurred during the summer of 2014, an extraordinary summer with record high temperatures, number of lightning strikes and amount of short-term precipitation. The thunderstorm on 26 June produced precipitation amounts of 44.5 mm in one hour and 72.8 mm in 24 hours, both records. It was short-lived in both time and space. The other thunderstorm on 4 August was part of a large organized convective system that traveled from southern Sweden to northern Norway, lasting 15 hours in total. Even though both thunderstorms were intense, the dynamical processes leading to their formation and evolution were starkly different. The necessary ingredients for deep convection are sufficient moisture, conditional instability and a so-called triggering mechanism. An in-depth synoptic scale analysis utilizing maps of the large scale flow, Lagrangian backward trajectories and stability indices is conducted for both cases to better understand how these necessary ingredients were met and their respective roles in the observed deep convection. The June case was characterized by large-scale persistent northerly flow associated with an occluding cyclone east of Norway and high pressure to the west. The flow pattern resulted in significant cold air advection and a suppressed tropopause (upper level cold anomaly). Meager amounts of moisture were found in this case, albeit with a local maximum in the thunderstorm region. Additionally, no synoptic scale triggering mechanism was present. The trajectory and stability index analyses indicate the primary cause of the thunderstorm was a steep vertical temperature lapse rate resulting from local surface heating and the large-scale upper-level cold anomaly. The surface heating on the day of the event was sufficient to create positively buoyant air parcels, thereby triggering the deep convection. The August case provides a remarkable contrast. The large scale flow characterized by low pressure to the west and high pressure to the east of Norway resulted in significant warm, moist advection and an elevated tropopause. The trajectory analysis points to two moisture regions: the Atlantic ocean and over the Baltic. The combination of anomalously high surface temperatures and moisture content led to large values of conditional instability as evidenced in the stability index analysis. Associated with a low pressure center over Great Britain, a frontal boundary draped over western Norway on the day of the event providing a triggering mechanism. The large scale (in time and space) nature of the organized convective system can be tied to the presence of significant vertical wind shear associated with low-level baroclinicity (i.e. the front). This study of two distinctly different cases of intense deep convection illustrates the utility of combining an understanding of the large scale flow pattern over the days prior to a convective event and the local scale conditions with respect to the necessary conditions for deep convection. Lagrangian analyses and stability indices were found to be valuable tools for the assessment of both events, tools that are available in a forecast framework.