An Investigation into a Squall Line over Complex Terrain

The British Columbia (BC) Interior is characterized by complex terrain which poses significant challenges to the meteorologist. Organized thunderstorms are infrequent over the region with only four squall lines being reported since 2000. On 10 July 2008, a squall line developed over the central interior in the morning and moved south-eastward into mountainous areas of the southern interior in the afternoon. Widespread tree damage and power outages were reported across southern BC as the squall line passed. The most severe property damage and downed trees occurred over the populated Okanagan Valley. Winds at Penticton reached 109 km/h breaking the old July record gust of 97 km/h set fifty years ago.

With good detection coverage from the Silver Star radar and a relatively dense surface observation network over the southern interior, this case offers a unique opportunity to study the structure and evolution of a squall line and the possible terrain effects on its evolution as well as the associated surface winds. Very little has been documented on these types of storms in BC. It is expected that this study will provide useful guidance for understanding the meteorology behind such storms and improving forecast skill for future similar systems.

An analysis of the pre-storm environment indicated that the squall line developed in an atmosphere of weak buoyant energy combined with moderate wind shear and dry conditions. The evolution of the squall line was short-lived compared to those storms occurring over more uniform terrain. There is little doubt that the complex topography over the southern interior along with a pre-squall dry air-mass suppressed new cells from developing.

This report will show that a very strong pool of cold air formed as a result of evaporative cooling from convective downdrafts and synoptic-scale cold air advection. In turn, this cold pool acted as a density current creating a strong north-south pressure gradient on the leading edge of the squall line. The combination of this and the channelling effect through the Okanagan valley were key factors in the severe winds reported at Penticton.