Evolution of a Wintertime Pacific Northwest Mini-Supercell and Tornado

On 10 January 2008, a rare “mini-supercell” developed over southwest Washington and produced a series of tornado touchdowns along a 19 km track in Clark County Washington. The classification of this storm as a mini-supercell can be made by its compact vertical and horizontal structure with maximum echo tops around 8 km and a mesocyclone diameter of 2 km.

The storm developed around midday in a cold, conditionally unstable, post-frontal air mass. As the storm moved east of the Coast Range into the lower Columbia River valley, it rapidly intensified with the concurrent passage of an upper level short wave, evident in satellite and radar imagery.

The close proximity of the storm to the KRTX Weather Surveillance Radar provided high spatial resolution data of the development of the supercell and tornadogenesis. Prior to storm development, radar imagery indicated the presence of persistent low level horizontal convective rolls oriented parallel to the mean wind. These rolls are suspected to have played a critical role in the development of the tornado providing a source of horizontal vorticity which was ingested into the storm after interacting with an outflow boundary.

The County Warning Area (57,000 km²) of the Portland, OR National Weather Service Forecast Office has averaged just over one tornado per year since 1950. The extended lifetime and track length of this EF-1 tornado made it the most damaging tornado in the region in 35 years.