The Use of Convective Parameters by the Australian Extreme Weather Desk in Forecasting the 16 December 2015 Tornadic Supercell

An independent review of the Australian Bureau of Meteorology identified the need for another layer of capability to respond to extreme weather events. In response to the review the Australian Extreme Weather Desk (EWD) was recently established with the aim to provide “a national focus for extreme weather intelligence” and “enhanced severe weather capacity during periods of sustained demand”. A complete forecast process for convection forecasting was trialled by the EWD during the 2015-2016 Australian Severe Weather Season. Forecasts of thunder, large hail, damaging winds, heavy rainfall and tornado were provided nationally. A daily verification product focussing on continual forecast improvement through verification completed the process. 

On the morning of Wednesday 16^th December 2015, a supercell thunderstorm spawned off the New South Wales coast. The deviant motion to the left of the deep layer mean wind directed the storm onshore as an EF2 tornado developed which subsequently impacted the Sydney suburb of Kurnell located just 5km south of Australia’s busiest aviation terminal, Sydney International Airport. Extensive damage resulted in an estimated $206m AUD in payments by the insurance industry. The Kurnell Automatic Weather Station was directly impacted by the tornado and recorded a maximum wind gust of 213 km/h. Hail with a diameter of 4 to 5cm along with heavy rainfall of up to 89mm in 1 hour was also reported. This event was unusual in that it spawned in a maritime environment during the morning and impacted a population dense area containing a relative abundance of observational data collection networks.

The end-to-end convective forecast process employed within the EWD is examined in detail. Anomalously high sea surface temperatures provided ample low-level moisture and energy to a conditionally unstable atmosphere. This was enhanced by very strong upper-level dynamics associated with cyclonic vorticity advection on the eastern side of a transient cyclonic PV-anomaly. United States National Weather Service Storm Prediction Centre normalised convective parameters highlighted environments conducive to tornadic supercell thunderstorms, while an analysis of near-surface kinematics identified the preferential ingestion of nearly pure streamwise vorticity.