On Christmas Eve, 1974, Tropical Cyclone Tracy made landfall at Darwin, Australia, with tragic loss of life and property. The event is part of meteorological folklore. The current study uses objective analyses from the NCEP/NCAR re-analysis project, and the Bureau of Meteorology’s operational Tropical Cyclone Limited Area Prediction System (TC-LAPS), to investigate Tracy’s motion and intensity change during the 3 days prior to landfall.

Although the Tracy circulation is not well-depicted in the original analyses, forecasts using a vortex specification technique and the NCEP analyses show remarkable skill at predicting the large scale environment and the storm’s track and intensity. This verification suggests that the analyses and forecasts are of sufficient quality to allow diagnosis of the event.

Landfall at Darwin coincided with the onset of the Australian monsoon and the flow changes associated with this large-scale event very much determined Tracy’s behaviour. Initially embedded in the monsoon trough located to the north of Darwin, Tracy at that time was moving slowly to the southwest. As the monsoon westerlies developed over Indonesian longitudes, the storm changed its direction of motion under the influence of the developing monsoon flow, and began to move to the east towards Darwin.

We propose that onset was triggered via an eastward and equatorward propagating wave train at lower and upper levels, which originated from high latitudes over the southwest Indian Ocean. The effect of this process was to establish a weak monsoon structure, which was then amplified by convection-related processes. We show that onset was mostly independent of Tracy’s development.

Intensification commenced just prior to recurvature and was associated with an upper tropospheric flow transition. A hierarchy of numerical models are used to illustrate that during this time, rapid and large scale flow changes : 1. resulted in an equatorward movement of the upper ridge and a reduction in environmental wind shear, 2. directly influenced the development of Tracy’s upper vortex via downstream development of a weak environmental trough, which overlay the near-surface circulation, and 3. provided a connection between the convectively-driven storm outflow and the midlatitude westerlies to form an extended outflow channel with enhanced eddy angular momentum flux convergence.

Discussion will focus on (i) performance of TC-LAPS, (ii) the physical processes operating during monsoon onset, and (iii) the upper tropospheric flow transition during intensification.