Based on long-term statistics from the Australian Bureau of Meteorology (BoM), an extreme rainfall fell near Canberra and nearby regions on 24 - 25 February 2018, which ranked as a top 1% extreme rainfall event when compared with all February days between 1900 to 2018 in New South Wales and the ACT. During that day, Canberra received more than the long-term statistical February total rainfall within six hours, causing overwhelming flash flooding in the city.
Diagnostic analysis shows that this relatively rare brief heavy rainfall was caused by the joint effects of lower-level warm, moist conveyor belts, the remnants of tropical cyclone Kelvin, surface cold fronts, and a mid-latitude trough at the mid-upper levels of the atmosphere. The lower-level northeast and west conveyor belts of water vapor contributed to the rapid accumulation of moisture and conditional instability over Canberra by transporting tropical moist and warm air from the Coral Sea and from the Kelvin remnant to Canberra prior to and during the rainfall period. Enhanced uplift was triggered by the arrival of a cold front in the warm, moist unstable air. The rainfall reached a peak 13.6 mm in 30 minutes soon after a low-level frontal updraft coupled with mid- to upper-level dynamical uplift ahead of the mid-latitude trough. After the passage of the upper-level trough, the dry, cold air in the rear of the trough stabilized and dried the atmosphere above Canberra. The rainfall decreased and ended rapidly. In this presentation the mechanisms associated with the heavy rainfall event will be discussed.