Severe Tropical Cyclone Seroja

3-12 April 2021
Best Track of Severe Tropical Cyclone Seroja
All times shown are in Australian Western Standard Time (AWST), that is UTC +8 hours.

Summary

A slow-moving tropical low developed near the southwestern end of the island of Timor on 3 April. Due to the low-pressure system remaining slow moving for several days, sustained and heavy rainfall caused extensive flooding and landslides on Timor and neighbouring islands. Widespread and devastating damage was reported including more than 150 fatalities.

The low-pressure system intensified and on 5 April it was named Seroja by Jakarta TCWC. It started moving initially west and then southwest, quickly intensifying into a category 2 tropical cyclone. As it continued moving southwest during 6 and 7 April the system weakened back to a category 1 system. During 8 and 9 April it began to interact with another tropical low, that briefly intensified into Tropical Cyclone Odette. Over a period of approximately 36-48 hours the two systems interacted via the Fujiwara effect, a phenomenon rarely observed in the Australian region.

The interaction with Odette is likely to have been a factor in maintaining Seroja's track towards the southwest, rather than recurving into the west Pilbara coast. As Odette circled around to the north and then the east of Seroja, it also made conditions more favourable for intensification by replacing the dry air that had been limiting Seroja's intensity with moist air that could fuel its intensification. The increased moisture combined with lower vertical wind shear resulted in Seroja re-intensifying into a category 2 while Odette weakened and eventually dissipated.

During 10 April, Seroja took a sharp turn towards the southeast and began to accelerate towards the Western Australian coast. The system further intensified into a severe (category 3) tropical cyclone on 11 April and maintained this intensity through to its coastal crossing just south of Kalbarri around 8pm AWST. It is very unusual for severe tropical cyclones to maintain their intensity this far south. Impacts at Kalbarri and the nearby town of Northampton were severe with around 70% of buildings sustaining significant damage, mostly consisting of lost roofs but with many structures destroyed. Many locations recorded maximum wind gusts more than 125km/h with the highest being 170km/h from Meanarra Tower near Kalbarri. Seroja weakened as it moved further inland, though due to its rapid motion destructive winds extended a long way inland before it eventually weakened below tropical cyclone intensity early in the morning of 12 April near the town of Merredin.

Widespread power outages were experienced through Western Australia's Mid-West region due to fallen trees and power lines.

Tropical Cyclone Seroja was the seventh tropical cyclone and the second severe tropical cyclone in the Australian region for the 2020/21 season.

**All information relating to intensity and track is preliminary information based on operational estimates and subject to change following post analysis.**

Extreme values during event (estimated)

Note that these values may be changed on the receipt of later information


Maximum Category: 3
Maximum sustained wind speed: 120 km/h
Maximum wind gust: 170 km/h
Lowest central pressure: 971 hPa