The Tropical Cyclone (TC) activity expected in the 2020/21 Tropical Cyclone Season to affect Tonga is likely to be around average (1 to 2 cyclones on average). Historical records indicate that Tonga could expect at least 1 cyclone and at most 3 cyclones to occur with a moderate chance (60%) of at least 1 Severe Tropical Cyclone occurring.
Any tropical cyclones passing close to the country, associated active cloud and rain bands may occasionally affect Tonga with marked rainfall and possible flooding, including sea flooding of low‐lying coastal areas.
Tropical cyclones affecting Tonga in the 2020/2021 season should expect to be mostly coming from the West and NW (Fiji Area – 60%) and the rest from the North, East and Northeast.
About 4 to 6 named Tropical Cyclones are expected for the South West Pacific in the 2020/2021 season with 1 to 3 reaching severe intensity (Category 3 to 5). Tropical Cyclone formation area will be shifted to the warmer ocean temperatures in the Australia and Papua New Guinea regions.
Current dynamics indicate the first tropical cyclone activity for the season in the South West Pacific could be around the beginning to mid-December 2020.
The 2020/21 Cyclone Season will almost certainly bring above average rainfall to all part of Tonga during the cyclone season due to La Nina conditions. In some cases, the average monthly rainfall can double.
Cyclone analysis and outlook for Tonga
The official 2020/2021 TC Season will begin on 01 November 2019 and will end on 30 April 2020. It should be noted that tropical cyclones have occasionally formed outside this period e.g. Tropical Cyclone “Keli” which affected Northern Tonga in June 1997. The peak time for the occurrence of tropical cyclones in Tonga is from January to March with most events occurring in February. Sea surface analysis indicates that the El Niño–Southern Oscillation (ENSO) is currently in a La Niña phase.
The tropical Pacific Ocean is expected to remain in La Niña conditions throughout the 2020/2021 Cyclone Season.
During La Niña events, Tonga normally receives above average rainfall activities. Climate models are also indicating that above normal rainfall over majority of Tonga during this Tropical Cyclone Season is highly likely. With a La Niña event established and in the view of the rainfall outlook for Tonga, there is an increased risk of heavy rain events and possible flash flooding in low lying areas in Tonga during this Tropical Cyclone Season.