2020–2021 Tropical Cyclone Season Updated Outlook for Tonga

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“There is a moderate chance (45%) for at least 1 Tropical Cyclone to occur for the remainder of the TC Season (08th March – 30th April 2021). If a Cyclone affects Tonga during the remainder of the Cyclone season, there is a 25% chance that this cyclone could be Severe” (Issued: 08th March 2021)

Recent Tropical Cyclone Activities

Since the official start of the 2020-2021 Tropical Cyclone (TC) season on the 1st November 2020, two TCs has occurred and has been recorded over our Southwest Pacific region including Tonga.

TC “ZAZU” (category 1) and Severe TC “YASA” (category 5). Both these TC systems occurred in our Tonga area of responsibility during the 13th – 20th December 2020.

TC “ZAZU” mainly affected the land areas and waters of the Vava’u group of islands with damaging gale to destructive force winds with heavy precipitation and flash floods. Severe TC “YASA”, however remained to the far western open waters of the Tongatapu and ‘Eua but had impacts of damaging gale force winds on Tongatapu and ‘Eua and with very rough to high seas accompanied by heavy damaging Northwesterly swells.

TC “ANA” (Category 3), also affected our Tongan Areas of Responsibility on the 30th January – 2nd February 2021. A series of Tropical Depressions (TDs) were also recorded during the last week of January to Mid-February 2021. These were TD 06F (28th – 29th January 2021) and TD 09F (9th – 10th February 2021). All these tropical system coincided within the last active period of our Southwest Pacific Region.

It was also being noted that towards the last weeks of February, the South Pacific Convergence Zone (SPCZ), the rain producing mechanism and cyclogenesis feature of our Southwest Pacific region was very active. It generated TD 10F and TD 11F (23rd -26th February 2021) which brought heavy rainfall events and strong winds over Tonga. Severe TC “NIRAN” (Category 5) also occurred in our Southwest Pacific region, 26th February – 06th March 2021 but had no direct threats to Tonga.

During every cyclone season there are active periods in which potential for tropical cyclone formation is increased. These active periods come around every 30 to 60days on average and lasts for a duration of about 2 to 3 weeks. There are at least three active periods in a typical TC season for the Southwest Pacific region.

The last active period for our current cyclone season occurred between the last week of January to mid-February 2021. This resulted and contributed to the occurrence of TC “ANA”, TD06F,
TD 09F, TD10F andTD11F last month. These active periods was the second active period for our current TC season.