Cyclone Vaianu is expected to strengthen slowly this morning as it moves through central Tonga.
The storm is expected to move south towards the main island of Tongatapu and the capital Nuku'alofa during the course of today and tomorrow.
In four hours' time it's expected to be 200 kilometres west of the Vava'u group of outer islands, bringing damaging gale force winds of up to 110 kilometres an hour.
Forecasters predict thunderstorms, heavy rain, high seas and heavy swells, leading to sea flooding of low-lying coastal areas.
