The Fiji Meteorological Service say Tropical Depression TD05F may develop into a Category One tropical cyclone by either later today or tomorrow morning.
It says TD05F was located 65km east of Port Vila in Vanuatu and is moving in a southeast direction towards the southwest of the Fiji Group.
The Nadi Weather Office said environmental conditions will not be favourable when the system approaches the Fiji Group.
The system is anticipated to pass close to the west or southwest of Fiji Group.
The associated active trough of low pressure is gradually approaching the country from the west, and it is expected to affect the Fiji Group from later this afternoon or evening.
Occasional to periods of rain, heavy at times and few thunderstorms are expected over most parts of Fiji Group from later today.
Consequently, a Heavy Rain Warning is now in force for the whole of Fiji Group.
With the anticipation of heavy rain, a Flash Flood Warning is in force for low-lying and flood-prone areas across the Fiji Group.
Moisture is abundant in the environment during this time of the year and with heating, afternoon and evening thunderstorms risks become elevated.
Heavy rainfall can be experienced in a short duration of a few hours.
The Fiji Meteorological Service will continue to monitor this weather event and update warnings and forecasts accordingly.