The system, to the country's west, is already whipping up strong winds and rough seas across much of the country.
The Vava'u islands have already seen winds in excess of 100 kilometres an hour.
Foleni Tu'uholoaki, from Tonga's Meteorological Service, said while the system isn't forecast to pass close, the islands are sandwiched between two pressure systems that are funnelling winds over Tonga.
"The current track is likely to go between Fiji and the main islands. But the strong winds and the damaging winds are to the right of the system, to the east of the system -- that's us. Whether it's going to be named or not, we're still going to have those strong and gale winds."
Foleni Tu'uholoaki said warnings for strong winds and rough seas are in alert for effect of Tonga.