* Storm upgraded overnight to Category 4
* Could make landfall as late as Saturday morning (Upgrades to category 4, updates timeline for landfall)
By Matt Siegel
SYDNEY, Jan 9 (Reuters) - A category-four cyclone bearing down on the South Pacific island nation of Tonga with hurricane force winds is expected to make landfall in as little as 18 hours, Tonga Meteorological Services said on Thursday.
Tropical Cyclone Ian, which has been hovering in a stretch of ocean between Fiji and Tonga since Monday, about 3,000 km (1,800 miles) east of Australia, was upgraded overnight from the less powerful category three and is now packing estimated average winds of 167 kph (103 mph), with gusts up to 231 kph.
The slow-moving storm system is believed to be moving towards the islands of Vava'u at a current speed of just seven kph, which means it could make landfall as late as Saturday morning local time.
There are five stages of tropical storm, with Typhoon Haiyan, which killed more than 4,000 people and caused widespread destruction in the Philippines in November rated at five, the most powerful. Category four is defined by wind gusts up to 251 kph.
Although less powerful than a category five storm, Ian can still be expected to severely damage solid buildings. It has the force to snap or uproot trees and down power poles, leading to what could be lengthy power outages for the remote islands.
Destructive winds could be felt in Tonga several hours before the centre of the storm makes landfall, the agency said. Residents of the Vava'u islands have begun boarding up windows, according to the Australian Broadcasting Corporation (ABC).
A forecaster from the Fiji Meteorological Service, Sanjay Prakash, told Radio Australia that the impact of the cyclone on Tonga could be "catastrophic".
Cyclone Christine ripped across Australia's northwest earlier this month, battering coastal regions and closing major iron ore shipping terminals. (Reporting by Matt Siegel, Editing by Jane Wardell, Ron Popeski and Alden Bentley)
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