The Port Hedland Port Authority has moved vessels out to sea from the Pilbara town's busy iron ore port in anticipation of a tropical cyclone forming off the region's coast.

A blue alert has been issued for coastal areas from Pardoo to Onslow, which includes Port Hedland and Karratha, as a tropical low threatens to develop into a tropical cyclone.

The Bureau of Meteorology predicts a weak low off the west Kimberley coast will move off the coast in a south-westerly direction and could develop into a tropical cyclone late on Tuesday or on Wednesday.

The bureau says the chance of the system developing into a tropical cyclone is moderate, which means a 20 to 50 per cent chance.

The alert, issued at 9.40am, says the tropical low is 195km east north-east of Port Hedland and 270km south-west of Broome.

It is moving west south-west at 12km/h.

Increasing swells, storms and showers are predicted for today and tomorrow.

"The low may develop into a tropical cyclone on Wednesday as it moves to the west south-west just off the Pilbara coastline," the Weather Bureau advised.

"If the system does not reach cyclone intensity then gales with wind gusts to 100km/h could develop between Pardoo and Dampier during Wednesday, possibly extending west to Onslow late in the day.

"Squally thunderstorms are expected with locally heavy rainfall."

A port spokeswoman said 35 vessels were given the order to move out to sea at 6pm Monday night.

At 9am this morning, there were seven ships in port but they will remain until further notice.

"We will continue to keep a close eye on the development of this system and watch for any signs of re-curvature towards Port Hedland and the closest point of approach, which at present is 38 nautical miles at 2am on Wednesday," the PHPA spokeswoman said.

"Vessels in the inner harbour will continue to load as per the DUKC (Dynamic Under Keel Clearance) windows until further notice."

Whilst China's Qinhuangdao Port is the world's largest bulk port, its coal imports comprise a significant percentage of its overall throughput, whereas Port Hedland's throughput is around 98 per cent exports, making it the world's largest bulk export port.

A blue alert warns people to prepare for dangerous weather.

Residents in the affected area are advised to prepare their home and family for a possible cyclone.

They should review their cyclone plan, prepare their home inside and out, organise an emergency kit and keep up today with community alerts through radio, television or the internet.

The Pilbara is on cyclone watch for the second time in as many weeks after Cyclone Narelle threatened, but never reached the mainland.

The West Australian

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