A tropical low brewing off the Western Australian coast could be a category two cyclone when it crosses the West Australian coast on Saturday morning.
The cyclone, to be named Stan if it forms, would be the first for the Australian cyclone season.
Perth-based Bureau of Meteorology tropical cyclone forecaster Joe Courtney says the low was about 435km north-northwest of Port Hedland and 480km north of Karratha at 5pm, heading south at 13km/h.
"The concern we have is that it has got a lot of time to intensify - tonight, tomorrow and tomorrow night before it reaches the Pilbara coast," Mr Courtney told AAP.
The low has sustained winds of 45km/h with gusts to 85km/h.
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If it peaks at category two, it will have destructive winds with gusts of between 125km/h and 140km/h, Mr Courtney said.
It's likely the cyclone will cross the coast between Pardoo and Karratha.
Even if the system does not develop into a cyclone, it's still likely to bring gale force winds and heavy rain.
A blue alert has been issued for Sandfire, Wallal, Pardoo, Marble Bar, Port Hedland, South Hedland, Point Samson, Wickham, Roebourne, Karratha, Dampier and Pannawonica and surrounding areas.
"Although there is no immediate danger, you need to start preparing for dangerous weather and keep up to date," the Department of Fire and Emergency Services warns.
Residents are warned to prepare their homes, secure or remove loose items and ensure their emergency kits are ready.
Gales are expected between Wallal and Dampier by Friday evening.
Tides will exceed high tide between Pardoo and Karratha, with very rough seas and possible lowland flooding.
Heavy rain is expected in the Pilbara and far western Kimberley, with a flood warning in place for the De Grey River catchment.
Vessels are being evacuated from anchorages at Port Hedland.
The Pilbara Ports Authority says it expects to complete the inner anchorage evacuation at Port Hedland on Thursday afternoon.
The inner harbour evacuation will take longer, with seven vessels departing on Thursday and another seven to leave on Friday.
The Port of Dampier outer anchorage evacuation should begin later on Thursday.
The Port of Ashburton remains open for now.
