The weather system is expected to cross the state’s coast between Port Hedland and Broom on Thursday night or early Friday morning as a category four system.
By that time, the cyclone could produce winds of up to 270km/h and rainfall of 200 to 400 millimetres, which is likely to cause flooding.
Heavy rain is expected to continue to inland parts of the Pilbara, with some falls possibly exceeding 100 millimetres, equivalent to a year’s worth of rain in some parts of the region.
On Wednesday night, the storm was still about 425 kilometres north-east of Port Hedland and 310 kilometres north-west of Broome.
Campers have evacuated from popular tourist spot Eighty Mile Beach, 250 kilometres north of Port Hedland, after warnings the sanctuary could take a direct hit.
Roadhouses along the Great Northern Highway are shutting up shop, including in Pardoo, which was hit by Cyclone Rusty 10 years ago.
Chef Robert Declaro served his final customers and headed for Port Hedland to seek refuge.
“Since yesterday I can’t sleep much because this isn’t ordinary. I’m really scared right now,” he said.
However others aren’t rushing off as quickly.
Queenslander Lisa Ashford, who is on a record-breaking ride around Australia, is hoping to outcycle the cyclone.
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“I’ll shoot past Port Hedland today and end up in Karratha,” she said on Wednesday afternoon.
“Tomorrow night I’ll be another 200 kilometres south.”
Emergency services said the winds were likely to pick up on Thursday, prompting Port Hedland residents to collect and dispose of debris on Wednesday.
An estimated $300 million of trade that moves in and out of the local port daily was halted.
“It’s just a case of securing best you can and get yourself in a safer place, away from the main impact of the storm,” Philip Christy from Pilbara Ports Authority said.
A State Emergency Service taskforce manned the ground with another small army of 10 volunteers arriving from Perth and packing sandbags.
Closer to Ilsa’s path, the De Grey River is about to be inundated with more than 200 millimetres of rain a day.
“Some of those areas in the inland parts of the Pilbara might see a year’s worth of rain in a couple of days,” Todd Smith from the Bureau of Meteorology said.
Residents are advised to stay up to date with the latest information on the Emergency WA website.
Communities can call the State Emergency Service on 132 500 for assistance and triple zero in the case of a life-threatening emergency.