Temperatures in Mandurah and Rockingham are set to hit the high 20C's this week.
The mercury is expected to reach 28C on Monday and 27C on Tuesday, Wednesday and Thursday.
Bureau of Meteorology's Jessica Lingard said warmer weather is due to the formation of a west coast trough.
"They bring down warm air from further north, so they drag that warm Pilbara air down the west coast into the southern parts and they bring us this beautiful bursts of warm temperatures," she said.
"We're only a couple of days away from the new month so definitely starting to see those warmer temperatures arrive.
"It feel a little different this month because it has been what has felt like a bit of a cooler month, it's taken a little bit of time for spring to really get going."
Temperatures will drop again on Friday to 23C, followed by 21C on Saturday and Sunday.
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