Spring has only just sprung, but Perth is set to get an early taste of summer weather this week, with a sunny day and top temperature of 30 degrees forecast for Monday — a full 10 degrees higher than the September average.

Bureau of Meteorology duty forecaster Steph Bond said it's "a little unusual" this early in the season.

"We've got some warm north-easterly winds moving over the Perth metro area tomorrow and we usually see 30 degrees in September about every 4 to 5 years, but usually they occur later in September," said Ms Bond.

"It's not a record though, the highest we've had recorded in September is 34.2 recorded back in 2014, and the earliest we've ever had a day over 30 degrees was on the 9th of September in 2015 as well, so we have had a day this early before."

Monday's spike is being caused by a trough developing off the west coast of the state, which is dragging dry warm air from central Australia over to Perth.

The forecast heat comes less than a fortnight after Perth recorded its hottest August day on record, when the temperature reached 28 degrees on August 28.

South coast also set to heat up

The state's south-west will also see temperatures 5 to 10 degrees above average for September on Monday.

It's going to be 30 degrees in Esperance on Monday and 28 degrees in Albany.

A trough developing off the west coast is responsible for the heat spike, according to the Bureau of Meteorology. ( ABC News: Andrew O'Connor )

There's also a possibility of near-record temperatures for the inland region on Tuesday, with a forecast of 34 degrees for Salmon Gums, north of Esperance.

Ms Bond said missing out on the sea breeze will cool the south-west coast back down.

Perth is also back to normal for the rest of the week, as the trough off the west coast moves inland.

"We see those temperatures drop on Tuesday as a cooler air mass moves through," Ms Bond said.

Perth weather forecast