Fire authorities will be on high alert over the weekend as scorching temperatures and gusty winds create dangerous fire conditions in the southern half of Western Australia, including Perth.

Key points: Perth is set to hit 35C on Friday and 40C on Saturday, with winds of up to 40kph

Perth is set to hit 35C on Friday and 40C on Saturday, with winds of up to 40kph There'll be a cool change on Sunday, but the hot and windy conditions will move east

There'll be a cool change on Sunday, but the hot and windy conditions will move east The hot weather is caused by a high pressure system in the bight

The temperature is forecast to hit 40 degrees Celsius in Perth on Saturday and east-to-north-easterly winds are expected to reach up to 40 kilometres per hour, prompting a severe fire danger warning.

If Perth reaches its predicted maximum, it will be the earliest 40C day recorded in the city in November and only the fourth time it has occurred since temperature records began in Perth in 1897.

"It's that combination of the high temperatures and the strong winds which mean that the fire danger rating for the Perth area for Saturday is going to be severe," Bureau of Meteorology spokesman Neil Bennett said.

The temperature in Perth is expected to reach 35C on Friday and 38C on Saturday. ( Flickr: strangejourney )

"We do have a cool change moving through on Sunday, so we should see those fire danger ratings ease right off.

"But certainly Saturday the high temperatures and strong wind mean that fire weather conditions are going to be quite bad."

The Bureau of Meteorology's six fire danger ratings in order of severity from highest to lowest are:

catastrophic

catastrophic extreme

extreme severe

severe very high

very high high

high low-moderate

Temperatures will ease later in the weekend with a maximum of 24C expected in Perth on Sunday. ( ABC News: Andrew O'Connor )

"One of the things we're trying to avoid is fires starting because once the fire danger ratings get up into the severe or the extreme range, they become very difficult to fight and what we want to avoid is fires starting in that environment," Mr Bennett said.

The hot and windy conditions will travel east on Sunday, but then a cool change will sweep over Perth, with the maximum expected to drop by up to 14 degrees.

However, the fire danger will still be present in many areas.

"For Sunday, as we go to the eastern part of the Wheatbelt, we'll be looking at many parts having severe-weather fire conditions, while extreme fire conditions can be expected through the Goldfields," Mr Bennett said.

"So it is really important that people are aware that we're going to have these hot and windy conditions for the weekend and just be careful when they're out and about."

The heat will be generated by a high pressure system in the bight that will direct warm easterly winds over western parts of the state and a trough that will form along the west coast.

"We'll see easterly winds pushing onto the west coast and then eventually swinging out of the north-east and as they swing out of the north-east, those winds are coming from very hot areas," Mr Bennett said.

"That hot air gets dragged down towards Perth and then we have this low pressure trough developing off the west coast and we get what we describe as a 'squeeze', where there's a lot of isobars in between that high pressure system and the low pressure trough, giving us those north-easterlies.

"It's that combination and the very dry air that leads to severe fire weather conditions."

Loading...

As the trough starts to move inland, it will allow onshore winds to cool the west coast on Sunday.

"The trough of low pressure, the west coast trough, will move inland during the course of Saturday afternoon, allowing the westerly and south-westerly winds to bring cooler air in," he said..

"We'll also have a very weak cold front moving up during the latter part of Sunday and into Monday, that could introduce a possible shower or two on Monday, but then the front goes through and we start to see a warming trend as we go into the middle of next week."

Perth weather forecast