Perth is about to experience a five-day streak of temperatures above 35 degrees Celsius for the first time in five years, and the temperature will not settle down until next week at the earliest.

Key points: A high pressure system is directing hot easterly winds onto Perth

A high pressure system is directing hot easterly winds onto Perth People are urged to stay hydrated and make sure they have sun protection

People are urged to stay hydrated and make sure they have sun protection There is an increased risk of bushfires due to potential lightning strikes

The hot spell kicks off on Wednesday, with temperatures in the mid-to-high 30s forecast until Friday.

But over the weekend the heat will intensify, with temperatures set to nudge 40C.

"We haven't had a long run like this for some time. In fact the last time we had five days or more of 35C in Perth was January 2014," Bureau of Meteorology spokesman Neil Bennett said.

Mr Bennett said on the heatwave scale, this one would be considered to be of "low intensity".

"The fact that we're going to have a prolonged period of temperatures above 35C for the daytime and temperatures of 20C or more overnight does mean that things are going to get a little bit uncomfortable towards the end of the spell," he said.

"So it's really a case of people being aware that it's going to be hot and hotter than it has been for a little while now.

"And [they need to be] just doing the right things — staying hydrated, making sure they've got plenty of sun protection, looking after the elderly and pets and so forth."

It comes off the back of Perth's coolest January in 12 years, despite the city recording three 40C days during the month.

What is causing the heat to stick around?

The build up of heat is due to a high pressure system directing easterly winds that have travelled through the outback onto Perth, combined with the position of one of Perth's key climate drivers during summer — a west coast trough.

When the trough is offshore, the sea breeze — if it develops — tends to be weak and does not penetrate very far inland, causing temperatures to rise near the coast.

But when the trough is inland, a sea breeze brings a cool relief to Perth generally in the afternoons.

Temperatures will stay above 35 degrees Celsius until next week in Perth. ( AAP: David Crosling )

"The west coast trough is going to remain close to the coast pretty much all the way through from here until probably Monday, and as a result of that we're likely to see temperatures rising as the air from the north-east starts to move down towards Perth," Mr Bennett said.

"The major reason for the mild January was because we didn't have one of these patterns where the trough was offshore for any length of time.

"The high pressure systems to the south of the continent were what meteorologists call 'mobile' — they were moving quite quickly from the Indian Ocean into the bight and dragging the trough along with them as they did so."

Mr Bennett said a significant low pressure system which brought wintry weather to the south-west of the state in January also dragged down the average temperature.

"That really put a lid on the temperatures for a few days there," he said.

"On top of that we think that the sea breezes may well have been a little cooler because the ocean temperatures off the coast are just a little bit below average at the moment, so that may have been a contributory factor as well."

Thunderstorms bring bushfire risk

Thunderstorms are also on the cards for Perth from Wednesday through to Friday, particularly over eastern parts of the city.

There is an increased risk of bushfires from thunderstorm activity. ( Supplied: DFES )

Mr Bennett said at this stage it was difficult to predict where and when any storms might hit.

"Obviously with any thunderstorm there is the risk of lightning," he said.

"So with the dry conditions that we have, the high temperatures and a possibility of an ignition source from those thunderstorms, fire weather conditions then become an issue so we do have to be very careful with that.

"But it's difficult this far out to pinpoint where the exact areas are going to be, but certainly the eastern parts of Perth are at risk."

Some gusty easterly winds are also expected on Thursday and Friday, which the BOM is monitoring closely.

"To get thunderstorm activity we need to have reasonably moist air, that often works against fire weather, so from that aspect it's not looking too bad."

"However if we get a thunderstorm that sets something alight and we have those fresh and gusty easterly winds overnight Thursday and into Friday, then even without very high temperatures we could have some problems.

"So it's a tricky one and something that we're looking at very, very closely and we will obviously be in close consultation with the Department of Emergency Services and we will be issuing warnings as needs be as we go into the week."

Perth weather forecast