The Weather Bureau says this week's unseasonal downpours in Perth and the South West are being caused by a combination of factors, including near record sea surface temperatures.

In October, the ocean off the WA coast recorded the second warmest temperature since records began in 1900.

However, forecaster Neil Bennett says while the warm ocean is contributing to the high rainfall, it is not a significant factor.

"The temperatures over the ocean will help with the amount of moisture that they can release but the actual mechanism to make it rain is that weather pattern, the deep surface trough, the low pressure system at the surface and this mid level system moving in from the West," he said.

Mr Bennett says the bureau's predicting more storms today before the weather starts slowly clearing.

"There's going to be more shower and thunderstorm activity through the South West of the State, so still some showers and thunderstorms for the metro area and even some morning showers still around on Friday, before we finally start to see things returning to what I suppose you could call normal weather conditions for this time of the year."