Fog covering suburbs across the Perth metro area has triggered a warning to drivers.

Motorists are being warned to drive with additional caution on Perth roads Friday morning with fog significantly reducing visibility.

The Bureau of Meteorology issued an alert early on Friday, stating fog will make road conditions dangerous in the southern Perth area.

Drivers are being encouraged to switch on their headlights and ensure a safe braking distance, to stay safe on the roads.

It comes after thunderstorms hit Perth on Thursday night, with heavy rain battering large parts of the city well into the night, as a cold wet front swept through the south-west of WA.

Meteorologists at BOM recorded the largest volume of rainfall in Mandurah, with the area recording 44mm of rain since 9am on Thursday.

Rottnest Island saw the second-most rain in the metro area, with 38.6mm and Bunbury measuring 35.8mm.

Heavy Rainfall is expected to continue well into Friday and Saturday morning, with the wet weather expected to ease by the evening.