Explanatory notes:
Perth has seen the coldest August day in 52 years that is since August 27 1968 with the maximum temperature at Perth Metro so far today being 12.1˚C at 12:47 PM and coldest winter day since June 16 2005 of 12.1˚C (draw) and 11.5˚C on June 12 2005.
The cold day today was due to the deep cold air mass over the region along with rain cooled low level air lying over the region creating a land breeze which kept the warmer maritime air offshore and clouds and showers which formed in the unstable atmosphere offshore enhanced by low level wind convergence of the land breeze and near the cut off low kept advecting onshore keeping a lid on temperatures.
Swanbourne recorded their coldest winter day on record with a maximum temperature of only 11.6˚C beating the record of 12.2˚C set on July 26 1998 and the coldest August day on record beating the record of 12.8˚C on August 4 1994. Records exist from September 1993.
The showers are likely to clear from south to north over the next few hours as the cut off low moves to the north. While the upper level cold pool typically results in the possibility of thunderstorms and/or small hail, the cold rain cooled low level air is fairly stable and would likely prevent the formation of thunderstorms over land of the coastal plain with the chances of thunderstorms being highest at the northwestern coastal areas.
On Tuesday should be partly cloudy and dry in the morning, clouds increase in the afternoon with convection trapped under a subsidence inversion, the slight chance of showers.