Thanks to a large sustaining high-pressure system and inland heat trough, many parts of southern Western Australia should either today or tomorrow record their warmest September day in four years. 

The high-pressure system which has been situated over WA since Thursday has brought settled and calm conditions for much of the state, with a lack of cloud cover allowing daytime temperatures to significantly warm. This has been aided by the formation of an inland heat trough, which has been directing warm air from central and northern Australia towards the southern regions. 

The majority of towns in the region are forecast for temperatures around 9 degrees above average for this time of year (Albany 26 degrees, average of 17 degrees); potentially making it their warmest September day in four years (2015), and warmest day since the beginning of April this year. 

Towns in the South West closer to the western coastline including Shannon, Bridgetown and Manjimup should today (Sunday) experience their warmest temperatures, while places more slightly east like Albany, Mt Barker and Hopetoun look to peak in temperature tomorrow (Monday). 

This is due to the onset of a cold front looking to arrive tomorrow, which will push the warmer airmass east, therefore creating this large spike in heat. This cold front looks to also bring showers and possible thunderstorms throughout Monday afternoon and evening, significantly dropping temperatures going into Tuesday.