Effective as of 12:00 AM Saturday December 25 2021
View the latest severe weather warnings from BOM at http://www.bom.gov.au/wa/warnings/ .
Explanatory notes:
An unusually intense heatwave, arguably Perth's worst December heatwave on record occurs until Tuesday as a very hot air mass lies over the region with the west trough deepening and lying offshore for several days and an upper level high leading to subsidence heating of the airmass. It will likely be Perth's hottest Christmas Day on record beating the previous record of 42.0°C in 1968 and Perth's highest December temperature on record, 44.2°C on Boxing Day in 2007 is also under threat this Christmas Day and Boxing Day. At least 3 consecutive days with maximum temperatures above 40.0°C are expected. Perth's longest December 40+°C spell on record was 3 consecutive days in December 2019. Prior to December 2019, 3 consecutive 40+°C days was underheard of in December.
Conditions would also be extremely bad in terms of bushfires with significant dry easterly winds expected. The easterlies would be particularly strong early on Christmas day and again later at night and early on boxing day where the strong easterly gradient combined with katabatic effect could lead to gusts in excess of 80 km/h over the hills and foothills. The strong easterly gradient with the west coast trough offshore will keep the sea breeze fairly weak, if it develops on Christmas and Boxing Day.
The most likely scenario is for the heatwave to end from next Wednesday as the west coast trough moves inland leading to earlier sea breeze onset and the marine layer advecing over the region.