Effective as of 12:00 AM Sunday December 26 2021
View the latest severe weather warnings from BOM at http://www.bom.gov.au/wa/warnings/ .
Explanatory notes:
Perth has seen the hottest Christmas Day on record with a maximum temperature of 42.8°C at the official Perth Metro station beating the record of 42.0°C set in 1968. In fact, it was the hottest Christmas Day on record for any Australian capital city and possibly any major city in the world based on maximum temperatures at the official stations.
The previous hottest Christmas Day in an Australian capital city was 42.1°C in Adelaide in 1888, but then Stevenson Screen's weren't widely used yet and so such a record is not directly comparable to modern times.
Today, there is a chance that it could be even hotter than yesterday. Perth's hottest December day on record was Boxing Day 2007 in which temperatures reached 44.2°C at the official Perth Metro station.
This heat is part of an unusual December heatwave, arguably the worst on record for December. 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. The heatwave is due to a very hot air mass lying 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.
Conditions are also extremely bad in terms of bushfires with significant dry easterly winds. The easterlies were particularly strong early on Christmas Day and again at night and continues early today where the strong easterly gradient combined with katabatic effect could lead to gusts in excess of 80 km/h over the hills and foothills. Strong gusts possible again later tonight and early on Monday, with gusts in excess of 75 km/h possible 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 today. A weak sea breeze developed yesterdaybut it did not deliver much relief.
Earlier onset of and a more significant sea breeze and the upper level anticyclone being displaced by an upper level trough means that maximum temperatures on Monday and Tuesday would likely be slightly lower, but still very hot.
The heatwave will end mid week as the west coast trough moves inland leading to earlier sea breeze onset and the marine layer advecting over the region.