Effective as of 4:00 AM Thursday December 30 2021
View the latest severe weather warnings from BOM at http://www.bom.gov.au/wa/warnings/ .
Explanatory notes:
While the severity of a heatwave is difficult as heatwave severity is a function of both intensity and duration and also on the humidity, minimum temperatures and cloud cover, this heatwave can be said to be Perth's worst on record for December and arguably the second worst on record for any month with the worst on record being the February 1933 heatwave.
During this heatwave, Perth saw some 4 consecutive days with a maximum temperature above 40°C which was the longest such spell on record for any month drawing with 4 consecutive days in February 2016 and February 1933.Perth has never seen 4 consecutive 40+°C days outside of February. The previous record longest 40+°C spell in December was 3 consecutive days in December 2019 and December 2020 where that was unheard of in December prior to December 2019.
In addition, it was also the hottest Christmas Day on record for any Australian capital city and perhaps for any major city in the world with a maximum temperature of 42.8°C while Boxing Day was the second hottest December day on record with a maximum temperature of 43.5°C and hottest since Boxing Day 2007 which had a maximum temperature of 44.2°C.
The heatwave was due to a very hot air mass lying over the region and the suppression of the sea breeze with the west coast trough having remained offshore for many days with a slow moving high in the Great Australian Bight where subsidence heating under and upper level anticyclone also contributed to the heat on both Christmas Day and Boxing Day.
The heatwave ends but more humid with the west coast trough having moved inland with the marine layer advecting onshore but not that much relief with the air mass aloft remaining quite hot.Likely hot again early next week but still plenty of uncertainty on the details this far out.
On average Perth is hotter in February than in December where December's tend to be milder with fresh afternoon sea breezes, however, Perth is on track to seeing 3 consecutive December's whose mean maximum temperature is higher than the February average. Perth is on track to see the second or third hottest December on record where the current hottest on record was December 2019 and second hottest on record was December 2020 (drawing with December 1931).
Maximum temperatures during this heatwave were:
December 24: 38.6°C
December 25: 42.8°C
December 26: 43.5°C
December 27: 41.0°C
December 28: 40.7°C
December 29: 38.6°C
The heatwave of February 1933 was arguably Perth's worst heatwave on record. That heatwave saw 4 consecutive 40+°C days which included 2 consecutive days with a maximum temperature above 44°C.
Looking at the temperatures during the February 1933 heatwave which occurred in a much cooler climate, it is clear that 6 consecutive 40°C Perth days are already a possibility in the current warmer climate, it is not a question of if but only when, highlighting the need for both climate change mitigation and adaptation. Heatwave frequency, duration and intensity are all increasing with climate change.
Maximum temperatures at the official Perth station during the February 1933 heatwave were:
February 3rd 1933: 39.8°C
February 4th 1933: 36.7°C
February 5th 1933: 38.8°C
February 6th 1933: 39.9°C
February 7th 1933: 44.3°C
February 8th 1933: 44.6°C
February 9th 1933: 42.3°C
February 10th 1933: 40.1°C