A heatwave encompassing much of south-east Australia to 20 December broke numerous maximum temperature records with the Bureau of Meteorology (BOM) issuing special climate statement No 53 (PDF).
This included an exceptionally hot night on 19–20 December breaking many minimum overnight temperature records.
The most extreme aspect of the event was the very high overnight temperatures which occurred on the night of 19−20 December. Temperatures didn’t fall below 30 °C over parts of northern Victoria and western New South Wales, and remained in the mid- to high 20s over much of central and western Victoria. Further south, many Tasmanian sites had minimum temperatures near 20 °C.
At Mildura, the minimum temperature on 20 December was 31.9 °C. This is a new
record for the highest overnight minimum temperature observed in Victoria, breaking the previous records of 30.9 °C set at Mildura on 24 January 2001 and Kerang on 26 January 2003. Ouyen (31.1 °C) also surpassed the previous Victorian annual record. Echuca (30.6 °C)—for which it was the second successive night above 30 °C – the first time this has occurred anywhere in Victoria—and Charlton (30.2 °C) were both above the previous Victorian December record of 30.1 °C, set at Swan Hill on 24 December 2009.
Most of the western half of Victoria, and the northern half of Tasmania, had its hottest December night on record. Along with Mildura and Ouyen, it was also the hottest night on record for any month in parts of north-central Victoria, with all-time records set at Bendigo and Castlemaine. In total, record-high minimum temperatures for December occurred over 58 per cent of the area of Tasmania, and 51 per cent of Victoria.
A few December records were also set in southern inland New South Wales. Weak onshore flow associated with evening thunderstorms resulted in Melbourne missing a minimum, but records were set at a number of suburban stations. Melbourne’s 9am temperature of 34.0 °C was, however, a December record, as was Adelaide’s 37.5 °C the previous day.
Read more on Rising summer night temperatures increasing risk.
But the distinguishing feature of the heatwave was it's widespread nature encompassing South Australia, Victoria, Tasmania and southern inland NSW.
A report by Newdaily.com.au provides information on the immediate health impact of the Saturday 19 November extreme heat event:
More than 300 people across Melbourne and Adelaide were admitted to hospital with heat-related illnesses on Saturday. According to Emergency Management Victoria, a dozen cardiac arrests were recorded, along with respiratory-related illnesses. In addition, four cases of children being left in cars were reported to 000.
More detail from the Bureau of Meteorology special climate statement:
"Statewide mean temperatures for 1−20 December were 3.54 °C above the 1961−90 average for Victoria, and 2.84 °C above the 1961−90 average for South Australia. Both surpass the previous records for the equivalent period, +2.94 °C and +2.67 °C respectively, both of which were set in December 1994. Statewide average maximum and minimum temperatures for the 1−20 December period have also set records in both States.
"The consistent warm conditions have also resulted in many locations having large numbers of days above thresholds such as 35 °C or 40 °C. Adelaide has had six days of 40 °C or above as of 20 December. This is a record for December (previously four days in December 1897 and December 1898) and equals the second-highest number for any month."
Here is how David Spratt tweeted the long hot night in Melbourne:
Record December maxima today. Avalon 45.1 (was 43.2), Geelong 44.4 (was 43.1), Aireys Inlet 43.0 (was42.1) Obs https://t.co/qzaZwMGZKT— BOM Victoria (@BOM_Vic) December 19, 2015
#Climate heat! Yikes! Last night Melbourne smashed overnight min. temp. record for December with 28.5C, 2.2C higher than old 1994 record.— David Spratt (@djspratt) December 20, 2015
Bureau of Meteorology senior climatologist Blair Trewin noted that heatwaves such as in South Australia usually occurred in late summer. "Systems tend to be more stable and slow moving," he said. "It's unusual to get a heatwave in December. We've had that a few times in January and February but never December." he told The Age.
Trewin provided some reassurance that the December heatwave does not necessarily mean hotter conditions in January.
"The seasonal climate outlook is leaning towards cooler conditions in much of Victoria and South Australia," he said. "We are experiencing a strong El Nino, but the main effect of that on temperatures in Southern Australia is actually in the second half of the year. El Nino effects on average temperatures disappear in Southern Australia from January onwards."
During El Nino Summers in Victoria are likely to experience both hot and cool extremes, so more hot days on the way, but likely punctuated with some cooler weather.
Strong current seasonal climate influences on Australia include a the strong El Niño in the Pacific and record-warm temperatures in the Indian Ocean with a positive Indian Ocean Dipole. Record warm Indian Ocean waters can act as a source of moisture, and may provide extra moisture for rainfall systems developing over Australia.