Melbourne’s thermometer was sitting at 24.7°C at midnight last night, a hot night indeed, but the city is expecting a mild day with a forecast temperature at midday below 20°C. We love to bring you stories when the weather plays games like this.

Image: Observations until 7:30am AEDT and Opticast forecast for Melbourne.

The chart above shows warm winds blowing from the north until the early morning, leading to an uncomfortably hot night. The forecast (after the dashed vertical line) shows winds shifting to blow from the west and then southwest. The warm northerly winds are due to a trough dragging a hot mass of air from the country’s interior. The southwesterly winds behind the trough will bring cooler air from the Southern Ocean.

Image: Near-surface temperature at 11pm AEDT Friday according to the ECMWF HRES model.

At 7:30am AEDT, winds were still from the north and Melbourne’s thermometer was reading 26.6°C but the winds shifted to southwesterlies at about 8:00am and the mercury dropped to 20.4°C at 8:30am.

If there was no cool change before 9am, then Melbourne Airport’s minimum would have been 23.5C (at 10:18pm last night) and it would have been its warmest November minimum in 36 years. But since daily maximum and minimum temperatures in Australia are recorded for the 24 hours to 9am, Melbourne Airport’s daily minimum is 19.4°C recorded at 8:46am, missing it’s chance of a 36 year record!

Image: Himawari-9 satellite imagery and temperature at 7am AEDT. Notice the cooler temps to the west of Melbourne where the cool change had already arrived.

This weather system was also responsible for causing Adelaide to swelter through a 40°C day yesterday which was the hottest November day in 14 years for Adelaide Airport. The city’s 40.3°C was the equal hottest temperature this early in the hot season since records began (equal with the same date in 1897). The cool wind change arrived at Adelaide early in the evening causing the mercury to plummet from 35.5°C at 7:30pm to 24.3°C at 8:30pm.