Summer is officially here, with the mercury will soar into the low 40s across parts of southeastern Australia, including several capital cities.

A prolonged period of northerly winds are being caused by a blocking high pressure system in the Tasman Sea and an approaching broad low-pressure trough. These winds will carry a hot air mass towards southern and southeastern Australia in the coming days.

Image: Forecast 850 hPa temperature and wind at 5pm AEDT on Thursday, showing northerly winds dragging hot air towards southern and southeastern Australia.

The burst of heat will impact SA and Vic on Thursday, before spreading east on Friday and into the weekend.

Adelaide is forecast to see three days above 30°C beginning on Wednesday, with the heat set to peak on Thursday with 38°C on the cards. Nightime temperatures will also make it difficult to recover from the heat, with minimums forecast to be in the low 20s for three nights beginning Wednesday.

Meanwhile western and northern SA will see temperatures soar into the low to mid 40s on Thursday and Friday before the cooler change flushes the heat out.

Image: Maximum temperature forecast for Thursday, December 5, according to ECMWF

While not as extreme, Vic will also be hot with temperatures forecast to reach the high 30s on Friday.

Melbourne should endure two consecutive 33° C days on Thursday and Friday, with the temperature only dropping to the low 20s on Friday evening.

Then the heat will creep to the northeast into NSW and Qld, with temperatures 3 to 9°C above average in the capital cities this week:

• Temperatures in western Sydney will reach the mid 30s on Friday and Saturday. 
• Sydney Observatory Hill should peak at around 33°C on Saturday.  
• Sydney’s minimums should be above 20°C for three nights beginning Thursday.  
• Brisbane will then see temperatures reach 34°C on Sunday. 
• Brisbane’s minimum only dropped to 23.7°C early Wednesday morning, with another warm night forecast tonight. 

The heat across eastern Australia will also be coupled with extremely humid conditions, making the temperature feel 2 to 4°C hotter than the actual temperature in Brisbane and Sydney later this week.

Looking ahead, temperatures should be cooler early to mid-next week ahead of another warm spell forecast at the end of next week.