Summer started out on a cool note for much of southern Australia, although a countrywide heat wave will send temperatures soaring this week. Many areas in southern and southeastern Australia experienced a notably mild start to summer this year. The temperature in Adelaide only reached as high as 24 degrees during the first nine days of the season. This was the city's coolest nine day spell during December since 1993 and the coolest first nine days of summer since 1909. Melbourne experienced its coolest start to summer in 16 years, with the first 10 days of December all staying below 24 degrees. The average maximum temperature in Canberra was just 22.7 degrees during the first 10 days of summer, which is well below the city's average December mximum of 26.2 degrees. Hobart's average maximum temperature for the first nine days of December was 18.2 degrees, two below average for this time of year. This spell included a 12.9 degree day, which was Hobart's coldest December day in eight years. After a notably cool start to the season, the first heatwave of summer will sweep across southern and eastern Australia this week. A pool of hot air sitting over Western Australia started spreading over South Australia on Sunday and will continue to extend further east in the next few days. Adelaide's top of 29.5 degrees at Kent Town on Sunday was four degrees warmer than any day so far this month and the city should reach 30-38 degrees between Monday and Wednesday. Days and nights will become noticeably warmer in parts of Tasmania, Victoria, NSW, the ACT and Queensland during the coming days as well. Melbourne's forecast top of 36 degrees on Wednesday would be 12 degrees warmer than any other day so far this month and the city's hottest day since February. Tasmania should register its first 30 degree day of the season on Wednesday, while Hobart's maximums are forecast to climb five to 10 degrees above average between Tuesday and Thursday. While parts of southern Australia will experience some uncomfortably high temperatures this week, the impending heat will be more persistent further north. Canberra is forecast to reach the low to mid thirties for the next seven days, giving the city an average maximum temperature of around 32 degrees during the next week. This is close to 10 degrees warmer than the opening nine days of summer. Severe heatwave conditions are forecast to affect parts of NSW, northeastern SA and south west Queensland from Tuesday onwards, where some outback will experience oppressive heat. Thargomindah is forecast to average at least 35 degrees for three consecutive days and nights from Thursday. Parts of western Sydney could have their hottest December spell in more than a decade this week. Richmond is forecast to reach 35 degrees or more between Wednesday and Saturday, which would be its warmest four day spell during December since 2005. The hottest day will be on Thursday, when the city could reach the mid thirties and western suburbs should exceed 40 degrees. A cool change will reach SA on Wednesday and parts of southeastern Australia by Thursday, although it won't penetrate into central and northern NSW and Queensland until early next week.