Much of southern Australia is about to be hit by the strongest cold blast since last winter and strongest this early in the season in five or six years. Some places may even be colder than any winter day in the past two years and possibly colder than any day this early in the season in more than 20 years. Temperatures will drop as much as 10 degrees below the early May average and be as much as 25 degrees colder than what it was a month ago. This will come as a shock to some, given it has been one of the warmest starts to autumn in many years in some areas. This Friday and Saturday will feel like the coldest of winter days in South Australia, Victoria, the ACT, New South Wales and southern Queensland. Temperatures will struggle to reach the mid teens due to cloud and frequent showers. The wind will make it feel colder than 10 degrees at times. Some showers will contain small hail and will fall as snow on some of the high country. If Goulburn fails to exceed the forecast eight degrees this Saturday it will be its coldest day since winter 2011 and coldest day this early in the season in at least 20 years. Mudgee, Griffith and Wilcannia may be colder on Saturday than on any day during last winter with forecast maximum temperatures of just 10, 11 and 14 degrees respectively. The current forecasts also suggest Friday will be the coldest day this early in the season in six years in Adelaide with a maximum of 16 degrees and five years in Melbourne (14 degrees) and Canberra (11 degrees). This Saturday is forecast to be the coldest this early in the season in six years in Sydney with a top of just 17 degrees and 18 years in Logan in Brisbane with a high of 20 degrees. The cold is developing as a result of a sharp tilt in the upper-atmosphere jetstream which is drawing cold air from the Antarctic as far north as the southern Queensland border. This airmass is originating from an area which has just experienced unseasonable cold itself. Casey station became a frigid minus 22 degrees on the Anzac long-weekend, seven degrees below its April average. The most intense part of the cold pool will miss Tasmania and Western Australia, where temperatures will be a bit closer to normal.