Issued 18th January 2019. Hopefully this extensive synoptic explanation helps. The extreme heat has been governed by an extensive low pressure trough stuck between 2 broad highs. This very slow moving trough has been feed in heat from the Pilbara, WA across into other parts of the country for the best part of a week now.
Please reference above image: The positioning of the trough has been vital for who gets hot and who doesn’t, with temperatures of up to 20ºc differing either side of the trough. This trough (marked in purple) earlier in the week was situated over the Bight and across South East, Central and North West WA. This allowed large parts of South East Australia, South Australia and Central Australian regions to bake under widespread significant heat.
Today that trough can now be seen over South East Australia, Central Australia and into North West areas. A large oblong shaped high is located over Southern WA and the Great Australian Bight. This high is feeding cooler air in behind the trough which is now impacting Southern SA, parts of VIC and TAS. It has been so dominant over Southern WA that places like Perth have largely avoided the heatwave.
Another high over the Tasman Sea has also been very stubborn and not moving. This high has been the reason why the trough can’t progress further East and why the heatwave is lasting 7 to 10 days rather than 3 to 5 max which would be more normal. This same high though is directly an onshore flow over QLD and that onshore flow has not only blocked the heat from coming across to populated areas of QLD, but due to no trough being in place across localised areas it has also hindered any storm chances from occurring. Thats why the heat has been so focused over Western and South West QLD as a State perspective with no rainfall.
Today you can also see localised surface troughs (marked in aqua) over Eastern NSW and the WA Coast. Both of these will prevent a seabreeze from occurring and thus places like the Pilbara should once again see hot temperatures and why places such as Western Sydney are expected to climb into the mid 40’s. Thankfully this is normal for the Pilbara and the seabreeze will eventually push the trough back Inland and allow for afternoon cooling, while the change (purple line) will move North and eventually reach those Central NSW regions overnight tonight.
The reason why it has still been humid over Eastern QLD is due to a number of factors. These include the high over the Tasman Sea which is producing onshore winds, while a temperature inversion and dry air aloft is trapping the humidity in the lower levels of the atmosphere. The monsoon trough and monsoon flow (blue line and yellow arrows) has become dominant and stationary across the ocean regions to the North of the Country.