There has been very little rain anywhere away from Australia’s coastline during the past week.
For the majority of the time we have a stubborn high pressure system sitting over the southeast or the Tasman Sea. This brings onshore winds for the east coast, where rainfalls have been good, but without any instability or low pressure, that rain can’t spread over the ranges to inland parts of the eastern states.
The pattern also means that warmer than average temperatures are more likely in the southeast, with the warmth carried there on northerly winds.
In order to make it rain you need two things to work together: instability and moisture. The blocking high means that any cold fronts that try to pass through are either blocked completely, or the strength of them is considerably reduced. This limits the amount of instability in that rain equation.
The moisture part comes from the oceans, and warmer water means greater moisture. Any cold front able to cross southern Australia brings rain to the coast, but in order for it to spread rain inland or on the other side of the ranges, it needs a feed of tropical moisture.
The blocking highs act to subdue both these things - firstly by weakening the cold fronts, and secondly by blocking that feed of moisture from the tropics.
One cold front is defying the trend in the middle of this week.
It’s a strong one, and it has significantly colder air behind it. That front is crossing Tasmania on Wednesday, bringing much needed rain to the west along with a chill in the air that lets it snow to low levels.
What about the rest of southern Australia? Is this front strong enough to make a difference?
Unfortunately, the answer is no.
The top of the front only clips southeastern SA and southern Victoria, and the blocking high means there is no connection to tropical moisture - so from Whyalla, to Renmark, to Swan Hill and further inland, you’ll feel a drop in temperature but no rain.
May 2025, overall, is highly likely to be drier than average across the majority of the country. The highs aren’t likely to lose their dominance over our weather pattern and this will really limit the rain.
The pattern may get a shakeup as we go into June.
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