A cold front and tropical humidity combine to drag rain from coast to coast, across the desert, this week.
- An atmospheric river is expected to bring widespread northwest to southeast rains this week
- Rain has started in South Australia and is expected to reach Melbourne on Wednesday, Canberra and Sydney on Thursday and Brisbane on Friday
- Recent flood-affected areas of Gippsland are unlikely to experience significant drops, but Bureau is monitoring cutoff behind system
According to Bureau of Meteorological chief forecaster Sarah Scully, thunderstorms are expected to develop in southwest Queensland, central New South Wales and even central Victoria on Thursday.
These storms could be severe, with gusts of wind being probably the most important factor.
Then there is the rain.
Rain is already falling on the desert and it is about to spill over into the South East and Queensland.
In winter, cold fronts along the south of the continent are almost weekly, but desert rain in the Dandenongs is not part of the usual scenario.
The difference this time around is what is called an “atmospheric river”.
What is an atmospheric river?
Kimberly Reid, a doctoral student at the University of Melbourne, studies atmospheric rivers.
She explains that they are like rivers of water in the sky that carry water, above our heads, in amounts roughly equivalent to the amount of water flowing in the Amazon.
“So they are huge and when they hit mountains or interact with cold fronts, as we are about to observe, they make that water rain down and can usually cause heavy precipitation, flooding, snow and strong winds. ”
Discerning Australian weather watchers may be familiar with the term northwest cloud band. Such a phenomenon is associated but not quite the same.
A northwest cloud band occurs when there is a cloud band stretching across Australia from the northwest and generally corresponds to a cold front, as is the case at this time.
Cloud bands in the northwest may exist without being associated with an atmospheric river, but they usually don’t bring as much rain.
But when an atmospheric river joins it, it brings extra moisture to the party.
As is happening right now.
An atmospheric river feeds on tropical moisture from the northwest just as a cold front crosses the south of the country from west to east.
“You have this hot, humid air in the northwest, then this cold, dry air coming up from the south.
“So what happens when they interact is that the cold air forces the hot air to rise and cool.”
As the water vapor rises and cools, it condenses into a liquid, generating a huge band of clouds and increasing the chance of rain.
According to Ms. Reid, atmospheric rivers are not particularly rare as there is usually one in our area about once a week.
And yes, there was one associated with all of that recent flooding in Victoria according to Ms Reid.
The rain has already started to fall on the Pilbara and the central desert.
This is a part of the world where rain gauges are scarce but they have already recorded 31mm at the Office weather station in Giles (From Uluru, continue west until you cross the western Australian border, then continue a bit more).
Not bad for the desert and a promising sign for those hoping for rain further south.
“This is very good news for manyâ¦ agricultural areas in South Australiaâ¦ as they have missed much of the rainfall that Eastern Australia has been flooding with in recent months,” said Ms Scully. .
But perhaps the biggest relief associated with the system this week is who will be missing.
After a few completely soggy weeks, the flood-affected areas of Gippsland are unlikely to receive significant precipitation with this band of rain, according to Ms Scully.
However, there is a low in the wake of the atmospheric river and its course is uncertain at this time, but the BOM is watching it closely.
“Particularly the west and north windward slopes of Victoria and New South Wales.”
This system is also expected to bring dangerous surf conditions along the southern coast, which are expected to coincide with the highest astronomical tides, increasing the risk of coastal erosion and possibly even leading to flooding of low lying areas.
Ms. Scully warns residents of the Gulf of Spencer and Gulf of St. Vincent, including Port Adelaide, to remain vigilant.
As usual, please keep up to date with warnings through ABC Emergency and consider bringing a set of dry socks if you venture out in the rain this week.