COULD THE RAIN BE BACK ON THE CARDS FOR SOUTHERN AND WESTERN AUSTRALIA??

We have been watching the battle on modelling between the easterly wind regime over QLD and NSW and the impending cold fronts moving through WA and then onwards towards SA and VIC.


2 week forecast - my notes below with model analysis and nation wide coverage.



Modelling has suggested that the initial indications of a blocking pattern may not be as strong as first forecast.


Euro modelling on the 21st of April 2021 for the period ending May 6th.

It now looks like that frontal weather is expected to win the battle to a degree, and knocking the blocking pattern that was forecast to dominate the synoptic scale.


So with that said the rainfall over the east will not be as heavy, but still falls of 50-100mm is likely in the onshore wind flow, with good model agreement on the coastal nature of the falls and the tight rainfall gradient, the further inland you go.


The latest guide has come into better agreement for the period ending around Monday morning...


GFS 18z run for rainfall ending Sunday afternoon

Canadian 12z run for rainfall ending Monday morning.

ICON 13km 18z run for rainfall until Saturday evening.

Euro 12z run for rainfall ending Monday morning.

That means that with a weaker onshore push and less robust stubborn blocking pattern in the east, that this will open the door for frontal weather coming in from WA through southern Australia and into the Tasman Sea.


The latest Euro model run showing the 500mb heights demonstrates this well. The frontal activity looks to knock the systems in the Tasman Sea out to the east.


Euro 500mb heights 12z run for the next 10 days. Note the upper low barging into the southeast inland knocking out the block. Will monitor trends over coming days.

With the upper low moving into the east and bringing that burst of rainfall and colder weather, it will allow a more zonal flow to develop (cold fronts moving west to east more easily).


Rainfall for the period from next week could be above average for pockets of southern Australia, which would be a welcome shift for parts of SA and inland VIC and NSW. It is usually during this time, that frontal weather does return to bring reasonable rainfall to the southern coastal areas and adjacent onshore inland regions.


Latest short term guide indicates the northward progression of the cold fronts and potential upper air lows through southern Australia.


Canadian 12z run for the next 10 days shows the potential for an upper low moving into the southern inland of NSW or northern VIC, bringing up the chances of rainfall for inland parts of southern and eastern Australia. This is an outlier.

The Euro 12z run shows a more vigorous frontal system moving through southern Australia initially, before becoming cut off from the westerly winds and moving over inland NSW and eastern VIC, with areas of rain and storms for the southeast. But with this configuration, SA would remain with only light falls for coastal areas.

The GFS 18z run shows a more zonal flow through southern Australia, however like Euro, has a pool of colder air cut off from the westerly wind profile through inland Australia, which eventually deepens off the coast of NSW in this particular run, bringing widespread rain and storms to southern and eastern VIC and eastern NSW. Again this would leave SA high and dry initially through the first week of May.

For southwest WA we have been watching trends increase rainfall for the region during the coming 2 weeks.


That is in line with the seasonal shift one would expect at this time of year.


So rainfall numbers will start to increase in the short term with the fronts making contact with the mainland during Thursday this week and another major front expected during the first week of May as a long wave trough lines up the region. That could be the most productive period of rainfall for the southern wet season so far.


Euro 12z run for rainfall over the next 10 days does support the Indian Ocean waking up for the next cold front during early May. Also has a strong cold front later this week. Rainfall numbers may increase in my forecasts over coming days.

Canadian 12z run is a little drier and has the Indian Ocean moisture lagging further back to the northwest, but that may come in, (if this was to verify) mid May with rain increasing, but the idea is similar to other global models. The front on this particular run is not as strong for Thursday.

GFS 12z run is indicating widespread showers and storms later this week and once again next week with multiple frontal bursts. Also has that Indian Ocean moisture plume that could impact rainfall numbers for not only WA but inland areas spreading into SA during the latter part of the first week or second week of May. Something to watch in coming weeks.


Longer term trends does support more influence from the Indian Ocean through the mainland into the southeast during the latter part of May. It also wants to rehash the easterly winds for NSW and QLD with the potential of heavy rainfall for the east coast at times during May. That does remain to be seen. One of these options will win out in the long run but I do suspect that the westerly winds might edge out the easterly influence.


Euro medium term rainfall outlook for the next 6 weeks showing a wetter signal returning to southwest WA and the southern coastline.

CFS ensemble also in line with the Euro modelling. Hangs onto the tropical moisture over northern Australia too, which could be something to watch over the next few weeks, especially if it moves off northwest WA.

Euro ensemble quite wet for southern and eastern Australia, which is a trend that has developed during the last 2 weeks. I suspect the wetter signal is for later in the month into June with fronts only starting to impact the southern and western coastal areas from next week.

GFS medium term modelling also wants to introduce that moisture coming in from the northwest through Central Australia and hangs onto the moisture over northern Australia. It is a lot wetter for southeast Australia through eastern NSW rather than QLD but that remains to be seen as this is an outlier in that idea, and keeping the southern coastline dry. This model does only go out to May 25th.

Overall the next 2 weeks of rainfall is looking fairly handsome for southwest and eastern parts of Australia, that carries a reasonably high confidence forecast with it. Lower confidence on rainfall for the remainder of southern Australia due to the potential of blocking slow moving patterns/systems developing.


The wild card will be the moisture plume from the Indian Ocean which the longer term modelling suggests above. That may bring widespread rainfall for parts of inland Australia if it verifies.


GFS precipitable water values for the coming 2 weeks showing the Indian Ocean coming alive and the tropics drying out.


My 2 week forecast below. This is not encompassing the moisture coming in from the Indian Ocean in it's full potential and may change on Friday. The tropics will start to see less rainfall on the charts from later this week and the south may see an up tick in rainfall numbers as we head into May and especially later May.


This forecast will be updated during Friday with a look at the coming 2 weeks of rainfall once again with a temperature forecast to boot as well.


As always comment on the post or jump onto the forum and we can most certainly chat away in there and discuss the weather moving forward. Want a more tailored forecast and information for your postcode, farm or town email me at karllijnders@weathermatters.org







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