A LOOK AT THE INDIAN OCEAN AND THE MOISTURE FOR THE MEDIUM TERM. WILL THE INLAND RAINFALL RETURN?

Updated: May 25

You know the deal if you have been following me for a long while, the Indian Ocean is key to inland rainfall for winter over inland SA, NSW and QLD. But we have seen a general absence of soaking inland rainfall in recent weeks. That is seasonal and what you tolerate against the climate averages at this time of year. July is the driest time of the year for many in northern and eastern Australia.


Lets get straight to the data sets for the coming fortnight and see whether we can spot any useful moisture surges.


Let's start in the Indian Ocean for the coming short term window to see the movement of moisture.


Euro - Simulated WV in the Upper atmosphere over the coming 10 days.

You can still see the elevated moisture levels over the northern Indian Ocean near Indonesia and over the south of India, that moisture being drawn south and southeast into the Central Indian Ocean and then dragged into Australia with the jet stream playing a big part in that. That pattern is expected to help spawn some cloud bands over the coming week through WA into SA, then southern NSW, VIC and TAS.

Euro 12z - Precipitable Water Anomalies next 10 days.

Elevated PW values are clearly evident north of Australia drifting west into the Indian Ocean and then being pulled south and then southeast in front of the fast flow westerly pattern over the Southern Ocean. You can see the frontal weather is fairly relentless. However there is another port of moisture within the westerly wind belt and that in itself, without the northwest moisture, will see elevated rainfall totals for southern Australia and I suspect we have already seen that over the past few weeks, that infusion of moisture into the westerly wind belt. There is evidence that it will continue for the next 10 days but below you can see that both the northern Indian Ocean moisture and the moisture within the westerly wind belt will both combine to bring elevated rainfall totals as we go through August for large parts of the nation. Short term modelling won't handle these elements well until we are about 5 days out from a system and then you will rainfall totals increase on charts in such a fast and complex zonal flow.

Euro 12z - Simulated Satellite Imagery next 10 days

We can spot 3 major cloud bands during the next 10 days according to both the GFS and the Euro this morning. That is being reflected in the fast flow pattern and frontal weather absorbing the moisture out of the northwest and seeing the transportation of that moisture southeast. So classic negative IOD behavior, with rainfall spreading through the northwest of the nation into the south and then southeast. But the dry spell does continue into the eastern inland....for now.

Euro 12z - Precipitable Water Anomalies next 10 days

Looking at the PW values for the coming 10 days and you can see those elevated values streaming across the country over the next 10 days. Note the impact those values have had on weather from WA, through SA and now today over VIC and NSW where rain has been falling and temperatures below average. Well that will be repeated time and time again as explained in my climate forecasting and long term analysis as long as these drivers remain in force. This will continue through Spring on all accounts and guidance.

GFS 18z - Simulated Satellite Imagery next 16 days

The medium term GFS also supports the elevated moisture streaming across the country during the next 2 weeks. Multiple attempts at rainfall, but note what happens towards the end of the sequence. A pattern flip, and this is what I am talking about in my national weather wraps - this flip in the pattern where we lose the westerly wind belt and the fast flow pattern over the south and the high pressure digs in and settles over this region, will then allow the jet stream to be aligned more northwest southeast over the north of the nation, so through the NT rather than over WA, opening the door for inland rainfall to develop for the NT, QLD and northern SA. This is the time frame to watch over the coming week to see if the models verify. I am leaning towards things shifting during this period.

Euro 12z M - Precipitable Water Values next 15 days

This is the ensemble data set from the Euro and so the resolution is not as great as the single run as it is comprised of 50 members. But note the moisture moving through the nation, in line with GFS, suggesting a pattern flip may be on the cards with the moisture pooling over the north and northwest, drifting west into the Indian Ocean and then being dragged over the northwest, central and eastern inland in the absence of strong frontal weather. That is what is required for rainfall to come back to the eastern inland. And long term modelling, again if you follow my analysis regularly, has suggested the second week of August.

Euro 12z - Rainfall for the next 25 days

Note the filling of the dry slots over the inland come the second week of August, the video runs fast so run it a few times, but I have reviewed all data sets this morning and there are a lot that have 4x the amount shown for the outback into QLD, NSW, SA and VIC. There a few members <10 that keep the westerly wind pattern going, so the bias is in favour for a pattern flip and rainfall returning to inland areas.

Looking at all of this, it is safe to say the following


  • The westerly wind regime over the coming 10 days will contain 2-3 cloudbands that will favour the southern states again. Some moderate falls are likely. The moisture will be deep enough from the Indian Ocean to support some inland rainfall, not necessarily heavy, but a repeat of what we have seen this week, next week.

  • The pattern likely to flip at the end of August with the westerly wind regime settling down and retreating south.

  • Moisture will reorganise in the second week of August to come through the Kimberly and through Central Australia and into the eastern inland of QLD and NSW. That will be a review point next week in my climate update.

How does that pattern flip look like on the modelling?


This is where we stand this weekend.


Saturday 24th of July 2021

Note the fast flow pattern to the south with multiple waves of weather passing through the southern states, dry hot weather over the north, sending dry air across the nation from the northwest to the eastern inland in those westerly winds over southern Australia. There is limited moisture being drawn out of the Indian Ocean, escaping through on the western flank of the upper high. This is bringing some moderate rainfall events to southern and southeast Australia with cold fronts but cannot penetrate further north due to the ridge over the central and northern parts of the nation.


Friday the 6th of August

The westerly wind belt has retreated well to the south. The ridging of high pressure over the central and northern parts of the nation has broken down and relocated south to the southern states. A cut off low pressure system now is allowed to deviate north of the flow and move into moisture sources from the northwest and northeast, lifting the chances of rainfall developing over inland areas.

Now for keen weather watchers as you all are - this is the period we need to watch in the coming week to see if the modelling verifies. I do think we will have more rainfall developing in the short term forecasts this time next week, for the following week over inland areas. So one more week of the fast flow pattern and then maybe, we will see the moisture start to have some impact on the eastern and central inland of the nation.


And a look at the Euro in the medium term


Sunday August 1st 2021

A system will have moved off the east coast of NSW with a trailing cloud band and patchy rainfall through inland areas of QLD, NT and WA. Probably more cloud than rainfall but will watch trends but the moisture plume north of the NT is what we need to watch. That is moving west with the upper easterly winds taking that towards Java.

Wednesday 4th of August

Upper level easterly winds have now shoved that moisture towards the northwest of WA and it is adopting a southwest track. High pressure in the east keeps things dry but the upper level winds out of the northwest will start to drag some of that moisture southeast. We are just waiting for a lifting mechanism to come into the picture in the form of a trough or cold front.

Friday 6th of August 2021

The front arrives over the southwest of the nation, this now pulling the moisture into the central parts of the nation, rather than coming through the Pilbara and or Perth. A much better setup for rainfall to return for SA, QLD, NSW and VIC. This again is a period to watch to see how it plays out but now we are seeing it in the medium term.

Like we saw with the models picking the long duration westerly wind dominance for July, the same models are now agreeing that the pattern should shift at the end of the first week of August.


And moisture will be in play too.





0 views0 comments