NATIONAL WEATHER UPDATE - MOISTURE WATCH FOR THE MEDIUM TERM.

We know that the fast flow pattern is fairly well entrenched over Australia, the frontal barrage continues to move through southern Australia for the next 5-7 days. That is going to see the best of the rainfall to continue falling over the same areas as has been reported over the past few weeks.


Rainfall percentages for month to date - BoM - From July 1-26 2021.

Away from interior and northern QLD, this is a classic winter month of rainfall. The classic winter westerly belt taking full control of the weather and leading to the southeast of VIC and east coast of NSW to turn dry and benign. The weather not changing much for the remainder of the month. Good to see some elevated values for parts of northwest VIC and the Flinders through to remaining Ag areas in SA. That will persist for another week. Dry season over the north and northwest holding firm with well above average temperatures.

Moving forward into early August, the moisture over Indonesia and Malaysia and through to south of India is increasing further with the monsoon strengthening. The above average rainfall for this region is expected to sweep south through the Indian Ocean, this meandering off the northwest of WA.


Now the moisture can go two directions, which the models are toying with, given that it is in the medium term, the guidance is quite poor but the theme is for, a pattern flip over Australia and for that moisture near the equator to shift south.


Rainfall for the coming 16 days Indian Ocean Basin.

Moisture shearing south from the equator is expected to spawn widespread rainfall through Indonesia and points west offshore through the northern Indian Ocean, some chance of a tropical low to deviate well south of the monsoon trough. That would mean a higher chance of moisture being dragged into the mid latitude westerly winds/jet stream, bringing moisture through Australia for the second half of August. So this is the period to watch not only the pattern shift over Australia, but watching what is shifting offshore over the Indian Ocean.

The two options for the moisture under current guidance for the second half of August.

Option 1 - The moisture is sheared south and southwest into the Indian Ocean west of the nation. The moisture is likely to then be pulled in by the jet stream and link into the cold fronts that are likely to be racing through the southern ocean during this period. That would mean even the moisture is going the wrong way, it eventually becomes embedded into the flow pattern that comes over Australia from the west. In this scenario, the frontal weather would be laden with deeper moisture and likely to produce above average rainfall, a bit like what we have seen this past fortnight and more recently overnight in the SWLD of WA where nearly 100mm fell from a strong front.


Option 2 - The moisture is sheared south then southeast, picked up by the jet stream and dragged southeast or east across the nation with widespread rainfall developing ahead of cold fronts and inland troughs. That would see the rainfall chances increase for the second half of August. The only mitigating factor against the moisture being utilised is if the SAM (Southern Annular Mode) is in a negative phase, which is possible, that meaning the moisture has to time itself perfectly ahead of the frontal weather spreading through the nation. If it all comes together and in phase, then widespread rainfall for inland parts of the nation would become more likely. There has been some signals of at least inland rainfall returning in the medium term modelling, but the models are struggling in the short term with each front, so again the low confidence strap applies.


The traditional region to watch for the negative IOD phase is in the region south of Indonesia, which is still quiet for the coming 10 days, but moisture can also come from further upstream as described above, so for those wanting a lot of rainfall, when that red circled area starts to get active, then we are on for more robust rain events. That most likely coming from September with the seasonal shifts.


I posted this yesterday but I will do it with regards to what to seasonally expect as we go through the coming months. The jet stream also moves in line with the seasonal shift through spring.


The jet stream and it's average movement through the nation from the northwest to southeast/east.

Note that what we have observed in recent months with the jet stream dragging moisture in from the northwest into the southeast through southwest and western parts of WA, then into the Bight and into the southeast. Occasionally rainfall has been linked to the moisture out of the northwest through SA, but a lot of the cloud bands have deviated south of SA. But looking at where the jet stream ends up going during the spring time, you can see the northern shift of the air flow will support the rainfall and moisture being spread throughout the inland of WA, through SA and into the eastern inland. October through November, moisture from the IOD comes through northwest WA and then into SA and often will bypass SA and feed storm events in the east, but occasionally the moisture can stream direct into SA ahead of slower moving frontal weather/troughs from WA producing widespread thundery rainfall.

Now back to what can be expected in the coming 2 weeks rainfall wise.


Rainfall for the coming 16 days - Valid Tuesday 27th of July 2021

There is little change in the guidance for rainfall over the next 2 weeks. There will be heavier falls coming back into SW WA and western TAS with the frontal weather to continue in the westerly wind regime this week, and then again at the end of the period. The GFS keeps the frontal weather going as you can see below, that would see heavier falls for the southeast and east, but I am leaning towards there being a high pressure belt coming in next week for a few days to squash the pattern over southern Australia. Then as mentioned above, we watch the moisture for rainfall potential for the second half of August.

18Z GFS - Upper air pattern the next 16 days.

The GFS is printing out some major weather events in the coming 2 weeks, with severe weather potential. I am not so sure on this as we go through the second half of the outlook so I caution you things are likely and will change. BUT, if these signals do continue, then forecasts for later next week will have to be adjusted to factor in possibly some larger scale weather events moving into southern and eastern Australia. This could interrupt the moisture streaming through the nation, and that is the mitigating factor once again to seeing a larger influence from the Indian Ocean.

18Z GFS - Precipitation and surface pressure pattern the next 16 days.

Note the fast flow pattern still expected for the coming 2 weeks. High pressure still cannot get entrenched and set up over the southern parts of the nation and is largely being pushed along at a rate of knots by that persistent westerly wind belt. This will stop the moisture over northern Australia to be utilised. The Euro is wanting to squash the westerly wind belt further south which would limit it's influence. You can find more on that in the evening updates later today.

So as you can see, there is plenty of moisture on the horizon. The issue, the fast flow pattern over southern Australia. The models still largely spit down the line in the short term, but better prospects are on the horizon with the seasonal shift working in the favour of those who want a decent downpour before the warmer months, but bad news for those harvesting etc. So mixed blessings ahead.







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