MEDIUM TERM UPDATE - THE MOISTURE AND THE PATTERN FLIP

Lets have another look at the potential for this pattern flip and the return of moisture to northern parts of the nation which I touched on in the morning updates.


Before we move on the term I use - PATTERN FLIP - means that the pattern literally flips, where in the case below, the frontal wavy pattern is replaced by high pressure, and where the high pressure was, the chance of low pressure forming begins.


We still have one week of strong and gusty westerly winds for southern parts of the nation, which will contribute to the heat building over inland this weekend, adding more thermodynamics to the eastern inland, creating some patchy rainfall, more extensive over the western slopes of GDR in the northern inland of NSW.


The cloud band could increase in coverage further given the thermal gradient being set up across the eastern inland. That is part one of the pattern flip that I need to see. So even though the fast flow pattern continues, how the warm air, higher moisture values and the colder air clash and how that results in observed rainfall will be crucial. Gives you an indication of what the season will hold.


Lets look at the modelling mid morning.


18z GFS - Upper air pattern at 18000ft - Next 16 days

This is a great example of a pattern flip. Note the wavy pattern with the long wave trough and embedded fronts within that long wave bringing the blustery wet weather to southern areas, but then come this time next week, the pattern continues to shift with high pressure developing bringing back easterly winds. By the end of the period, the westerly wind belt has retreated, allowing the pattern to shift back to easterly winds and moisture building up over Australia.

18z GFS - Precipitable Water Values - Next 16 days

This is a useful product, the precipitable water content of the atmosphere. I will point out the shift in the conditions the high pressure has next week which resets the nation, so again watch it a few times to see how important high pressure can be in these situations.

Weekend Synoptic Pattern - GFS 18Z Thursday 29th of July 2021.

The wavy flow is still in place from today through to mid next week, however there is likely to be more moisture pulled into this flow pattern once again with widespread showers and rain likely to form over the southeast inland. Another rain band will likely form in response to the clash of the airmass over the eastern inland of NSW and southern QLD which could see moderate falls develop over the western slopes of the GDR.

The details beyond the synoptic can be found in the moisture values, and you can see there is a lot of moisture to work with. This is why the models are increasing rainfall for southeastern inland areas, especially over the northern slopes in Victoria and northeast of VIC and southeast NSW. So this weekend while we are in our last week of the westerly wind dominance, some clues are already underway as to how the medium term will likely evolve, with moisture building north and northwest of the nation, that will continue to drift west into the Indian Ocean later next week.

By next weekend you can see the pressure pattern has resolved, the westerly winds have retreated southwards. The high pressure is now anchored over the southern states, that is leading to a settled period of weather. On the northern flank of the high, the winds likely to shift into the east and you can now see the modelling printing out rainfall returning to eastern QLD and moisture building over the eastern inland. IF the Southern Annular Mode can trend positive, then this forecast will likely have more merit.


Synoptic Pattern - Saturday 7th of August 2021

The easterly winds are really critical to bring in the moisture from the Coral Sea. But also the movement of the moisture to the north and northeast of the nation drifting west, the weather will turn more humid over the tropics, this leading to more rainfall chances increasing over the inland.

The high pressure system coming through the nation is key, yes it may turn dry and benign, but that reset is required to reconfigure the moisture surrounding the nation, organising it to come through the nation in concentrated fashion. This chart shows moisture relating to the surface pressure pattern above. Note the two large ports of moisture, large rain bands likely forming through this period with fronts moving through the southern parts of the nation. The frontal weather is further south than this week, so you will see them acting as puling mechanisms to drag in the tropical moisture more south than east, as we have been seeing. Again key to producing more inland rainfall. And lastly it allows for moisture to become stuck over inland areas as there is less chance of a cold outbreak blasting the moisture out of the nation.

Taking the modelling out further to the middle of the month, looking at where the pattern flip becomes realised through the nation, and we are seeing evidence of that occurring still mid August. So for those in QLD and northern SA and parts of northwest NSW who are wanting rainfall, there is a good signal still on the board for this to happen mid month, but again will watch the mechanics below as the date becomes absorbed into the short term. But there are key differences being pointed out in the charts between what is happening now than what is happening in 10 days let alone 14 days time. So there is a shift happening.


Synoptic Pattern - Thursday 12th of August 2021

Moisture continues to build in relation to the long fetch easterly winds. The westerly winds which have dominated the last month, are even further south which is allowing the ridge of high pressure to dig it's heels in further south and more of an easterly taking hold of the northern third of the nation. Moisture is still moving east to west north of the nation, but may creep south to bring more build up type weather into the far north. That may kick off afternoon pop up showers and storms. Then keeping an eye on 2 areas of moisture surging southeast which you can see below.

Note the moisture continues to build further mid month which will play a part in bringing rainfall back through inland areas via the jet stream. However modelling has also been on the idea of introducing upper level troughs over inland areas of the nation, and if they form, well that is the last piece of the puzzle to bring widespread rainfall for inland QLD, NT, SA, NSW and VIC. The second trough over WA with a front approaching will also produce a large cloud band if it was to verify, so we could go from benign weather, to two large rainfall events ongoing at the same time if GFS is right. So as I keep saying the potential for the tap to be turned on full blast is quite possible.

Rainfall for the coming 10 days.

Note the rainfall for the southern parts of the nation is connected to the vigorous westerly wind regime. That will break down as the upper ridging of the north breaks down, the pattern flipping. But before then, 3-4 fronts to pass over the southwest of the nation will produce moderate falls which could increase the flooding underway along some river systems. The frontal weather tapping into moisture streaming in from the northwest, also bringing more rainfall for southeastern areas of the nation Saturday and Monday. There may also be some areas of rain for southern inland QLD and northern NSW with the best falling over the GDR in NSW. Then some colour returns to the tropics, signaling the moisture is increasing and there is the chance of a few pop up showers and thunderstorms coming later next week.

More weather details to come and another update on this later this evening.




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