RAINFALL EVENT NEXT WEEK FOR SOUTHERN AND EASTERN AREAS.

There is some promising signs coming via model data this morning, that moisture running through the eastern inland of the nation may be picked up by a stronger front passing from WA through SA, leading to widespread rainfall developing.


OVERVIEW


The atmospheric dynamics are expected to be running pretty high with the combination of well above average temperatures, moisture being drawn in from easterly winds and this being lifted by a strong cold front and upper trough to produce rain over WA from next Wednesday and this then travelling through SA by late Thursday and into TAS, VIC and NSW by next Friday onwards.


A large band of rainfall is anticipated to develop.


MODELLING AND INGREDIENTS.


There is reasonable agreement in the modelling to bring areas of rainfall from west to east through next week. A strong cold front looks highly likely to pass through WA mid next week, and as that front passes through, a large scale temperature gradient is expected to develop. This will be the zone where a low pressure system forms as the airmass clash.



Moisture during this time in the warmer airmass (remember the warmer the air the more moisture it can hold = larger and more productive rainfall events), will be drawn west across the country in broad scale easterly winds, that veer into the northeast, rotating around the top of a strong high over the Tasman Sea. The difference in modelling is the Euro has more ports of moisture coming into the system, but is quicker in passing the system through the south as you can see below. The GFS is slower as you can see in the pressure pattern allowing more moisture to pool and then be lifted, the rainfall event slower to move from one point to another means a longer duration event = more rainfall.

The system next week while large scale does move through quicker and does not quite maximise all the moisture available, but for now the signals across the deterministic data in the global modelling is strong for a front to pass through the southern parts of the nation.

The simulated cloud forecast is also a useful tool to reflect how the moisture is being utilised in terms of cloud and rainfall coverage/potential. There are cloud bands galore in the coming 10 days. It is now a matter of whether they can lift into areas of rain.

And here is the mechanics to get the cloud and rainfall developing later next week over the east.

As mentioned, the GFS is a lot slower with the system, this is next Thursday showing the speed of the system holding it back over SA where the Euro has it about 1000km further east. The system then is allowed to lift the moisture that is passing through the eastern inland of the nation. That leads to widespread rainfall developing. GFS is more aggressive in the idea of low pressure development. The better rainfall is determined by the timing of the system, the timing of development and whether there are any blocking complications over the Tasman Sea. In the latest guide, this appears to be not an issue but as pointed out yesterday, if a block forms in the Tasman, it may force the system south! That is still a chance, but a lower risk today.

The latest GFS shows quite widespread rainfall moving through the region to kick off September over southern Australia. As I forecast two weeks ago, a wet end to August over the south and east may continue into early September and on current guide, that seems fair.

Lets look at the rainfall data


The GFS continues to show widespread rainfall developing next week for wide areas of southern Australia. Now don't get caught up in the colour shading over you as it will chop and change, I want you to note the coverage of the rainfall potential, note that it is shifting from coastal domination and now being introduced to inland areas more frequently. So this is signifying two things. 1. The Indian Ocean influence may be improving. 2. The seasonal shift is also on early this year and you will feel that next week. So looking at the rainfall spread once again the colours will chop and change as we get more clarity on each system, but for you farmers and those on the land, it is likely to be an early spring with useful rainfall for some, annoying rainfall for others.

The Access Run this morning which takes a more focused look at next week connects the dots still, bringing the ingredients together to produce widespread rainfall, but note where the rainfall initiates, over central SA, that is where the front runs into the moisture pouring in along the western flank of the high. It meets that moisture, lifts it and then propels it back over the east and southeast as a rain band. So perhaps we flag the idea that rainfall may be better the further east you are. However if you are in SA, all it takes is for the system to slow down, a low to form closer to the EP and you could see a months worth of rainfall for much of the states Ag areas. This is the kind of weather we are dealing with. Higher confidence of follow up rainfall for large parts of VIC and NSW at least.

The Korean Model has less rainfall in terms of totals, with the heavy rainfall smear for the southeast connected to the coming few days. It has got widespread falls, but lighter rainfall. That suggests that the ingredients are there, but they are not coming together, they are out of phase so this is the outcome if it does not all come together. We have to leave that solution on the table for now as we are a week out and things can fall out of phase. Again, all we need if there is a block in the Tasman Sea and then lighter falls are likely.

CMC shows a similar spread to ACCESS and indeed GFS and Euro in the front running into the moisture rushing west bound through the nation, then crashing into the front moving in from the west and again, the same principal, lifting that moisture and throwing it back east and south as a band of rain.

For now I will leave it there, and I really wanted to outline the science at a useful level so that as my forecasts come out with the rainfall charts in the coming days, which will chop and change, you have a few of the mechanics and the understanding under your belt to understand why that is, and why it is not personal....it is just weather.


My medium term forecast charts to follow after lunch.

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