A BRIEF LOOK INTO SEPTEMBER

The data is starting to improve a little as we edge into the back half of this month. There are some indications in the medium term modelling that a shift to easterly winds are possible. However being so far out, we just flag that idea and place it in the back of our minds as we move forward during the coming week.


I will state at the top, the two drivers to watch during the coming 4-6 weeks are the Indian Ocean and the Southern Annular Mode.


The Southern Annular Mode

The next fortnight, the SAM is expected to remain fairly neutral, however given the volatility in this driver, it can change rapidly in a matter of days so I am constantly reviewing this data. At this stage, the neutral forecast is fair given the spread of modelling from high positive to deeper negative, and not a tight cluster in the forecast members. So this expresses an ongoing level of high uncertainty, we see that in the short and medium term forecasts and the global modelling being quite poor in forecasting beyond 5 days at the moment. That does happen. Into early September, the SAM is expected to be slightly biased to the negative side of neutral. This will mean near normal westerly wave activity for now. The tighter the cluster in the modelling, the higher the confidence in that modelling.

The Indian Ocean - BoM Forecast current.

The Indian Ocean according to BoM has now moved into negative territory, but only marginally. The Indian Ocean is somewhat difficult to forecast ahead of time and again this year, we are seeing some big shifts from the fortnightly data sets as they update. At this time, you can see that the Indian Ocean northwest of Australia is not quite shaking and baking as I expected it should be at this time, which has got one of my eyebrows up, however given the volatility of this driver this guidance will continue to bounce around. As expected the data suggests that the BoM model has been incorrect so I would be leaning to the Euro model based on observation. That expects the driver to peak during October into November before waning. So while it is quieter from the NW at the moment, I am still of the belief the driver will have an impact during September and more likely through October into November.

So lets have a look at the rainfall expectations for the coming month ahead.


Euro Rainfall for September 2021

The rainfall spread across the members are starting to show that bias of rainfall returning to the eastern and northern parts of Australia in line with the seasonal shift, but also in line with the moisture building up over northern parts of the nation. There is a curious dry area over the NW of the nation which is not what I would expect under this current climate guidance so that will be monitored over the next fortnight, this again a signal that the negative IOD is still in the process of deepening, or it could be that the IOD is not as strong as forecast models had it, we wait and see. Otherwise you can see the frontal weather is expected to continue through the month with more wet weather for those areas exposed to the westerly wind regime, but the good signal is that moisture does return to northern and eastern parts of the nation which will dampen down the fire season with some luck and introduce rainfall back to dry areas of QLD. The month is relatively looking as forecast 3 months ago.

Euro Rainfall Anomalies for September 2021

Rainfall anomalies are looking fair and this is a good forecast given the current observations from the Indian Ocean and the constant dry air training through WA. The wetter end to August and start to September from the northwest of the nation I was expecting might not be on the cards, but will continue to watch. QLD may see more influence on rainfall from the easterly wind regime. The easterly winds may become more entrenched as we go through the month with a higher chance of moisture creeping west of the divide bringing up rainfall chances for the eastern parts of NSW and QLD. Most of the nation expected to see at least seasonal rainfall with many areas likely to see more than that. Tasmania may begin to dry out, which suggests the bias towards the easterly wind regime through September.

CFS Rainfall for September 2021

This climate model shows a much wetter trend through southern and eastern Australia with the tropics waking up as well. Like the Euro, a bias to have dry weather over northwest Australia continues. The Indian Ocean according to CFS expected to peak during mid Spring. The frontal weather being forecast by this data set is expected to be more robust and frequent over southern Australia as compared to Euro.

CANSIPS - Rainfall Anomalies for September 2021

The current climate models are pinging a wetter signal for much of inland Australia during this time, and I have mentioned this again, that it only needs to rain once for the outback to jump above average. Where the above average rainfall signals are more impressive are in areas where the rainfall is much heavier during September. So it could be a much more soggy month for the north and east. This model does not have much in the way of strong frontal activity and more of a dominance towards easterly winds.

Temperatures for September 2021.

CFS Temperature Anomalies

There are near seasonal values for most parts of the nation for September, not the blotchy nature of the signal, that represents the uncertainty regarding the impact of easterly winds over the eastern inland. Cooler than average weather the southern coastal areas would be connected to the frontal weather rolling through southern areas of the nation. I think this model is too cool for the northwest of the nation on my current thinking.

For transparency, this was the previous update from yesterday showing a cooler look, so again this shows the uncertainty in the spring time modelling which again is normal.

Euro Temperature Anomalies

This is probably closest to the post, warm to hot weather over the northwest of the nation and over the northern tropics that is due to elevated humidity values keeping the daytime and night time temperatures up. Some of that warmth may drift south and southeast at times, we are seeing that this week and that will repeat itself. The east coast may have below average temperatures in relation to the easterly wind regime developing, so that is something to watch in the coming week.

I will have a seasonal update coming up for September through November on Friday and the signals are largely unchanged across global modelling, but as always modelling is not 100% fool proof, in fact it is full of flaws and with the current observations,


I would be expecting more weather to be developing from the IOD region in the coming fortnight. If we do not see that then we have got a problem in the projected rainfall forecasts, and as such forecasts should be adjusted to allow for that trend.


We won't know that for another 2 weeks - but I will flag it here, it is critical to see that moisture source from the negative IOD to start influencing the rainfall chances in the coming fortnight. Not too concerned but I will be by the end of this month as then we have to adjust rainfall forecasts ( this is why I update so frequently).


AREAS TO WATCH AND COVER OFF FRIDAY

The air pressure anomalies are pinging for an active season ahead through September to November over the northwest and north of the nation, this will impact the remainder of the nation during this period despite seeing a white covering over the nation.

Rainfall anomalies from the Euro continues to show a decent chance of rainfall being above average throughout much of the nation bar the southwest which is seasonally expected. The north very damp with that early onset rainfall.

And more details on the SSTs - the area that we need to see become active to verify not only my forecast of a wetter spring through much of the nation but the global modelling is the elevated sea surface temperatures offshore WA which is the negative IOD zone. And in the green zone, the Euro not totally on board with the ENSO turning into a La Nina, other global agencies suggest a La Nina is on the table.

More to come and that next climate update in 2 days.






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