CLIMATE UPDATE - AUGUST 2021

Headlines


  • The winter westerly to kick off the month, the pattern we see across Australia at the moment likely to continue into the first week of August.

  • The pattern begins to break down with moisture beginning to increase over inland areas through the second to third week of August, bringing up rainfall chances for inland Australia.

  • The wet signal is beginning to extend from the southern coastal areas and spread to inland areas as we get to the second half of the month.

  • Temperatures will be generally above average for the north of the nation and that will spill down into central and southeastern areas with frontal weather dragging it down. I do think the temperatures are a little too warm still, but the signal is largely unchanged so August temperature wise will be similar to July.

  • Rainfall signals improve greatly for QLD, SA, NSW and VIC as we get into September with the moisture increasing over the north of the nation, early build up likely for the NT and northern QLD and WA.

Rainfall Outlook - August 2021

% chance of deviating above and below the median rainfall for August.

Rainfall for August will be a tale of two halves. The first half of the month, particularly the first week of August, will still be heavily impacted by the westerly wind belt so the rainfall spread we have seen over the course of the past two weeks will continue once again for this week coming and likely the week following. The SAM is currently expected to move around the neutral to negative phase over the coming week, the negative shift coming this weekend and that is reflected in the short term forecasting. To finish the month and to begin August, there is no change from this driver. The Indian Ocean data which is yet to come in, is still suggesting a wet signal from the northwest for the coming 4-6 weeks and we can see that in the WA rainfall today and that will repeat a number of times during the coming 4-6 weeks.

They are the major influences.

Temperature Outlook - August 2021 % chance of deviating above and below the median day time temperature for August.

Very similar spread in the temperature outlook for the month of August, and the temperatures are likely to remain above average over the north with humidity values also reaching above average proportions due to the influence of the Indian Ocean Dipole. The temperatures along the southern coastline will remain near to below average for the coming 4-6 weeks with the first week of August suggesting more dynamic winter weather is possible for the southern states. The temperatures will warm at times through the month for southern areas, but the signals are for more active weather to pass through the southern parts of the nation so that feeling of early spring may be apparent with large fluctuations in the day time temperatures. Overall the SAM and IOD are playing a large part in the temperature outlook for much of the nation, with cloudy skies over the nation and moisture drawn into cold fronts rushing through southern areas, bringing longer duration of cloud and rainfall events.

CLIMATE DRIVERS

Southern Annular Mode - SAM - Remaining neutral but negative pulses in the coming fortnight.

The neutral phase of the SAM can be seen on the satellite today, cold fronts spaced out but moving through at pace through southern parts of the nation. We see the phase shift swiftly negative during this weekend and there is an increase of frontal weather over the southern parts of the nation. Then we see the phase turn neutral before another negative pulse mid next week. This will mean frontal weather and a zonal flow with southern areas picking up the bulk of the national rainfall is set to continue into early August. And indeed the driver beyond this continues to show neutral phases through to the middle of August. Once the phase tends positive, the frontal weather should ease and this could open the door for more rainfall to develop over inland areas.

INDIAN OCEAN DIPOLE - NEGATIVE

The IOD remains largely unchanged in the coming 4-6 weeks with the global data coming ahead of the BoM update today suggesting no shift in the wet signal for the month of August. The moisture continues to pool over the basin and is being drawn south and east into cold fronts and troughs, which we have seen in recent week and we see it today. Another pulse of moisture next week confirms that the IOD will remain active in phase with the neutral SAM, supporting the above average rainfall signals for the southern states. Inland areas will have to wait for the SAM to turn positive for the moisture to build up over the eastern inland of QLD and northern parts of SA, this bringing rainfall chances up. This is likely to occur during the second half of August and the global models do show that rainfall filling that dry zone. You can see that below.

Indian Ocean Dipole Outlook - Negative phase to continue through Spring with more reliable agencies peaking this driver through September through October.

ENSO - NEUTRAL.

As explained in the previous few weeks, this driver is having little impact on the weather currently and likely for August with the driver expected to remain neutral for now. However as pointed out in previous climate updates (head to the climate section) the Pacific could shift into a La Nina, with some agencies suggesting a La Nina developing in our spring or summer. That would see the Pacific and Indian Ocean both in wet phases which would enhance rainfall forecasts further. Some agencies have a lot of green on the board for August, and I am not satisfied that is reasonable for QLD, given the current medium term outlooks, so it is best to keep it calm. The rainfall anomalies will likely increase from the second half of this month into September over northern parts of the nation which is reasonable against climatological expectations under this current guidance.

BoM ENSO Forecast - Valid July 6th (this updated today or tomorrow).

THE RAW DATA


Rainfall


Euro M Rainfall for the coming 6 weeks

The easiest way of showing the above impacts of the drivers on the rainfall is this video below. Note the SAM influence being neutral and somewhat negative at times during the coming 2 weeks, keeping the south wet and the north and central parts relatively dry and warm. But then a pattern flip with the SAM becoming more positive towards the second half to the month, now allows that high moisture value come in via the jetstream through the inland areas of WA, SA, NT and QLD. This combining with easterly winds over the eastern inland would increase rainfall for these areas for the second half of the month and there are signals for that to occur, while the south gradually sees a reduction in rainfall frequency, not dry, but a reduction in frequency. This video takes you through from now until the 3rd of September.

Euro C Rainfall for the coming 6 weeks

This is the single control member of the above data set showing the rainfall events spread out over the nation during the coming 6 weeks, the south copping the most of it first up but notice there are events for inland areas of QLD, NSW and SA through the month, especially the back half of August, which is fair given what the data is saying today.

Euro M Rainfall Anomalies for the coming 6 weeks

The drier signal over the north is starting to erode, that will shift through QLD moreso as we go through September, and that dry signal is largely thanks to the SAM over the coming 2-3 weeks, which is also likely to resolve during the second half of August.

GFS M Rainfall anomalies for the coming 5 weeks

This model which has been running way too dry continues that signal, but I must say it has suddenly decided to bring in rainfall over the NT and inland WA, so perhaps it is starting to get with the program surrounding the influence of the SAM and IOD for the next few weeks and then the pattern flip coming into the back half of August. It is not impossible that it will be this dry, but I am leaning more with the Euro which has performed better.

CFS M Rainfall anomalies for August.

The tie breaker between the models is the Climate Forecast System, which is suggesting a wet signal for August, probably a little too wet for inland QLD, but again like the GFS not impossible, but I will think that the rainfall signals from all the data sits with the Euro outlook.

CFS M Rainfall anomalies for August through October.

This is the outlook for the seasonal period August to October, and continues to support my forecast from last week for the season ahead, that the wet weather will increase, the only thing I disagree with is the dry signal for northern parts of the NT and QLD, which is not what I would expect under the current climate guidance and the previous analogues I have reviewed when we have seen the drivers in this phase.

Temperatures

CFS M Temperature anomalies for August.

This is an interesting signal from the Climate Forecast System, once again I believe running a little too cold for much of the nation, but not out of the realms of possibility. But I believe the above average temperatures will continue for the bulk of the NT, northern and central WA and QLD with that warmth being drawn south and southeast with northwest winds developing into frontal weather. That said, if there is more rainfall, more cloud cover than the below is possible but I am think this is a touch too cold.

Euro M Rainfall anomalies for August.

This is probably closer to the post in terms of temperatures for the coming 6 weeks which takes in August. I do think we will see the heat continue to build as the SAM continues to dominate the south over the coming 2-3 weeks. Then the heat values will come down when the pattern begins to flip. Once the pattern flips, more cloud and rainfall potential is likely for central and eastern parts of the nation and thundery showers may form in the hot and humid air over the northwest of WA, through the Top End and into Cape York.

So as you can see there is a feeling of ground hog day if you are looking for a break in the conditions, whether you are over the windy rainy weather in the southern third and or the dry hot and sunny weather over the northern two thirds of the nation. Conditions likely to persist for another 2 weeks at least and then a pattern flip more likely to evolve in the second half of August.


More details to come and another update on the climate drivers coming up tomorrow when all the data is in overnight into Wednesday.

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