CLIMATE UPDATE - AUGUST 2021

Headlines


  • The month to be wetter as we go through much of the southern and eastern inland of Australia. The bulk of the rainfall likely to fall from the second half of the month though a rain event may break out over QLD and WA during the second week of the month.

  • The temperatures likely to remain above average as we track through the first half of the month over the tropical north, with that warmer air filtering down at times to the south and east.

  • Temperatures near seasonal for the southwest and west of WA, with rainfall likely to be above average over the southwest once again, but the rainfall frequency may begin to decrease.

  • A brief look into September and the weather looks cooler and wetter than August. More on that next week.

Rainfall Outlook - August 2021

% chance of deviating from the median rainfall for the month of August 2021.

Rainfall outlook is fairly consistent with the movement of moisture through the northern and western parts of the nation through the southeast and eastern inland, all in relation to the IOD strengthening further. The influence on rainfall increases as the nation warms further so the signal gets wetter as go through the month. Two to three rainfall events over the eastern and southeast inland through August may produce a flood risk, especially in catchments currently in minor flooding and those that have been on watch. This wet month coming may also place other catchments on flood risk. Farmers and graziers should be aware of the rainfall signal and prepare for a wet spring ahead with flood risk.

Temperature Outlook - August 2021

% chance of deviating from the median temperatures for the month of August 2021.

Temperatures are largely unchanged for the nation with a near certainty of temperatures coming in above average for the Top End and parts of the Kimberly and Cape York. Above average temperatures also likely through the remainder of northern Australia extending southeast through to the eastern inland. But the rainfall and cloudiness that may develop through the month may mitigate and offset the heat signals coming through from the north and west during the course of the month. The cooler signal over WA is in relation to the a good chance of a persistent long wave trough sitting over the region with further cold fronts.

CLIMATE DRIVERS UPDATE


SAM - Southern Annular Mode -Negative but trending Neutral

The SAM is expected to stay negative until early next week and we can see that reflected in the pressure pattern. The frontal weather continues to early next week. It is beyond next week that the members are split on where the SAM is going which has been consistent over the past week, with low confidence forecasting continuing. But the mean of all the members is to keep the SAM neutral throughout the first half of August, which would reduce the frontal weather but not clear the westerly wind belt completely. If the SAM can trend more positive, then we may see the easterly winds increase further for QLD and NSW pumping in moisture. There is some evidence that the weak positive shift next week could flare a rainfall event IF an upper trough moves in and links into moisture. Otherwise the SAM will continue to have a large impact on rainfall distribution for August.

IOD - Indian Ocean Dipole - Negative

Not much change to this driver and the guidance with moisture still expected to spill southeast and south through the jet stream from the IOD during August. I will mention that I am leaning towards the Euro data being the most reliable, with this model picking the influence of the IOD well so far, where the BoM model had peaked it too early. If that was the case, we would have received widespread heavy rainfall through the central and eastern interior, more than what we have experienced thus far through June and July. We should start to see the impact of the negative phase as the nation warms more and more likely from mid August onwards, however another cloud band is likely to develop over WA next weekend.

BoM 6 month IOD forecast - this model will be updated again on Tuesday.

ENSO - El Nino - Southern Oscillation - Neutral

New data will be received in the next week or so from the climate modelling with this to be updated during the coming fortnight. For now I still leaning for the ENSO to play no part in the rainfall or temperature outlooks for the coming month, though as mentioned during the past 4 weeks, there is the chance of a La Nina forming during the spring and summer, with a 60% chance analysed across all the global climate modelling agencies. Some of the climate agencies have declared a La Nina watch which is fair on current guidance.

MODEL DATA FOR AUGUST


RAINFALL


Euro 12z C - Rainfall for August with anomalies underneath.

Note that the trend of rainfall becoming more widespread through the month of August appears to be a fair forecast against the data. The data set below is the control member of the Euro this morning, which shows that the influence of the IOD and the pattern shifting away from the westerly wind dominance with a neutral phase of the SAM, opens the door for moisture to pool over northern Australia and be transported south and east through the nation. We are seeing an example of that today through the weekend before the long wave passes eastwards, but the moisture is about what you would expect to see this time of year. But I really want to make the point that the weather will get wetter as the nation warms, the air holds more moisture as the air gets warmer, that leads to more widespread rainfall over inland areas as troughs and fronts, like the one coming through this weekend, move through. It appears that from mid August onwards, some parts of NSW, VIC and SA could be in for a drenching if this verifies. If the rainfall verifies for the southeast, this will lead to flooding over northeast VIC and possibly northern VIC and southeast and southern NSW.

GFS 12z C - Rainfall for the coming 5 weeks.

The GFS also has the same trend with the moisture building further through the month as the nation warms, especially over northern and western Australia. The moisture spread on the GFS today suggests that an early build up may kick off from mid August over the Top End and Cape York with higher humidity and rainfall developing before that moisture spreads south and east through SA, NSW, QLD and VIC. The west will see cold fronts continuing, but the configuration according to GFS would keep the high pressure over the southeast meaning the frontal weather will peak over WA resulting in more wet and cool conditions for the coming 4 weeks or so. The east would be milder, more humid with elevated rainfall chances as we go through the month.

CMC 12z M - Rainfall for August 2021

The Canadian Model is a lot more robust for rainfall to spread throughout much of Australia, picking up on the IOD a lot more than other models, with many areas of the east and south seeing above average rainfall. Also above average rainfall for the outback and through to the southwest of WA. Some flood risk remains for the southeast and southwest with these rainfall totals being forecast so will continue to monitor the risks, but as we are approaching harvest season, it could be quite wet.

The climate forecast system also shows the influence of the above average moisture lurking through the northern parts being transported south and southeast with heavy rainfall totals for parts of August and continue into September. Some of the rainfall being expressed by modelling today would bring up the risk of flooding through southwest WA, southern and mountain VIC and southeast NSW, the ACT and northeast NSW. So will take a look at that in the next 2 updates to see how the risk stacks up against this forecast because as I keep saying, once the tap is turned on, it might be turned on full blast for some areas.


Modelling has failed to pick the potential for rainfall over northern and central QLD late next week so that could be an adjustment that gets factored in at the next update as well.


CFS 12z C - Rainfall for the coming 6 weeks

CFS 12z M- Rainfall for the coming 6 weeks

TEMPERATURES


CFS 12z C - Temperature Anomalies for August

This model may not be too far away from the truth, given that it is suggesting a significant lift in rainfall spread during the second half of the month, and now with the other climate models suggesting a similar outcome for August, then above average temperatures may be moderated by the above average rainfall signal. BUT we need to see the rainfall and cloud cover increase in response to the moisture building in the warmer airmass over northern and western Australia then being transported southeast and east across the nation.

GFS 12z M - Temperature Anomalies for August

GFS continues the warmer weather for August with the coastal areas still looking at a relatively seasonal run of weather for the coming 4 weeks, but this may change if more moisture gets involved as per the model above, where more cloud is drawn across the country and suppresses the heat levels that are being drawn in from the northwest. If there is less rainfall for inland areas, then a warmer month is likely. That could cause more volatile weather to develop through September as the IOD peaks and more moisture gets involved in a more volatile atmosphere. For now I am thinking this run of the GFS is a wee bit too warm for southern inland areas and about right over the northern tropics with higher humidity and early build up conditions.

Euro 12z M - Temperature Anomalies for August

Euro is probably closest to the truth for the month of August, with an early build up contributing to the above average temperatures for the month. The higher temperatures will filter down at times (as per today and tomorrow through the eastern inland) ahead of frontal weather and troughs, which will then set up battle grounds between the airmasses and moisture intersecting this zone then breaking out into areas of rain with moderate falls over inland areas. The CFS model at the top suggests heavy falls are expected through August, especially the second half of the month where the other models are a little less robust, but still offer some heavy falls for southeast inland areas of the nation for August.

Looking into September and the weather looks wetter and cooler than August for parts of the southeast and eastern inland, with the flood risk increasing further, if the rainfall verifies through August. The Indian Ocean Dipole is yet to peak and this will lead to more rainfall developing for areas further inland over central Australia and then into QLD.


The build up may also increase through September, with scattered showers and storms from time to time, but the humidity values will be well above normal, this also being transported south and east and adding to the moisture being sent in from the IOD.


Cloud cover becoming more widespread may lead to cooler weather through a lot of the southern and eastern inland.


CFS 12z C - Rainfall Anomalies for September

The rainfall anomalies are quite elevated for parts of the southeast and eastern inland which would continue to exacerbate the flood risk IF the heavier rainfall develops through August. So once again, no dry spells really expected for many parts of the nation through mid August onwards.

CFS 12z C - Temperature Anomalies for September

The temperature anomalies are very much pointing to a colder month with widespread cloud and rainfall to persist from late August into September. At this stage this is a low confidence forecast, but we need to see how August verifies, and we won't know how that looks until we are about 3 weeks from today. Then we can have a better look at September off the patterns observed and see which model is performing better. But this model below is in line with the idea of wetter and cooler weather as we go through Spring.

I will have another climate update during Tuesday next week.









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