I like to look at the influence of the climate drivers when they are in phase to have major impacts on the weather nation wide. The Indian Ocean is certainly a major player as we go through August and to around the end of December this year.
The rule of thumb for August is that as the nation warms, the more moisture atmosphere can hold. So the moisture plumes coming out of the northwest Indian Ocean will likely become more robust over inland parts of WA through northern SA and depending on their bearing (whether they move east into QLD/NSW or southeast into NSW/SA) provide more rainfall for the eastern states.
I do firmly believe the second half of August, following the pattern flip over the next 10 days, will become wetter as we go. That is expected against all the climatological data sets as we move from winter into spring, lose the winter westerly belts and see high pressure move further south. This allows moisture to pool over northern parts of the nation, especially the northwest when the IOD is in a negative phase, this becoming a constant port for widespread rainfall development in spring time.
Lets look at the data as it stands.
The IOD remains NEGATIVE - this again assists in the development of rainfall through late winter, peaking through spring and then waning during summer as we move from season to season.
The influence of the IOD, when either negative or positive, across the nation. You can cross check the impact status against the time line at the bottom to track when you can expect to see influence on average, but not one event is the same. This years event looks to peak through the mid spring period.
When and where the moisture plumes come from, during the phases of the IOD and the seasonal impacts on that moisture. A you can see the moisture is likely to lift further north through WA over the coming weeks. We have seen the moisture further south with a high impact over the southern states with frequent rain bands and above average rainfall.
And when the Indian Ocean is in negative phase combined with a negative SAM (more frontal weather impacting the nation) the rainfall can be well above average. Pockets of southern Australia has experienced this. The weather will be very wet if we get a negative phase of the SAM through late August into September.
I will have more on the flooding risks this spring in the next week or so, but if you are living west of the divide in NSW and north of the divide through VIC and parts of northeast SA and southern QLD, flooding is an elevated risk this spring.
Let's look at the next 2 weeks to see how the Indian Ocean is performing in relation to the weather in the medium term.
18Z GFS Precipitable Water Values - Next 2 weeks
The moisture is still a little further north than where it has been through June and July, which happens with the patterns moving around a bit. We are seeing a reset of the pattern throughout the basin and this will lead to a period of quieter weather for the IOD region. But that won't last. The moisture further west is also having influence, being drawn into the westerly wind belt and propping up rainfall chances for frontal weather that will move through southern Australia. We will see that this weekend, with the front carrying a large rain band and then that larger front moving into another shot of moisture coming through WA from the NT. Next week we can see a tropical low that is being analysed by GFS moving southwest through the Basin, but again this will change from run to run. There overwhelming trend in modelling is that moisture near the equator will start to drift south next week, with the jet stream likely to pick up the moisture and sling shot it east and southwards into mid month over Australia.
This is looking into the middle of the month with what is happening over the Basin, there is quite an impressive tropical low that IF it forms, would pull down deep tropical moisture into the Basin, quite close to the jet stream that would then funnel that moisture over the nation in week 3/4 of August. So for now, the influence from the Indian Ocean is quite low with the basin resetting.
At the same time over Australia, while the Indian Ocean is resetting and moisture is surging southeast and south through the Basin, the moisture over the north of the nation will also be well above normal, and this will now provide us with the chance of rainfall for QLD and follow up falls for the remainder of the northern and eastern states. There will be frontal weather continuing through the coming week, but it won't be until mid month that those fronts start to encounter moisture surging south from the NT/WA to provide inland rainfall. So again watching the north now with anomalous humidity values for August.
12Z Euro Simulated IR Satellite - Next 10 days
The satellite shows the quieter period of weather across the Basin heading southeast into the nation. But do note the moisture surging southeast into the front later this week over southern Basin before that sweeps into WA with a band of rainfall. So while moisture is not streaming from south of Indonesia into the frontal weather, the fronts are being seeded further west of the nation, bringing the chance of widespread rainfall to the western parts of the nation. Those fronts then bringing patchier falls to SA and then the eastern states. So while the influence is weak for the coming 10 days, there is still some rainfall connected to the action we are seeing thousand of kilometres further west.
Friday 6th of August 2021
The next parcel of moisture to impact Australia associated with the Indian Ocean is later this weekend for WA with a front bringing a band of moderate rainfall.
Wednesday 11th of August 2021
The pattern has shifted further south if this is right, by the middle of next week, with the moisture drifting deeper into the Central Basin, that seeding the jet stream to bring moisture into the nation from the middle of the month. The traditional area for where above average rainfall emanates from generally, is expected to remain quiet for the coming 10 days.
So with that said the current observations along with the PW values and transportation of that moisture is seasonal next to the averages, even when the IOD is negative, bringing an above average rainfall signal. You do get breaks in the onslaught of frontal weather or rainfall associated with the IOD. BUT THIS WON'T LAST LONG.
The IOD is likely to bring up rainfall chances as the nation warms and the season tilts towards spring.
Rainfall for the next 2 weeks.
I have highlighted the area that could see rainfall develop in coming updates.
Temperatures remain above average for many locations away from the southern coast, with the westerly belt slowly moving south, a flat ridge over much of the nation keeping the warm air over much of the nation. The west to east movement of frontal weather reducing the risk of colder air surging north over the nation post this last front. That will help moisture build over northern areas of the nation.
I will have another update on medium term weather on Wednesday. Tomorrow I will have a climate update for the states for August.