Updated: Jun 22
For those that are waiting for rainfall and needing to repair their water table after a dry period (this is for areas such as the Murraylands, Riverlands and Mallee areas for example), rainfall numbers and chances are likely to increase dramatically as we go through our spring. This is following very similar conditions to what we saw last year through this time.
The Indian Ocean Impacts - For the remainder of the year. It is a general guide and not a rule of thumb but based on current guidance this seems to be fair.
TIMING OF INFLUENCE - GENERAL RULE - NOT FIXED!
WA - You have felt the full impact of those warmer waters through May and June but has eased in recent times, but it will be back through the months ahead.
SA - Starts from later July into August and then peaks in the September/October so the minimal impact so far, seasonal and expected.
VIC - Similar to SA, impacts gradually increase from July onwards and then more prominent through August and into Spring when the nation warms, and the jet stream becomes more wavy sending in the moisture.
NSW - Increasing influence from July onwards but more prominent from August through November, especially on and west of the divide with the dividing range undercutting the influence for the coastal fringe.
QLD - Really impacts the southern third of the state and this starts to be felt from later August through Spring but the easterly winds that return push back a lot of the influence through the state and shoves it southwards for the remainder of central and northern areas, doesn't mean we don't get moisture influence, but it is more frequent the further south you go.
NT - Influence greatest over the western Top End and you should start to see increasing showers and storms from as early as late August and certainly an increase in humidity coming back in about 6-8 weeks.
Blue Zones - High Impact from a negative IOD event.
This area can expect to see above average rainfall chances through July onwards to the end of the year, that's based off the current guidance from the climate models and other data sets I have scoured through in recent days. I am of the belief that the further we go into July, the wetter the phase will get likely to increase through August and September.
Flooding becomes a risk through parts of NSW, VIC and SA with the frequency of the rainfall and the intensity increasing as the nation warms up adding more atmospheric dynamics. That will see the rain become more thundery as well in line with the storm season, so the risk of not only riverine flooding, but flash flooding becomes an issue.
I will be talking more about the flood risks over the coming week for inland areas of QLD, NSW, VIC and SA because this could be on those situations where many who don't get the information could be caught with the pants down around their ankles what the hell is going on.
Light Green Zones - Moderate impact from negative IOD event.
Rainfall has a higher chance of being more frequent and heavier, but the risk of exceeding the average is not as elevated for July. Generally, the rainfall is connected to the westerly wind belt and there are signs that will be present through early July with rainfall for a number of days for the region.
Longer term, the rainfall can be heavier through these regions but generally the above average rainfall risk is slight decreased off climate history. But with the IOD event continues into the late spring and summer, we could see rainfall continue through to the end of the year for interior regions where flooding may develop.
With early onset rainfall further moisture will likely pool from the north and northeast over these regions, not just connected to the Indian Ocean. That is more likely for central and eastern interior regions.
And in the east, an active and very wet storm season is likely to unfold, with not necessarily more severe thunderstorm events, but if forms form, thunderstorms may be more productive and lead to flash flooding events.
Dark green zones - Moderate impact from a negative IOD event.
High chance of more humidity values interrupting the dry season over the coming months with significant chances of pop up showers and storms through the dry season from time to time between July to August.
The build up likely to be earlier through the NT and Cape York and then spreading into the Kimberly. The build up likely to start with a protracted period of higher humidity and not much rainfall then a more active storm season from October through November.
Early onset rainfall is highly likely for large areas (I will have more to say about this is a couple of weeks). This may suppress day time heat indices which are excessive during this period.
The IOD lingering into early summer could elevate the risk of early cyclones over the Indian Ocean in particular ahead of the monsoon developing later in December.
Rainfall looks likely to be above average from October through December through these regions.
Indian Ocean - 9 Month Outlook
The Indian Ocean is forecast to turn more negative (more warmer waters to appear throughout the basin) throughout June into July. The strength of the negative phase is forecast to increase from July into August, and you think it is wet across the eastern inland at the moment? Well, it is about to get wetter if the outlook verifies, and the rainfall will spread to much of SA and through VIC with higher frequency and intensity. So, while the weather is a little dry through some pockets of SA, this is expected to flip as it normally does through late May into June. The negative phase of the IOD is forecast to peak around September through November and the drawn-out phase of the IOD is of concern, with flooding risks increasing through late Winter and into Spring. There is some chance we will not lose the warnings over the inland parts of QLD and NSW throughout the year given the moisture content and rainfall forecasts/observed falls in recent times. The northern tropics will likely see above average temperatures through the dry season, higher humidity and above average rainfall chances and early onset wet season activity from August. This will play out through parts of QLD as well with higher falls for cattle country against seasonal averages through the north and west. In WA, widespread rainfall is forecast to emerge through the Central and Northern Interior along the jet stream which will be dragging in larger and above average rainfall chances. This could at times feed higher moisture content into the SWLD with increased rainfall chances through the southwest wet season. Crops in the east have been drowning in recent weeks and sadly the growing and harvest season is going to be challenging for many locations on this current guide.
WINTERTIME - WHAT TO LOOK OUT FOR TO SEE IF THE FORECASTS ARE VERIFYING.
This was taken from June 2021, but it is important to point out that the similarities to last year are quite high. So, for the forecasts to verify off some of the members above, the moisture has to start being drawn in ahead of cold front as per the video example below. We are already seeing this occur in the cloud bands emerging through the short and medium term forecasts. But always good to go back and reflect on past events and compare to what is going on now.
This is from July 2021, note the fast flow pattern to the south with multiple waves of weather passing through the southern states, dry hot weather over the north, sending dry air across the nation from the northwest to the eastern inland in those westerly winds over southern Australia. That should have been a dead giveaway that the IOD was struggling to develop, as there should be more moisture running over northern areas of the nation. There is limited moisture being drawn out of the Indian Ocean, escaping through on the western flank of the upper high during this period. This is bringing some moderate rainfall events to southern and southeast Australia with cold fronts, but cannot penetrate further north due to the ridge over the central and northern parts of the nation as the SAM was out of phase. We will need to see a negative phase to really assist in distributing the moisture around.
Into early August 2021, the westerly wind belt had retreated well to the south. The ridging of high pressure over the central and northern parts of the nation has broken down and relocated south to the southern states. A cut off low pressure system now is allowed to deviate north of the flow and move into moisture sources from the northwest and northeast, lifting the chances of rainfall developing over inland areas. But the difficulty for moisture to spread through the jet stream is the slack pressure pattern and fast flow pattern well to the south. It needs to be further north. So will be watching that.
Into mid-August 2021, A system moved off the east coast of NSW with a trailing cloud band and patchy rainfall through inland areas of QLD, NT and WA. This caused limited rainfall and more cloud cover as there was little forcing, but moisture started to develop over northern areas for the first-time last Winter. Once again, the IOD which was forecast and being analysed as negative was certainly supporting more neutral conditions.
Mid August 2021, the upper level easterly winds have now shoved that moisture towards the northwest of WA and it adopted a southwest track. High pressure in the east kept things dry but the upper level winds out of the northwest started to drag some of that moisture southeast. Some rainfall moved through but once again, this showed that the IOD was not building, however this year, all models are in on a big outbreak of rainfall and below average temperatures.
Finally, a front arrives over the southwest of the nation, this now pulling the moisture into the central parts of the nation, rather than coming through the Pilbara and or Perth. A much better setup for rainfall to return for SA, QLD, NSW and VIC. This did indeed bring a band of rainfall throughout southern and eastern Australia mid August last year but it was a long wait!
During August last year we saw temperatures wane a bit, which interrupted the full impact of the IOD event for southern and southeast areas of the nation, leading to SA missing out on a lot of above average rainfall forecasts, and so this will be an important feature to watch. At this time, the SSTs are exceeding these values at present! Whether they are retained through to Spring is what is being forecast to occur, but there are no guarantees. So will be watching closely.
SPRINGTIME - WHAT TO LOOK OUT FOR TO SEE IF THE FORECASTS ARE VERIFYING.
In September last year we saw excessive moisture pooling near Indonesia, and this being drawn southeastwards. The difference in this year, the Indian Ocean is likely to be a lot stronger and in place earlier. So we should expect that this will unfold from say August onwards, with those very high moisture levels being drawn into frontal weather approaching WA. This was taken from early September 2021.
Watching the influence of early onset rainfall spreading through the jet stream across northern Australia and being propelled into the jet stream and lifting rainfall over QLD and NSW. This was taken in early October 2021
Moisture from the Coral Sea and through the north via the early onset rainfall is something to watch. Then late season cold fronts that also should be bringing in moisture form the Indian Ocean will likely merge with this moisture, lifting rainfall across the nation from west to east. This was taken in late October 2021 and you can see the seasonal shift away from the westerly wind dominance and the return of the easterly winds. So between now and October, the IOD needs to deliver for southern areas who need to fill ahead of the Summer season.
TAKE AWAY FOR THE DATA SO FAR?
The rainfall frequency looks to persist into December as well, and then there is the talk of La Nina possibly developing which I have discussed yesterday at some length so look out for that. But we are likely walking into another pretty wet period for the nation, wetter as we go through Winter and into Spring.
I will be highlighting this climate driver more and more as we move through the months ahead but wanted to give you some background and context to when things are in phase, but I have not really touched on when things are not in phase and the IOD does not go to plan, and last year we felt when things didn't go to plan!