NORTHWEST CLOUD BAND SEASON - ARE WE SET TO SEE AN INCREASE IN THE MONTHS AHEAD?

We are moving into Northwest Cloudband Season, and while things are currently quiet (under the present climate drivers that is to be expected with the neutral SAM and out of phase MJO) the season is yet to peak and that will pick up through May and into June through July before we see conditions easing in later Winter.


The overall season ahead is pinging above average rainfall through May and into June (latest data sets) across large parts of inland WA, through Central Australia and into the southeast. While it is not overwhelming, the signals are still present for the cloudbands to make a comeback which should provide some hope for those in the very dry southern Ag areas that have missed out on all the rainfall events over the past few months.


Good thing to do is compare with last year which is running very similar in terms of the climate drivers in play and how they are resolving this year in comparison to how they did in 2021.



Mid April 2021

TC Seroja brought the first major cloud band of the year and brought the Autumn Break to parts of WA and then onwards to pockets of the southeast of the nation. This system heralded the shift into a cooler season pattern for southern areas and the wet started to east from this time. But the major component that helped bring this system through at full power to Central West of WA was the first major frontal pattern that was helping to strengthen the jet stream aloft. These elements then created that large cloud band that passed through the nation. Now in this medium term forecast, we need to watch the potential for the next tropical wave moving through northern Australia from this time next week and whether it can get drawn into the jet stream around Easter and spread throughout the nation. If a late season cyclone forms out of that wave over the eastern Indian Ocean then this is possible.

May 2021

In May last year we saw the first major cold front that emerged through the Southern Ocean via a negative SAM phase and this drew in a large plume of moisture and formed a large cloud band with light to moderate rainfall that impacted large areas of WA through SA and then into the southeast of the nation. This brought some Autumn Break rainfall to some in SA and through western VIC and NSW. But the heaviest rainfall with this event was over inland WA with many areas recording a month's worth of rainfall. In May we start to see the northern projection of the cold fronts leading to widespread rain bands ahead of cold fronts. In this case we had a large scale tropical system over the Central and Northern Indian Ocean so once again, late season cyclones/tropical lows do provide some major rainfall influences as we move into the latter part of Autumn.

Indian Ocean Basin - Precipitation Anomaly 2 weeks. GFS 18z Valid Tuesday 29th of June 2021.

High levels of rainfall anomalies by as much as 400-500mm during this period saw large quantities of moisture being drawn into the jet stream over northwest Australia through this period which brought widespread above average rainfall for northwest Australia and higher rainfall chances spreading thereafter into central and southeast Australia.

Water Vapour Satellite - Next 10 days - Euro 12z run - Valid Tuesday 29th of June 2021.

This is a great way to identify the amount of fronts rolling through the southern ocean and to also follow the moisture plumes through the mid latitudes, to see whether there will be connection between the frontal boundaries and moisture plumes. You can see multiple bursts of moisture coming through the Indian Ocean, but not quite connecting with the fronts. This is where timing is everything, but all the ingredients are there for cloud band formation. But this is another useful product to look at in terms of scale and depth of moisture heading into frontal weather through the cooler season.

Water Vapour Transport and Direction through the upper levels with surface pressure pattern. GFS next 2 weeks - Valid Tuesday 29th of June 2021.

This product generally shows the movement of moisture from one location to another. We want to see the moisture of those yellow and red tones heading towards Australia and feeding further south and east towards the continent to indicate the formation of cloud bands in these areas is quite possible.

Precipitable Water Anomalies % above normal- Next 10 days - Euro 12z run - Valid Tuesday 29th of June 2021.

Larger moisture plumes coming through the Indian Ocean signify the impact of the northern movement of the frontal passages over the Southern Hemisphere leading to widespread rainfall opportunities moving from west to east.

KMA Rainfall 12 days - 12z run - Valid Tuesday 29th of June 2021

Was more aggressive in utilising the moisture from the northwest for that coming 9-12 day window with a large cloud band being picked up by this model in advance of the cold front spreading eastwards.

Note the moisture being drawn south as we move into the cooler season. July 2021

The moisture is more likely to be drawn south and southeast through the mid latitude westerly winds, ahead of cold fronts, broadcasting that moisture throughout Australia, lifted by cold fronts and troughs to increase rainfall chances nationwide, especially southern and central parts, from later this month. Also note the ingestion of moisture east of Africa through the roaring westerly winds over the Southern Ocean. (PW values of 60-70mm does not mean you will get 70mm of rainfall, but says the airmass is saturated and very productive in terms of rainfall)

BACK TO THE CURRENT PICTURE!

It is fair to say we will see the Indian Ocean playing a part in bringing in larger moisture plumes into the jet stream as we get more frontal weather into the picture, and out of that, cloud bands will begin to be forecast as we move forward through the coming weeks.


GFS Moisture Content through the Indian Ocean next 2 weeks

For now there is not a whole lot of influence from the Indian Ocean with a lot of drier air through the northern and central Basin. You can pick out the westerly wind belt is also somewhat retarded to the south for much of this period so that will also suppress the formation of Northwest Cloud Bands. We need the SAM to be negative to see a higher chance of a wavy jet stream and that will lead to more interaction between the subtropical jet and the polar jet which forms these larger cloud bands as well. That appears more in late May through Winter.

Note the very dry air over the northwest and north of the nation which would suppress the ability for cloud bands to develop at this time. So the region to watch is clearly off the northwest of the nation through northern Australia and there is not much luck at this time for tropical weather to develop and have an influence on the formation of cloud bands.

The simulated satellite imagery also shows you how quiet the Indian Ocean is expected to be during the next week with minimal impact expected through Australia.

The INDIAN OCEAN DIPOLE - CREATES OR STIFLES NORTHWEST CLOUD BAND DEVELOPMENT.

The upcoming season is expected to turn more negative as we go through Winter, therefore suggesting we should see a generally average Northwest Cloudband Season for April, May and into June but the activity will likely increase through July and into Spring as being advertised by climate modelling.


JULY TO AUGUST - LEANING NEGATIVE.

Current model data and reviewing the information suggests that the IOD will likely remain neutral through much of Winter but will likely start turning negative through August, which is from this point I would anticipate the rainfall and temperature observations to be realised across parts of the country. It is also important to point out the forecast confidence is low but this is where the data current stands and the risk for now is leaning negative.

AUGUST TO SEPTEMBER - DEVELOPING NEGATIVE IOD INTO EARLY SPRING

The current guidance is strengthening the signal for a negative IOD in around 6 months. Now it is 6 months out, but for the purpose of shifting the focus from the ENSO through to the IOD which will be our driving force for Winter and Spring weather which is crucial to many, this is the current data sets. THIS WILL CHANGE. At this time, the data is suggesting a strengthening IOD will lift rainfall chances above average for large parts of the nation and elevate temperatures and humidity over northern Australia, increasing the chances of early onset northern season rainfall. The filling season in SWLD of WA would be quite productive and could be protracted into early Spring, depending on how the IOD evolves. These specifics will become clearer as we get into May but more likely to get better accuracy from June and July.

DATA SETS

Sea Surface Temperatures Last Month.

So where are we with regards to the SSTs through the Basin? We are warm, but not as warm as it can be at this time of year. The signal is for the Indian Ocean to remain at about 0.5C-1C below normal over the coming 4-8 weeks which is normal. If there is any late season cyclone mischief, then we may see the values cool further but at this time, on current guide, expect not too much influence in the traditional sense of the traditional Indian Ocean Dipole. The values are similar to this time last year and the year before and that suggests to me that we are sitting in a situation where we could be looking at another negative IOD (wetter) more than a positive IOD (drier) on the current guide.

A closer look shows that slight warmer bias out over the Indian Ocean, but it is sitting at about 1C below where it should be at this time of year. That is on the back of the Summer Season. But overall, the basin is fairly warm and may maintain this warmth through the Central and Southern part of the basin before spreading back towards Australia in Winter and through Spring.

So lets move along to the various parameters that give a sense of where conditions are heading and look at some of the data sets (I have looked at about 60 parameters over the past day or so) which illustrate what we may expect from July onwards.


THESE CHARTS ARE FROM JULY INTO SEPTEMBER.

Euro Seasonal Forecast - July through September - Sea Surface Temperature Anomalies Forecast.

Noting the shades of deep red over the oceans northern, northwest and through the southern parts of the nation. That in itself will lead to excessive moisture loads being propelled into the atmosphere and linked into the westerly wind belt through southern areas of the nation over Winter. But relating this back to the Indian Ocean Dipole, these values are maxing out the likelihood of a developing negative IOD through Winter and into Spring 2022. This is far more aggressive than last year's signals at this time of year for the record so it is with some merit that we now pay attention to these outlooks more regularly and shift the focus from the ENSO region to the IOD for the coming 6 months. This should raise the eyebrows of farmers throughout the nation. The last time I saw such strong robust signals relating to SST anomalies was back in 2016/17 and that lead to cold season flooding over inland areas of the nation.

The elevated SSTs are developing through the IOD region south of Indonesia and northwest of Australia and likely in the process of strengthening from August onwards in my opinion but it could be earlier than that if this is right.

Euro Seasonal Forecast - July through September - Deviation from seasonal surface level pressure.

Noting that the shades of blue over much of the nation which denotes the chance of a higher probability that the period is dominated by lower air pressure. Now in saying that, it does not mean it will rain everywhere every day, but this would lead to a higher chance of rainfall being above average, increased cloudiness and temperatures leaning below average over parts of the nation. The interesting feature which to me signals that this model is suggesting a negative phase of the IOD, is that the air pressure is below average back through much of the northern basin bordering Indonesia. That signals that we could see a robust phase of the IOD which could lead to a very expressive late Winter and early Spring rainfall pattern across the nation.

This is quite unusual across northern and northwest Australia and it is not very often you see the anomalies so much below average across large parts of the nation. So will watch these trends.

Euro Seasonal Forecast - July through September - Rainfall Anomalies.

Another signal that Euro is leaning towards the development of a negative IOD is found in this rainfall dataset. You can clearly see that the values over the nation are elevated but it is turning your attention to Indonesia. When you see values that elevated across northern and northwest parts of the nation, this suggests a negative IOD is place and would likely see moisture broadcast right through the nation via the upper level winds feeding into troughs and frontal weather that accompany winter. That means frequent rain bands and above average rainfall potential. At this stage, this forecast is not as aggressive as we saw back in 2016/17 when we had the last major cool season flood for VIC, NSW and parts of SA. But being this far out, the confidence in specifics are low but we need to use this as a base in future updates to see where the models are going.

The values over the nation represents the higher rainfall associated with a developing negative IOD. Remember that the peak of the IOD influence is not realised until about October through early November. So the influence here is quite significant.

Euro Seasonal Forecast - July through September - Temperature Anomalies

The temperature anomalies are also another hint that a negative IOD is developing, when you see elevated temperatures being forecast over Indonesia and northern Australia, that signifies a more humid dry season with a chance of rainfall developing throughout the period which is anomalous (see above). Conversely more cloud and rainfall across the nation leads to the chance of seasonal to below seasonal temperatures across the period. The Euro is not as aggressive as the other models for the period but does give credence to the idea that we are looking at another Winter and into early Spring where we may not be looking at baking temperatures if this is right, but as always we have to watch trends moving through the coming months

Note the temperatures are above average over much of northern Australia which is line with the IOD developing and strengthening through the second half of Winter. Note the warmer bias about coastal areas in the south. That may be related to the overnight temperatures not so much day time in my opinion.

Euro Seasonal Forecast - April through September - Tropical Cyclone Forecast - Next 6 months.

I thought this was an interesting product as it does show that late season cyclone activity running over those warmer waters through the Basin is possible during April and May and while we are talking more about July through September above, it is possible that we could see enhanced rainfall chances and higher humidity based off this influence moving through the back half of Autumn. IF we see late season cyclone activity, it could cause some of the SSTs to moderate in pockets of the IOD region so will be watching closely. Not an overwhelming enhanced risk but there are signals for tropical weather to remain a feature through May.

There will be two phases to watch out for, the decline of the ENSO influence in the coming 6 weeks and the increased influence during Winter of the IOD. Now in saying that the IOD is forecast to be negative is misleading this far out, but as you can see above, we could see one develop and IF one develops, we may need to prepare for more severe weather issues related to flooding more than any other element again for large parts of the nation.


A negative IOD phase - Average impacts across the nation.

This phase is very important when coming out of drought as it enhances the rainfall across the nation and starts to repair deficits from previous dry seasons/years through an El Nino or more impactful, a positive IOD.


More moisture is generated through the Indian Ocean Basin, increased convection through convergence sits further south, generally over Indonesia and south of the archipelago allowing highly anomalous rainfall to develop through the region. This convection shears of elevated precipitable water values or moisture through the upper atmosphere via the fast moving jet stream and thus drags that moisture south and east over the nation during the winter and spring.


That moisture generally can float across the nation consistently with out too much fanfare, appearing as high and mid level clouds with not much rainfall in the absence of a trigger such as a cold front or trough.


But when the cold fronts are more active thanks to a negative SAM phase, the we see that moisture being lifted into widespread areas of rainfall. We have seen this happen multiple times this year so far.


Frontal weather interacting with a deep layer of moisture surging southeast from the Indian Ocean producing widespread rainfall.

The impact of the southern ocean influence begins to wane through later September with the retreat of the winter westerlies, and the elevated moisture that is then pooling of the nation generally builds up to very high levels, inland troughs returning thanks to the build up of hot air, then generates more inland rainfall and storm outbreaks and the rainfall then increases for areas such as QLD and NSW, inland SA and northern and eastern VIC before the influence wanes further on closer to summer.

In context we have seen the impacts play out over WA numerous times this year with above average winter rainfall and that has spread through to the southeast a few times. But that frequency does increase as we get into August through October as the phase peaks.


I will say that the one key ingredient for rainfall to be widespread and above average is connected to the SAM being persistently negative and allowing the winter fronts to be frequent, taller and move through large parts of the inland. We are seeing that to a degree this week the frontal weather could be further north than this for more widespread falls.


But what does the IOD look like when it is positive and how does that influence the nations weather


The Positive Indian Ocean Dipole - Average Impacts across the nation.

The inverse is true with the positive IOD phase where the cooler than average temperatures over the Indian Ocean mean there is a lack of moisture that spreads across the nation via the jet stream, leading to lower than average rainfall for extensive areas of the nation.


It also leads to a warmer winter and an early start to the build up temperatures over the northwest of Australia through the heat engine, not the build up to rainfall, but scorching temperatures. We have seen that in years go by, that with a positive IOD, parts of inland WA can reach 40C very early on in the September and this transported down into the south and southeast with very warm temperatures.


Back in 2009 we saw some locations hit 30C through southeast Australia in the late May and early August with fire weather warnings issued in both of those months and a very brief respite from the fire dangers, but lack of rainfall lead to a dangerous fire year that year over southeast Australia.


The positive IOD also leads to below average rainfall across northern Australia with a prolonged build up and higher than average day time temperatures with more sunshine hours.


The positive IOD usually follow periods of negative IODs but that rule is not fixed in. They will happen again, and they usually have a stronger impact on rainfall than the El Nino for WA, SA and parts of the NT.

More weather to come through the week.




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