A warmer and wetter spring likely for many parts of the nation.
The Indian Ocean Dipole is expected to play a large part in enhancing rainfall numbers over the east 2/3rds of the nation including the tropical north.
A La Nina is a moderate to high chance of developing through the latter part of spring which may enhance rainfall for QLD and NSW in particular during the latter part of spring into the Summer 2021/22.
A La Nina developing may enhance the early onset rainfall for northern Australia and persist into the monsoon of 2021/22.
Rainfall Outlook Spring - September to November 2021
% chance of exceeding the median rainfall for Spring 2021
Heighted rainfall chances remain in place through Spring 2021. The moisture is expected to continue to build early through the northern tropics sparking a near certainty of receiving above average rainfall through this period. That moisture will be drawn south through spring. Again the warmer the atmosphere, the more moisture it is able to hold and therefore the rainfall will increase as we go through spring. Flooding is still a MODERATE risk for inland NSW, NE VIC in particular, as well as portions of outback SA if the falls verify from the Indian Ocean Dipole. The west will return to seasonal rainfall as expected with the dry season winding down during October. Most of the nation has an excellent chance of seeing a good season of rainfall.
Temperature Outlook Spring - September to November 2021
% chance of exceeding the median temperatures for Spring 2021
The temperatures are expected to be warmest through September nationwide before we see moderation thanks to extensive cloud and more frequent rainfall events. That may tip some parts of the eastern inland into the below average range for October and November if the rainfall and cloudiness expectations are realised. The Indian Ocean Dipole being in a negative phase will peak in the October and early November with that resulting in deeper moisture passing over the nation from northwest to southeast. The current signal from the Indian Ocean is weak at the moment and this will be monitored for the coming 2-4 weeks and forecasts will be adjusted based on observed weather. The phase is expected to deepen through the coming two months and this will bring more humidity and cloud to the nation suppressing the chance of elevated temperatures for much of spring.
LETS LOOK AT THE DRIVERS
INDIAN OCEAN - NEGATIVE
This driver is unchanged and likely to remain negative for the coming 2-4 months, with a peak expected October into November. Now in recent weeks I have not been satisfied with the influence on the nations rainfall and this could be in part with the MJO being out of phase with the monsoon flow back over Africa through the Americas. The MJO is expected to sweep back into the Indian Ocean in the coming weeks so this will be a critical time to see how much of an influence this has on rainfall drawing southeast through the nation. For now I am mildly suspicious of the forecast signal surrounding the IOD and it's influence but not enough to change the outlook at this time but will update accordingly in the coming weeks.
ENSO - NEUTRAL
The ENSO is remaining neutral, a cool neutral, through spring. That is my official forecast at this time gathering all the data, however there is a greater than 50% chance of a La Nina forming during the late Spring into Summer which may enhance rainfall chances for QLD, NSW and the tropics from November into Summer 2021/22. There may be an overlap between the IOD phase and the ENSO being in La Nina. IF that occurs this will elevate widespread heavy rainfall chances for the nation to finish the year and to begin 2022. For the first half of Spring, minimal impact is expected from this driver at this time.
THE MODEL DATA
CANSIPS - Seasonal Rainfall Outlook Spring 2021
This climate model is all in on bringing up extensive rainfall for much of the nation with heavy rainfall for northern Australia kicking off earlier this year. The influence on this model is from the IOD and the development of a La Nina through later Spring into early Summer 2022. This impact starts to become evident during November with values of 300-400mm above normal over the north. Enhanced rainfall over eastern Australia through the central and western inland of the nation is connected to the Indian Ocean Dipole, again I need to see more evidence of that influence in the next 2 to 4 weeks to be satisfied it will play a large role in this season ahead. So this outlook is banking on all drivers turning wet for the last half of this year into early 2022. This a MODERATE chance of occurring.
CFS- Seasonal Rainfall Outlook Spring 2021
This other climate model is suggesting a drier picture for the north of the nation and mixed odds on rainfall for the southern half of the nation. This could be due to the weaker influence of the IOD phase and the continuation of the cool neutral in the ENSO region. This combines to bring the forecast below. Now while it is not impossible, the forecast chance of this occurring is LOW.
Euro - Seasonal Rainfall Outlook Spring 2021
This is likely to be closest to the truth. The rainfall over the north will be likely elevated but I do not think to the point of the CANSIPS modelling. The rainfall over the southeast being about 100mm above normal for the season is fair on average given the drivers that are in place. This model is banking on the IOD peaking in October into November before waning in December. This model does not develop a La Nina phase at this time. The chance of this outlook verifying is HIGH.
Euro Temperature Outlook - Spring 2021.
This is the likely outcome for the season, the moderation of that heat build up that brought above average temperatures throughout July and August will continue into September, and then there will be moderation in the temperatures through October into November as the cloud cover and rainfall is expected to be heightened during this time. I think the northern third of the nation is probably a little too cool so I have held over the warmer than normal temperature values for October, then I suspect the moderation into November. There will be warmer spells over the nation, it is just they won't be long duration at this time. What you will likely experience is higher humidity values coupled with the warm temperatures this spring. This is a HIGH chance of verifying.
CFS Temperature Outlook - Spring 2021.
This is again likely too cool! And considering the mixed rainfall odds across the nation during this period I do not buy the colder spread throughout the nation as advertised below. So if the rainfall outlook was more aggressive I would support this but at this time I am thinking this is a VERY LOW chance of occurring. Not impossible, but unlikely.
CANSIPS Temperature Outlook - Spring 2021.
Fairly good agreement with the Euro for the season ahead. More cloud cover, more frequent rainfall events, especially as we go through Spring, will lead to a moderation in temperatures for the second half of spring. September still very warm for the nation with those elevated humidity values coupled with above average temperatures over the north. There will be warmer days for all parts of the nation similar to what is being experienced now but the duration will not be drawn out with humidity and rainfall breaking out in response to the higher heat values that do eventuate. This outlook is a HIGH chance of occurring.
So this is the preliminary Spring outlook for 2021.
A further update on the coming medium term forecasts will continue through the weekend and a focus on the major rainfall event early next week with that strong cold front. Another Climate Outlook expected Tuesday, taking a look at the Indian Ocean once again.