The weather is likely to turn wetter as we finish 2021 and kick off 2022 for large parts of Australia, in line with shift to wetter weather over northern Australia with the monsoon possibly returning by mid January. Ahead of that occurring, the wet weather increases and drifts southwards. The outlook also supports the return of easterly winds as well, dominating the synoptic scale with areas of rain and thunderstorms coming back to eastern QLD and into NSW as we round out the year and into January.
The Bureau updated their Summer outlook yesterday and no surprises they are only following their data set generally and are calling for a short lived La Nina, but that is not necessarily the case if you look at the global model data sets.
The main points you need to know.
La Nina is still developing and expected to peak sometime in January and to begin waning during February into March.
Some global agencies support the La Nina being stronger, peaking in late January and then persisting into March before easing in April. These agencies were the first to pick the ENSO event ahead of the regional agencies and persist in this idea.
A persistent +SAM phase will enhance the northern and eastern wet seasons through to the end of March, leading to enhanced rainfall chances, similar to what we saw last year.
Remember, the lag between the La Nina developing, peaking and waning and the atmosphere responding to these phases, is about 4 weeks on average. So we are likely to feel the impacts of La Nina for some time to come.
%chance of exceeding median rainfall for Summer 2021/22
As we move through the season, the monsoonal activity will drive the rainfall chances for northern and central as well as eastern Australia. The monsoon is likely to arrive in early January at the earliest to the north of Australia and then pass through during January. Every time you hear that the monsoon is being forecast to be over Australia this Summer, the impacts are likely to be felt through southern and eastern Australia. The longer the monsoon stays over the nation's north, the longer the elevated rainfall chances remain over Australia. This will lead to elevated flood risks for the north and east of the nation in particular. There is growing evidence that February could be the wettest month out of the Summer. The wet weather has a good chance of lingering into March as well.
%chance of exceeding median temperatures for Summer 2021/22
The cooler bias over central and eastern inland parts of the nation will continue into the second half of Summer with a strong chance of below average temperatures persisting into the February. The temperatures over the west trending above average is fair based upon the current strength and depth of La Nina with the easterly winds eventually taking over, these running over land will warm up and dry out so a good chance of higher temperatures over the nation.
%chance of exceeding median rainfall for Autumn 2021/22
With the lingering La Nina from the Summer still influencing the rainfall distribution through the early Autumn, there is a real risk of a repeat of flooding for the east coast in line with the peak of the wet season for the subtropics which is usually March through May. So pay attention to the guidance in the coming weeks over Christmas and New Years (yes I will still be here). The weather looking less wet for southern parts of the nation for now, but the Autumn break may be moot point with the wet Summer running into March before conditions ease through the season. The tropics may have a prolonged wet season with above average rainfall taking us through to May. Cyclone activity peaks in the Feb to April range through the Indian Ocean and this could influence rainfall for the interior and into the northern and eastern inland.
%chance of exceeding median temperatures for Autumn 2021/22
As per last season, the east and north were below average temperature wise with above average humidity values and rainfall across the east and north expected at this time, so the reduction in temperatures is a logical conclusion. The west and south I think will trend warmer, not necessarily drier, that average across these areas. We may have some impacts of late season cyclones running through the northern and western parts of the nation.
ENSO Spaghetti Plot - Model Predictions November Plume.
I will use this update from last month ahead of the new data set coming in from later this week (I will have that data set on Friday with the 6 week outlook). Note the plume generally sits lower that the ACCESS model run - so that means that looking at the data set, the La Nina will not be short lived and could be a stronger than what is being advertised across the Summer before waning in the Autumn. That idea is still probably the right solution at this time, so be aware, looking at the BROADER data sets is crucial to forecasting these major climatic drivers. As we saw with the botched IOD forecasts from mid to late this year, looking at the full data set assisted me in forecasting the very wet Spring and nationally the WETTEST November on record well ahead of time. And I stand by the stronger and longer duration La Nina for this Summer.
A brief deep dive into the data sets, and I have outlined the international agencies that persist the La Nina into Autumn 2022. And while there are more that resolve the La Nina by then, this highlights that the event overall will be of normal duration, not short duration with a risk of the La Nina persisting into Autumn
With that said, that means a persisting La Nina phase has a major impact on the rainfall and temperature forecasts moving forward, especially at a critical time for farmers in the east where the ENSO impacts are felt more.
The tropical weather would also be prolonged and higher than average rainfall would be a high chance. That would then increase rainfall down through central Australia and into the southeast states as well.
If this occurs, we may need an Autumn Break from the rainfall!!! So while there is some suggestion in the media landscape that the La Nina will be rather short in duration, there are other solutions that suggest the opposite.
I will update this again during the festive period plus have the latest model predictions hopefully by Friday from ALL agencies!