Optimistic signs point to more useful rain

Optimistic signs point to more useful rain | The Outlook


Most international models now believe all necessary thresholds for a La Nina will be met before the end of October.


ANOTHER rain event in the past week - this time bringing the heaviest falls in the far west of the state - continues to support our long-term prognosis that rainfall for the rest of the year will be above normal in many parts of eastern Australia.

However, this will continue to be dependent on occasional one off events.

This scenario is supported by the continued drift towards a La Nina event in the Pacific.

Most international models now believe all necessary thresholds for a La Nina will be met before the end of October and that La Nina conditions will then persist into 2021.

Some indicators are already there.

The 30-day running mean of the Southern Oscillation Index remains close to plus nine, which is in the La Nina range.

South-east trade winds are close to average for this time of year, but analysis by the Bureau of Meteorology indicates recent trade winds across the north-west South Pacific have been the strongest for the past three years.

A sustained strengthening of the trade winds is something that occurs in a La Nina event. Also, a further cooling of the tropical eastern Pacific is expected and this will result in a full La Nina developing.

To the west, large parts of the Indian Ocean have warmer than average sea surface temperatures (SSTs) and as a result, the Indian Ocean Dipole (IOD) is averaging out at just negative.

However, lack of widespread convective activity over the tropical north-east Indian Ocean to the north-west of Australia has meant reduced cloud in that area, so the north-west cloud bands have not been forming in recent times.

However, there is nothing preventing this development as there was for much of 2018 and 2019 so we will have to wait and watch.

The signs for later spring remain good, but it must be remembered that the IOD has little effect on rainfall in south-east Australia from November to April.

To the south, the Southern Annular Mode (SAM) remains slightly positive and is favoured to remain that way in the coming weeks.

A positive SAM in spring can also increase rainfall potential in eastern Australia a little, but its use as a forecasting tool weakens as we approach summer.

So to our east, west and south, there are more optimistic signs than negative ones for those looking for a continuation of useful rainfall patterns in the coming months.

All we need is for the local synoptic patterns to do the right thing at times in the coming weeks.

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