THE skill level of medium range weather outlooks (one to six months) has improved with the advent of more detailed satellite information and a realisation that the ocean temperatures and currents play an even bigger role than previously thought.
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But, in the past year or two new factors have occurred to make the prognoses more challenging.
The main additional factor has been a general increase in sea surface temperatures worldwide.
For example, the slow development of an El Nino pattern in the Pacific has been expected for some months now and its development in the coming weeks is still likely.
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However, in a usual El Nino, SSTs increase in the tropical eastern Pacific and decrease in the western Pacific near Australia.
Temperature increases are occurring in the eastern Pacific but closer to Australia’s east coast, the SSTs have remained a little up on the long-term normal.
This has rarely occurred in an El Nino event in the past but it is likely to result in decreasing the effect of this El Nino on eastern Australian rainfall.
In other words, in the past, about 30 per cent of El Ninos had minimal effect on eastern Australian rain and it is likely this could be the case in the current developments.
Overall, the progress towards at least a weak El Nino in the Pacific basin before the end of the year still appears likely with about 70pc of models indicating at least a weak El Nino by the end of November or early December, but it looks increasingly likely that it will only persist for a few months.
Rainfall deficiencies throughout eastern Australia in 2018 have been locally relieved by good rain in some parts of NSW and southern and central Queensland in October, but even if the developing El Nino does not have much impact on rainfall potential in the coming months, rain is still unlikely to be sufficient to overcome these deficiencies already established.
With SSTs up a little on normal, and cloudiness in the “heat engine” area of north-western Australia lower than normal, there is a strong chance that summer temperatures in eastern Australia will exceed normal – although they might not be quite of high as over last summer.
The warmest summer weather is likely to be later in the season further south.
The weak El Nino pattern also encourages severe storm development and these are likely to persist into the new year, especially in north-eastern NSW and Queensland. Rain from this source will be patchy and very variable.
However, most models indicate that tropical cyclone frequency could be down a little this coming summer
