THE ability to predict weather trends for more than six months ahead to even years in the future, could be very useful but it is one area of weather analysis that has remained poor.
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One reason for this is lack of detailed data and information over extended periods.
Reliable rainfall data for a sufficient number of locations only become established in eastern Australia around 150 years ago and temperature data for only the last 100 years.
With temperatures, a significant upward trend commenced around the late 1970s.
Before then, there were periods of relative warmth in the 1940s and in some areas in the 1910s, while even making allowances for poor exposure conditions, it appears as though some years in the 19th century, such as 1896 were exceptionally hot.
These were one-off years and the significant warmer trend over the past 30 years has swamped all earlier fluctuations.
With many believing such a dramatic warming has been linked to atmospheric chemistry changes, there is no reason to believe this warming trend will not continue well into the future, interspersed by only occasional years (such as 2010) when an La Nina was strong.
Looking for long term rainfall trends is a little more interesting. It appears that the eastern states had plentiful rainfall in the past 30 to 40 years of the 19th century, broken by a very occasional extremely dry year such as 1888.
The great drought at the start of the 20th century was probably more a correction, made worse by the many good years leading up to it.
Rainfall was generally lower for most of the next 40 to 50 years.
This changed in 1949, with a wet cycle returning for the next 50 years, although again some years (such as 1957, 1967-68, 1979-81) were very dry. Drier than normal years have been more frequent than not since the start of the 21st century, especially in the south of NSW, Victoria and the SW of the country, where the dry trend commenced over 30 years ago.
However, overall trends with rainfall patterns have not been as clear as the temperature trends.
So, what does all this mean to the future?
Across NSW, for example, 2018 was the 22nd consecutive year with above average temperatures and there is no reason to indicate that there will be any change in this pattern.
So the next decade will be warmer than average, but extended spells with dry air will result in slightly lower but still above average overnight minimum temperatures.
As for rainfall, average rainfall will most likely end up about 10 per cent below the average of the past 50 years with occasional La Nina years briefly boosting totals but this is unlikely in 2019.
