March was a remarkable month as far as rainfall patterns in Australia were concerned. The country was divided, with well above average to near record falls in the north, especially in the Northern Territory and Queensland, while well below average rainfall to near record lows were a feature in the south - south east Australia and Victoria. NSW was in between with variable falls but generally a little drier than normal.
Variable rainfall patterns are favoured to persist in the coming months with the likelihood that by the end of May, most parts of eastern and north east Australia will have had at least average autumn rainfall but in the south, falls could still be patchy and variable. Below average falls would normally be associated with a decaying El Nino but because of the complication this year with the warmer than normal sea surface temperatures around continental Australia and especially in the Coral and Tasman seas, then variable rainfalls have become the slightly favoured scenario. It is almost impossible to predict exactly which areas will be "favoured" by this variability and which areas will miss out.
Most models still forecast a continuation of the El Nino event during the rest of the autumn months, but rapidly weakening after that. After a brief neutral period, a La Nina becomes the most favoured scenario after mid-winter, with the chances of a La Nina increasing further by spring. It is interesting to note that the models indicating the greatest chance of a La Nina developing during late winter or spring are all the northern hemisphere models, but that might be just a coincidence.
To add to the challenging long range forecasting conditions, the Indian Ocean is also different to any time in recent years. The Indian Ocean Dipole is now slightly positive, although officially well within the neutral range. Remember a negative IOD is preferable for winter rain events in south east and eastern Australia.
International models have changed, now preferring a positive trend in the IOD in early winter mainly because SSTs have cooled a little recently off north west Australia. However, this could be a temporary feature due to tropical cyclone activity. Also, SSTs across the Indian Ocean are generally higher than normal, so the confidence level of this assessment is fairly low at this stage.
Finally, to the north the Madden-Julian Oscillation is expected to re-strengthen a little around mid-April with a possibility of renewed tropical activity in Australian longitudes. This could last one to two weeks. However, after that it will have minimal effect on our weather until spring.
