LITTLE has changed in the long-term developing global patterns in the past few weeks.
The El Niño–Southern Oscillation (ENSO) in the Pacific is still in a neutral phase with the Southern Oscillation Index (SOI) towards the lower end of the neutral range. However, the majority of international models still indicate it will trend towards an El Nino reaching the required thresholds in mid to late spring.
The only minor changes recently have been (1) a slowing down in the sea surface temperature (SST) increase in recent weeks and therefore a slowing down in the trend towards an El Nino and (2) an interpretation that if the El Nino develops (which is now about a 70 per cent chance), then it could persist well into 2019.
It is still worth noting that in most but not all El Nino events rainfall in eastern Australia is below normal. In about 30pc of past events, there has been minimal or no impact on NSW rainfall.
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In addition, on this occasion, SSTs especially in eastern Australian waters, remain above normal which is rare in an El Nino situation and the effects of this could be to produce briefly significant rainfall in the occasional “one off” events which can also be a feature of an El Nino pattern.
This is perhaps the only slightly optimistic feature of the current set up. However, on this occasion, we are entering a potential El Nino with already established moderate to severe moisture deficiencies in most of NSW, so there is a need for above average rainfalls to overcome this and that is unlikely.
In the immediate future, Southern Ocean frontal activity will remain a feature, pushing the high pressure belts across northern NSW and resulting in dry weather in all but the southern inland of the state.
In the medium term (one to three months) temperatures are likely to increase to be regularly well up on normal by day and the potential for localised severe storms will also rise by later spring.
In the longer term (four to eight months) a continued El Nino pattern well into 2019 will hamper a return to at least average rainfall for some time.