Localised, 'one-off' weather events tipped

Localised, 'one-off' weather events tipped

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We can expect to see lower than average rainfall

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It is always satisfying, as a medium to long term weather forecaster, to see your prognoses being reinforced by actual events.

The weather patterns continue to be influenced by a weak or developing "El Nino" - one in which the atmospheric response to the sea surface temperature (SST) patterns has been minimal and a continuance of above average SSTs off the east coast of Australia.

This combination was forecast to provide a scenario where overall, rainfall was below average but it also provides the mechanism for short term, one-off but quite significant, local rainfall events. In other words, occasional spells of 1-2 days where a restricted area can receive worthwhile rain but the majority of the eastern states remain dry. This has occurred in recent months and at this stage, there is no reason to suspect that it won't continue to be the case for at least another couple of months.

So we continue to have a situation where the SSTs in the tropical Pacific Ocean are still close to El Nino thresholds but the atmosphere is yet to show a response. The Southern Oscillation Index (SOI), which typically drops below -7 when an El Nino pressure pattern develops across the equatorial Pacific, remains neutral with the current 30 day means sitting around -3 compared with -7 in March and -13 in February. In addition, the SE trade winds in the tropical western Pacific are currently close to normal strength and they normally weaken in an El Nino. Even equatorial cloudiness near the International Date Line which increases in an El Nino event has been near normal for some weeks now. So, in other words, normal climate indicators used for forecasting are variable and not that useful as a predictive tool at the moment. That is why any outlook is unchanged - namely, overall rainfall will be struggling to reach average in coming months apart from a few areas where localised 'one-off' significant events occur. Also overall, temperatures will continue to be a little up on normal leading into winter with daytime anomalies a little greater than overnight ones. Brief but significant cold spells are likely to become a feature during May and early June in Victoria and at least the southern half of NSW.

The SSTs in the Indian Ocean are becoming more important now. Currently the Indian Ocean Dipole (IOD) is very slightly negative at the moment, and if this continues through May, then the potential for worthwhile rain from those before mentioned one-off events will increase a little... a glimmer of optimism but not significant at this stage.

- Don White, Weatherwatch

The story Localised, 'one-off' weather events tipped first appeared on The Land.

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