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Why you may not get any rain during the next two days in Sydney, but your neighbour might

Jacobus Cronje, Friday October 11, 2019 - 11:04 EDT

On Friday morning, heavy rainfall was observed over the lower North Shore of the harbour city, while at the same time, barely a drop fell at Observation Hill. Similar, patchy occurrences of rainfall are likely during the next 48 hours. 

At the moment, an inland trough over inland NSW is broadening eastward, enhancing instability over the eastern parts of the state during the next few days, while this unstable air is being fed by moisture from persistent onshore winds. This is increasing the likelihood of showers over Sydney between Friday and Sunday.

Weather forecasts throughout the week have been indicating a very high chance of showers over Sydney today and Saturday, but weather app users will likely have noticed how much the forecasts for Friday and Saturday have been changing over the last few days.

The devil is in the details when it comes to forecasting terminology. For the meteorology community, the term 'showers' has a very specific meaning. It represents precipitation that has rapid onset, uncertain duration and that only occurs from convective clouds that can have very patchy spatial distribution.

Forecasting the rainfall amounts and timing associated with showers can therefore be quite difficult, given that rapid falls can occur over a very isolated area.

Another factor contributing to the difficulties in forecasting showers is that convective clouds are not steered by winds near the surface, but by the circulation around 10,000ft, or 3km, above the surface. This can be particularly problematic when these steering winds are weak and disorganised, as is the case today.

During the remainder of Friday and Saturday, the most likely parts of Sydney to receive rainfall will still be eastern suburbs, but localised heavy falls are indicated further inland for places like Canterbury, Blacktown, Parramatta and Penrith late on Friday and early Saturday.

Steering winds are expected to strengthen and become more noticeably onshore overnight on Friday and during Saturday, meaning that any rainfall that occurs out to sea is a bit more likely to move onto the coast and reach even further inland.

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