Neutral outlook for majority of the country

What does a 50 per cent chance of above median rainfall mean?

Weather
Victorian rainfall deficiencies, January 1 to March 31, 2019. Image courtesy of the Bureau of Meteorology.

Victorian rainfall deficiencies, January 1 to March 31, 2019. Image courtesy of the Bureau of Meteorology.

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A forecast of a 50 per cent chance of above median rainfall means there is less chance of extremely wet or extremely dry conditions over large areas.

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For May to July, the Bureau's climate outlook indicates there is a 50 per cent chance that rainfall will be greater than the median across most of the country; the Bureau calls this a neutral outlook.

Many readers might be asking "what exactly does this mean?" To understand, let's first look at how the Bureau's climate outlook is made.

Every day the Bureau collects millions of observations across the globe, from ground stations and ocean buoys to weather balloons, aircraft and satellites.

This huge amount of data goes into our climate computer model that uses mathematical relationships to represent the physics of the oceans, land, ice and atmosphere, and their interactions, to calculate how the weather could change over the next few months.

Using a model helps us understand which future weather patterns are more likely than others.

We run our model multiple times with small changes to the initial observations to account for uncertainty in the measurements.

For example, we might run the model 100 times, If, say, 80 of these 100 forecasts predict above median rainfall developing in an area, we say the chance of above median rainfall in this location is 80 per cent. If only 50 of the forecasts turn out above median, we give a 50 per cent chance.

A neutral outlook often occurs when the patterns in the oceans and atmosphere that drive Australia's climate (like El Nio, La Nia or the Indian Ocean Dipole) aren't strong enough to steer us towards a wetter-or drier-than-average season, as they are currently.

This can also happen if different climate drivers are showing opposite impacts-they can cancel each other out.

A forecast of a 50 per cent chance of above median rainfall means there is less chance of extremely wet or extremely dry conditions over large areas, but it doesn't mean you should expect median rainfall.

The natural variability of Victoria's rainfall means it can be very different from year-to-year, even under average conditions.

If we just consider the middle 50 per cent of past May-July rainfall totals for the state (ignoring the driest and wettest quarters of the rainfall record) there's still almost a 70mm difference within the range of closer to average seasons, which is more than enough to have a large impact on agricultural businesses.

For the complete climate outlook, visit www.bom.gov.au/climate/outlooks

- Jonathan Pollock, BOM climatologist

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