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How to interpret the winter forecast

Ben Domensino, Thursday May 17, 2018 - 14:13 EST

The Bureau of Meteorology issued a seasonal forecast for winter on Thursday.

When interpreting a seasonal forecast, it’s important to know how they differ from day to day weather forecasts, so you know what to expect in the months ahead.

Forecasts at the seasonal scale look at how broad-scale patterns in sea surface temperature are likely to influence the development of weather systems during the months ahead.

A seasonal outlook won’t tell you how much rain is going to fall at your place throughout winter, or what the average daytime maximum temperature will be during the season ahead.

What they can tell you, though, is whether you’re likely to see more or less rain than you typically would during the coming months, and whether days and nights are likely to be warmer or cooler than usual.

The forecast for winter is made a little more difficult this year due to an absence of any major climate drivers affecting Australia at the moment.

In the Pacific Ocean, the El Nino Southern Oscillation (ENSO) index is currently neutral, meaning that neither La Nina or El Nino are present.

In the Indian Ocean, a neutral Indian Ocean Dipole (IOD) means that this often useful index is also of little value at the moment.

Under neutral ENSO and IOD conditions, Australia’s weather can go either way.

In the absence of these two main climate drivers, the seasonal forecast models are relying heavily on fluctuations in local sea surface temperatures to determine what’s likely to happen this winter.

Warmer-than-average seas near Australia can cause more evaporation and help low pressure systems develop, which can cause more rain. By contrast, cooler waters can limit atmospheric moisture and hinder the development of rain-bearing systems.

According to the Bureau’s outlook, the chances of above and below average rainfall are roughly equal across most of Australia during winter. However, there are some exceptions.

Western districts of WA and northeast Queensland may experience below average rainfall during the next three months. There are also indications that parts Tasmania and eastern Victoria could have a wetter-than-usual winter, although it’s worth noting that the accuracy of rainfall forecasts in this part of the country is historically quite low for winter.

These predictions are largely based on the influence warmer-than-average waters sitting at the surface of the Tasman Sea and cooler water off the nation’s south and west coasts, and near Cape York Peninsula.

In terms of temperatures, days and nights are expected to be warmer than usual across southern Australia. While there is no strong indication that days will be unusually warm or cool for most of northern Australia, nights may be colder than usual for most of Queensland.

As with all seasonal forecasts, you’ll need to consider what the temperature and rainfall are usually doing at your place during winter to know what above average and below average means for you.

You can find climate averages for many Australian locations here: http://www.weatherzone.com.au/climate/stationdrill.jsp

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