BoM's winter outlook shows above average rain is likely across much of Australia - but not in the southwest, or southern parts of southeast Australia. Here's why.
The latest rain and temperature outlooks have been issued by BoM for the winter period.
I'll take you through what those forecasts are based on, clarifying what you are hearing about another La Nina and the Indian Ocean Dipole. There's another really important one to focus on too: SAM.
The outlooks (issued May 26th):
The map shows there is a high chance of above average rainfall across much of mainland northern and eastern Australia.
For those that have dealt with floods so far this year, this appears as not great news. However, much of northern Australia actually has very little rain during winter on average, and it's 70%+ chance of above average, not heavy rain.
Winter rain is usually heaviest in the southwest, and over southeast Australia.
These are actually the parts on BoM's winter outlook that show the lowest chance of above average rain this winter.
The southwest and southeast (in correlation with lower chances of rain) have higher chances of above average daytime temperatures overall this winter. The rest are starkly in the opposite, thanks to the cloud that is associated with the potential for above average rain.
Remember, these are 'overall'. There will still be warmer than average and colder than average temperatures as the temperature swings based on the weather systems moving through.
So, what is driving these outlooks?
There are three main drivers controlling our weather.
The Pacific Ocean remains in a state to push tropical moisture towards Australia. As long as it is blue in the box out to sea, and orange off Queensland, this will continue to occur.
This is reflected in the Pacific Index.
We were borderline in the green area all summer (known as La Nina), and projections show that we are likely to remain in a similar pattern through winter and spring. No model likes the drier area (known as El Nino).
This means that the Pacific Ocean should continue to push tropical moisture towards Australia for the foreseeable future.
The Indian Ocean operates in a similar way. When waters in the box off Africa are blue, and the waters off Indonesia and northwest WA are orange, tropical moisture is pushed towards Australia from the Indian Ocean (known as a Negative Indian Ocean Dipole).
The box off Africa only went blue in the past fortnight (see the ocean map above), a projection all the models had forecast. And now that the index has plunged into the green area, all the models expect it to remain there for winter and spring, keeping a steady supply of moisture coming in from the Indian Ocean.
So, that is both oceans sending moisture our way, but that doesn't mean anything unless there is low pressure to turn it into rain.
That is where SAM comes into it.
SAM tells us what that low pressure could look like.
SAM has spent much of the year in the top green area (known as positive), and very little time in the bottom green area (known as negative).
La Nina's in the Pacific Ocean encourage SAM to be in the top green area.
When that occurs, we are likely to see the weather pattern show troughs and low pressure systems over Queensland and NSW, possibly coming into Victoria. Instead of cold fronts in the southeast, there is blocking high pressure, encouraging drier conditions over southeast SA, southwestern VIC, and western TAS.
If the centre of the high is over the Tasman it is warmer than average, if the centre of the high is over the Bight it is cooler than average. Either way, they block the strong fronts that bring this area most of it's rain, and it is a similar set up in the southwest of the country too.
You can keep up to date with all these maps each week in our Seasonal Forecast, and rain maps day by day for the week ahead in our Rain Forecast. Snow Forecast's are up and running soon - check out what we have for you this season. And as always, stay up to date with hourly conditions for your spot under Forecast.
So, if you're in eastern NSW and have already seen too much rain, keep in mind that the oceans will continue to send tropical moisture, and whenever low pressure moves through it brings heavier rain than usual. In winter we see this in the form of east coast lows, and if you are located just to their south, the wind, surf and rain are damaging.
For inland parts of NSW, VIC and SA look for breaks in between the high pressure systems, and a northwest cloud band indicating a supply of this tropical moisture from the Indian Ocean. Troughs, fronts and lows will turn that moisture into steady, soaking rain - but in a positive SAM strong cold fronts are less likely.
For southwest WA, coastal parts of SA, southwest and central VIC, and western TAS, if SAM continues to be mainly positive, then there will be less strong cold fronts than in a usual winter, and that leads to less rainfall than average. On a positive note, it means it is less windy without the strong fronts.
For those looking towards a bumper snow season after two years stuck at home, the Indian Ocean sending moisture is a great sign. We have better snow seasons in a negative Indian Ocean Dipole. However, the Pacific Ocean's La Nina can lead to SAM being positive for much of the time - and that makes it harder to get air cold enough for big snow storms/dry snow/snow to lower levels. I'll have more on this in my next article.