Friday 29th Jul, 2:56 AM

29th July Seasonal and Rain Update - juicy northwest cloudband to soak parts of south

In this series I'll take you through the drivers of our weather, highlighting any changes over time and things to watch out for (every Friday). It covers weather elements like temperature and rainfall, and how they are driven by moisture from the Pacific and Indian Oceans, as well as bursts of energy from low pressure (SAM and MJO).

Jane's commentary on the big picture drivers of our weather, updated weekly on Friday's.

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THE NEXT WEEK

The current weather maps shows a large high moving across southeast Australia and a cold front crossing the southwest.

This map should be looked at in conjunction with the satellite today, as there is a river of tropical moisture coming in from the northwest (the cloud band sweeping in from the cyclone located a long way out over the Indian Ocean).

The 8 day rain outlook has limited wet weather in the north, and quite a bit of rain through inland NSW, northern and SW VIC, NW TAS, southern SA and southwest WA. That's a typical rain pattern when there is a cloud band like that. However it's a slow starter: look at the day by day rain maps to see that most of the rain is coming next week rather than in the next few days. The peak of the rain is over 50mm, with lots of areas seeing more than 10mm.

Now compare the projected rain to average. This is from Monday 1st to Sunday 7th.

Large areas with a greater than 60% chance of above average rain (ie whenever we've seen a pattern like this in the past, it has been wetter than average 6 out of 10 times, and drier than average 4 out of 10 times). Some of those areas are up above 80% chance.

It's a decent drop - with 30 to 50 % chance of unusually high rainfall through these green areas.

While it doesn't spread north - much of far northern WA, NT and QLD have a 30 to 80 % chance of unusually low rainfall. Gippsland in VIC, and southwest TAS are also low - in a rain shadow from a northwest cloud band.

The nights map shows a lot of warmer than average, as you would expect with all that cloud.

The days map shows cold air really only coming into the far west (and parts of QLD). If you're a skier/snowboarder you won't like this outlook (and you can see just how ugly it's likely to be at each snow resort unfortunately - big rain mid week, then snow to follow).

THE DRIVERS OF THIS PATTERN

Looking at the two boxes of interest in the Pacific and Indian Oceans, the Pacific continues to be mostly blue, only a little bit yellow. See the Pacific Ocean outlook graph for more on this.

The Indian box is half blue half yellow - but the blue is deep. See the Indian Ocean outlook graph for more.

The Pacific Ocean shows we are well out of La Nina (green area), properly sitting in neutral, and nowhere near El Nino (brown area) - but heading back towards La Nina (green). You may hear different things from the USA as they use a weaker threshold, putting that observation right next to La Nina.

The Consensus of the models shows that we may return to be closer to Australia's La Nina (green) threshold during spring, and remain there for summer. If it crosses the threshold that would be our third La Nina in a row. Even if we don't cross the threshold, the ocean continues to show La Nina-like characteristics (ie the blue out there and warm orange off QLD), and no model likes El Nino. This all acts to send moisture from the Pacific Ocean towards Australia.

PACIFIC OCEAN (EL NINO/LA NINA)

The Indian Ocean models all have a similar outlook - known as a negative Indian Ocean Dipole (green area). The Indian Ocean box (from the sea temp map) has the blue 'winning' over the yellow. We crossed the threshold into this negative IOD in late May and should remain there until the end of the year. This acts to send a lot of moisture from the Indian Ocean towards Australia (with the next week being a great example of that in action).

INDIAN OCEAN (IOD)

So:

- the Pacific Ocean pushes a little moisture towards Australia this winter and spring (less moisture than we had over summer and autumn, but not taking the moisture away)

- the Indian Ocean pushes a lot of moisture towards Australia this winter and spring.

- this is helped by warm oranges around much of Australia to help drive that moisture in... and the very warm Tasman Sea encourages low pressure to form just off the coast (one of the ingredients for flooding rain).

- next summer we may have a lot of moisture pushed towards Australia from the Pacific Ocean, if we go back into La Nina.

But... moisture is just one part of the rain equation. You can have bucket loads of it, but if there isn't any low pressure in your area, it won't produce any rain (except for lighter falls near the coast in onshore winds).

So, we look at SAM and MJO to see what low pressure is likely to do.

When the SAM signal is:

- in the bottom green we are more likely to see strong cold fronts coming up from the Southern Ocean and crossing the south. They usually only hit one area with force... if they peak in Perth they slide over Melbourne, while if they peak in Melbourne they missed Perth.

- in the top green we are more likely to see troughs and lows in NSW/QLD latitudes, with weaker fronts in the south. A low may travel into the south if the highs move out of the way.

- in white means no push either way.

There was a strong negative SAM at the beginning of winter, and our brief return to negative around the 11th July (you may notice these are the only two times this winter we've had good alpine snowfalls...). Otherwise the signal is neutral to positive and expected to continue that way.

SOUTHERN ANNULAR MODE (SAM)

The MJO is a pulse of tropical energy, and whenever that comes near Australia it acts to bring low pressure and moisture together. It is currently in the brown zone (well out over the Indian Ocean), but the satellite shows it can have far reaching capabilities!

MADDEN JULIAN OSCILLATION (MJO)

MONTHLY OUTLOOKS

The rain outlook for August.

I'll update these each week on Friday's. Make sure you are signed up (free or premium membership) to get them delivered to you. And as always, you can see each of these graphics as soon as they update as well as more information about them under our Rain Outlook and Seasonal Outlook pages within Jane's Update.