Friday 12th Aug, 3:45 AM

12th August Seasonal and Rain Update - negative IOD cemented, next La Nina even more likely

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 complex area of low pressure over the southeast (one centre near Adelaide, the other west of TAS), with cold fronts stretching well up into QLD (courtesy of a negative IOD - see later for more on that). High pressure is cradling the lows, so this won't be moving anywhere fast. Another front is approaching the west.

This our second 'inland' rain system in a week and the satellite shows where these bands of rain are located today. In the eastern states the rain falls mainly west and north of the Great Dividing Range, and the east coast, Melbourne and Gippsland are in the rain shadow (the rain dries up as it crosses the ranges).

The 8 day rain outlook shows this pattern in action, however, you may notice higher falls in TAS and southeast VIC. The cradling high means the lows aren't moving fast, but one of them is likely to form off that coast later in the weekend, with big falls for those locations if the low is close to the land. When this part occurs, the inland areas of the mainland dry up (ie they are now in the rain shadow).

See the day by day maps for each day in detail, including when the west is affected by weather systems. Saturday's isn't strong, but early next week could have a big feed of moisture.

Comparing the projected rain to average confirms that the east coast and northeast are taking a break, but there may be yet another rain system crossing inland parts.

Some of this rain may be significant, and next week has a 30 to 50 % chance of unusually high rainfall in these parts:

Much of the country is expecting warmer than average nights overall in the week beginning on Monday.

Much of the country is expecting warmer than average days overall too. This pattern indicates no strong cold fronts coming up into the southeast, but widespread rain may cool things in the far west.

THE DRIVERS OF THIS PATTERN

In order to make it rain you need two things to work together:

- Tropical moisture

- Low pressure

Let's begin by looking at the moisture part, in the tropical areas of the Indian and Pacific Oceans.

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, offset by warm water in the western Pacific. See the Pacific Ocean outlook graph for more.

The Indian box was gaining yellow, but this week the blue is taking over again. It is offset by warm water off NW AUS and Indonesia - the pattern shows a proper imbalance setting up. See the Indian Ocean outlook graph for more.

This map screams oodles of tropical moisture is available for Australia.

The Pacific Ocean graph shows we went out of La Nina (green zone) early in Autumn, but the index is heading southwards again, and most models show La Nina or close to it, later in Spring and into Summer. This would be our third La Nina year in a row.

PACIFIC OCEAN (EL NINO/LA NINA)

Looking under the surface of the Pacific Ocean, out in the middle the warmer colours have disappeared over the last two months, replaced by a large area of blue. This will act to keep the boxed area cooler than average - and gives much higher confidence in a developing La Nina.

BENEATH THE PACIFIC OCEAN SURFACE

The Indian Ocean has been in the green zone for two months now, so the has BoM declared that we are in a negative Indian Ocean Dipole (IOD). The observation line rose in the past few weeks (as the yellow took over the box a bit more) but it remained well below the threshold, and all models keep us in a negative IOD until it naturally decays in summer. This is our third negative IOD year in a row. The recent inland rain, and the potential for more, are great examples of this in action.

INDIAN OCEAN (IOD)

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).

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, again around the 11th July, followed by another dip in late July/early August - negative SAM is when the alps are most likely to see big, cold snowfalls.

Now we are heading positive, so the low's take over - this time sitting off Gippsland and eastern TAS, rather than Sydney or Brisbane.

There may be another negative dip in the short term, otherwise this model like positive as it seems to do all the time.

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. We've just moved very slightly into the green zone, but the signal is heading into the neutral or brown zone from here. The next big connection from the tropics may be in early October.

MADDEN JULIAN OSCILLATION (MJO)

MONTHLY OUTLOOKS

The rain outlook for September.

I 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.