Friday 16th Sep, 2:29 AM

16th September Seasonal and Rain Update - La Nina and negative IOD combine to increase rainfall, but only to those in the path of low pressure

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

WEATHER MAP AND SATELLITE

A deep and complex low is slowly moving across the Bight, controlling the nation's weather. The troughs and fronts connected to this low bring waves of wet weather when they pass through.

The first trough has now moved off the east coast, and the next front is over South Australia, about to head through the southeast. You can see yet another front lining up just west of the low, due in the southeast on Saturday. The separate front southwest of Perth only clips the southwest on Saturday (the high is in the way), but it strengthens as it crosses the Bight to push through the southeast on Sunday into Monday.

There's another rain system due next week. It begins in the west on Monday (again not much thanks to the high), and increases as it moves eastwards mid to late in the week. This one may travel further north, missing southern VIC and TAS.

8 DAY RAIN OUTLOOK (Friday to Friday)

That pattern leads to heavier rainfalls concentrating on the southeast, up into southern QLD. In the west, the high limits what rain can occur, so the heavier falls are only over the far southwest.

See the day by day maps for timing and movement of the rain, with each day in detail.

NEXT WEEK RAIN COMPARED TO AVERAGE (Monday to Sunday)

Looking at the period from Monday through to next Sunday, and that amount of rain is below average for western TAS, but above average for much of QLD and the interior. It is also slightly below average for this time of year in the southwest.

Stay ahead of what is most likely at your location by setting your Weather Summary alert for a day by day guide, or any of our Rain alerts for more specific details - sent to your inbox every morning and/or evening. Try our Good Spraying Conditions forecast too, to see the breaks in the weather.

NEXT WEEK CHANCE OF UNUSUALLY HIGH RAINFALL

This projected rain is likely to be unusually high for this time of year in eastern parts of QLD and northern NT, but also in northern VIC and southwest NSW, and parts of southeastern WA.

NEXT WEEK CHANCE OF UNUSUALLY LOW RAINFALL

There is a large area in the north that the projected rain is well below average for this time of year. Also included is southwestern TAS.

NEXT WEEK NIGHTS COMPARED TO AVERAGE

The cold nights pattern continues over the interior and into the east. The southwest looks above average, along with the tropics.

Set your cold temperature alerts to see which night's are a concern at your place. Our frost risk alert (that takes into account not just temperature, but cloud and wind as well) will be operational shortly.

NEXT WEEK DAYS COMPARED TO AVERAGE

The Bureau's model continues to show a strong signal towards colder than average days, centred over central and eastern Australia, not including the tropics. We'll still see the odd warm day, but this indicates that overall it is colder than we expect at this time of year.

THE DRIVERS OF THIS PATTERN

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

- Tropical moisture

- Instability from low pressure

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

1. TROPICAL MOISTURE

SEA SURFACE TEMPERATURE ANOMALY

When the boxes have cool blues, and the waters around northern Australia have warm oranges, then tropical moisture is encouraged to push towards Australia rather than away from it. When the boxes have warm oranges then tropical moisture is encouraged to push away from Australia.

Both oceans are currently set up to push tropical moisture towards Australia.

PACIFIC OCEAN (EL NINO/LA NINA)

When the signal is in the bottom green area, then tropical moisture is pushed towards Australia from the Pacific Ocean (known as La Nina). When the signal is in the top brown area, then tropical moisture is pushed away Australia from the Pacific Ocean (known as El Nino).

We came out of La Nina in Autumn, and the signal rapidly fell in early Spring, with the Bureau declaring the next La Nina has begun, our third in a row.

6/8 models push the signal further into the green area in the months ahead, and the remaining two keep it borderline.

La Nina means there is an abundance of moisture from the Pacific Ocean, and whenever low pressure gets involved it encourages rain in the low's path that can be significant. There is also a break in the rain when there is no low pressure nearby.

All models have us coming out of La Nina early next year (rather than in Autumn), so this one may not last as long as in previous years. However, it will take a season or two for the ocean to remove that huge blue patch (see the sea temp map) so we'll continue to have moisture pushed towards Australia for a while (rather than away from us).

INDIAN OCEAN (IOD)

When the signal is in the bottom green area, then tropical moisture is pushed towards Australia from the Indian Ocean (known as a negative Indian Ocean Dipole [IOD]). When the signal is in the top brown area, then tropical moisture is pushed away Australia from the Indian Ocean (known as a positive IOD).

The Indian Ocean has been in a negative IOD since late May (sometimes known as the 'La Nina of the Indian Ocean'). All models keep us in a negative IOD until it naturally decays in summer.

This is our third negative IOD year in a row. A negative IOD means there is an abundance of moisture from the Indian Ocean, and when low pressure gets involved it encourages rain in the low's path that can be significant, but when there is a break in low pressure nearby it is dry.

Rain as a result of moisture from the Indian Ocean doesn't just affect Western Australia. Tropical moisture is great at travelling long distances, and often fuels weather systems once they reach the eastern states.

Parts of WA sometimes miss out, as the rain activity can be highly pin pointed as it crosses that part of the world. But as always, if a low sets up in the west, a drenching soon follows for those in its path.

2. INSTABILITY FROM LOW PRESSURE

We look at SAM and MJO to see what low pressure is likely to do.

SOUTHERN ANNULAR MODE (SAM)

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 and a belt of high pressure. A low may travel into the south if the highs move out of the way.

- in white means no push either way.

We have been positive for much of the time this year, with only the occasional dip into neutral, and even rarer dip into negative. The forecast shows positive is likely to be the dominant condition in the months to come.

This means that lows and troughs gravitate towards southern QLD, NSW and northern VIC, and occasionally off Gippsland in VIC. High pressure is more likely to be the dominant feature in TAS, southern VIC, southern SA and southern WA.

Watch the signal for the occasional dip, signalling a shake up to the weather pattern, and cold outbreaks in the south with strong cold fronts.

MADDEN JULIAN OSCILLATION (MJO)

The MJO is a pulse of tropical energy, and whenever that comes near Australia (the green zone) it encourages low pressure and moisture to work together. When it is away from Australia (the brown zone) it can suppress this relationship.

This signal came near Australia recently, but the latest forecast shows the MJO won't be much of a feature in the next few weeks.

MONTHLY OUTLOOK

RAIN OUTLOOK FOR OCTOBER

Negative IOD, La Nina and positive SAM all add up to increased odds for higher than average rain in October in the blue and green areas.

The southern coastline only sees this rain if a low can wander through... it is not from a series of strong cold fronts... which is why southwest WA and western TAS continue to be orange and likely to be drier than average.

Stay up to date with the forecast for your area in our hour by hour outlook for the next 10 days.

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.