Friday 7th Oct, 2:27 AM

7th October Seasonal and Rain Update - big rain Friday to Sunday, next big rain begins next Wednesday

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

A strong cold front crosses through the southeast on Friday, and affects NSW and southern QLD on Saturday into Sunday.

A dry break, thanks to high pressure, follows the front, lasting for a few days.

Meanwhile, a weak trough sets up in the west over the weekend. It slowly moves eastwards next week, increasing in instability as it goes. That's the east's next rain system.

8 DAY RAIN OUTLOOK (Friday to Friday)

The focus of rain is over the southeast and into southern QLD.

It comes through in two bursts. The first is Friday through to Sunday. A break follows, then the next burst is likely around Wednesday to Friday next week.

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)

This map shows increased chances for higher than average rain in central parts and the east, but less rain over the west.

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.

CURRENT FLOOD WATCHES AND WARNINGS

As of 12pm on Friday, these are the current flood warnings, and catchments under flood watch (a heads up that flooding may develop or renew in the next day or two), over southeast AUS. This is an evolving situation and the current warnings are available on the BoM site.

With the rain the east has had so far, many catchments are already well and truly soaked, and dams are full. The rain is falling on an already dangerous situation.

Continue reading to see why this is occurring, and why it will continue this spring and summer.

NEXT WEEK DAYS COMPARED TO AVERAGE

The pattern of colder than average days for this time of year continues for a large part of the country - except in western TAS, western WA and the NE tropics.

Rain doesn't let the land bake, so we're missing the usual 'heat up' that we get in spring. It means that when winds turn northerly they are missing the jump up in temperatures we usually see. Read on to see why this continues into summer too (hint... La Nina and positive SAM).

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, had a neutral Winter, and went back in to La Nina in early September. Our third La Nina in a row. It's sitting borderline on the threshold now - but the pattern of temperatures in the Pacific, and how the atmosphere is behaving indicate we are well and truly in La Nina.

7/8 models push the signal further into the green area in the months ahead, and the remaining one keeps 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.

Southwestern parts of WA sometimes miss out, as it all depends on where the low actually moves, as to who gets the rain. 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 neutral to 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 and northeast TAS. High pressure is more likely to be the dominant feature in western TAS, southern VIC, coastal SA and southern WA - but if a low breaks through this, then rain follows the low to these areas too.

Watch the signal for the occasional dip in the chart, signalling a shake up to the weather pattern, and the potential for cold outbreaks in the south with strong cold fronts (yes, even in summer too) - we have one of these now.

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. When it is in the inner circle or two white wedges, it has limited influence on our weather.

This signal has been weak, but the latest forecast shows the MJO is making a beeline towards Australia, and likely to remain in our area for a few weeks. This encourages rain, as the connection to the tropics strengthens.

MONTHLY OUTLOOK

RAIN OUTLOOK FOR NOVEMBER

La Nina and positive SAM lead to increased odds for higher than average rain in November in the blue and green areas. The Indian Ocean Dipole starts to weaken, but the oceans off northwestern Australia are still warmer than average, so there is still a source of moisture - but it's no longer constantly pushed towards Australia.

This leads to higher than average rain over central and eastern parts of the country, while much of the west is a weak/neutral signal (no strong push to either wet or dry).

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.