Friday 30th Sep, 4:15 AM

30th September Seasonal and Rain Update - soaking rain moving east next week, another big rain system

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.

Sign up for Free or Premium Membership and have it sent directly to your inbox.

THE NEXT WEEK

WEATHER MAP

The low that brought the most recent rain is moving away over the Tasman Sea.

Now, the high near Tasmania pushes a southeasterly airflow over the eastern states, and when that air runs into a trough it encourages showers/thunderstorm activity. This continues across the weekend, but it's fairly weak, so this is also a generally sunny break before the next big deluge arrives next week.

The low in the northwest is another big one. That'll very slowly progress eastwards over the weekend and next week. It misses southwest WA, but large parts of central and eastern Australia are in the drenching zone from this one as shown on the next map.

8 DAY RAIN OUTLOOK (Friday to Friday)

See the day by day maps for timing and movement of the rain, with each day in detail. The rain reaches the east mid to late next week.

NEXT WEEK RAIN COMPARED TO AVERAGE (Monday to Sunday)

This is another big one. The map shows the week from Monday (so it doesn't include the big rain in the northwest from Friday to Sunday).

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 (Monday to Sunday)

From Darwin through to Melbourne, it's likely to be another big week - in terms of how much rain it could be in comparison to extremes we've had in the past. Yet another week amongst the wettest 20% for this time of year across a huge part of the country.

CURRENT FLOOD WATCHES AND WARNINGS

As of 2pm 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. There is also a flood watch in northwestern AUS.

With the rain the east has had so far, many catchments are already well and truly soaked, and dams are full. Next week's rain will be falling on an already dangerous situation. Continue reading to see why this is occurring, and why it will continue to occur this spring and summer.

NEXT WEEK NIGHTS COMPARED TO AVERAGE

With rain comes warmer nights, while those in the west go into another dry phase.

Set your Frost Risk alert to see which night's are a concern at your place.

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, SW 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 should continue 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. The Pacific blue has intensified over the past month.

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).

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. This may be encouraging this latest rain system, as the connection to the tropics strengthens.

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.