Friday 23rd Sep, 1:08 AM

23rd September Seasonal and Rain Update - next week's low is likely to take a more southerly path

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

The big low pressure system that began in the west, moved across SA, then slowly through NSW, is now right on the QLD/NSW coastal border this morning, stretching a trough back across NSW and into central VIC. The position of the low now, means the heavy rain (and storms) are around Lismore, but have cleared SE QLD (as the heavy rain with a low on the coast falls just south of the low, while it's dry to the north).

The trough is also bringing lighter rain (and some storms) to it's east as well.

All of this will continue to move eastwards and clear Australia today and tomorrow.

Next... a cold front clips the southeast today into tomorrow (but if you're an AFL fan, the wet weather clears in time for the Grand Final).

After that, the next low slowly moves through. This one bypasses the west, then affects SA, VIC, TAS and southern NSW. It takes a more southerly path than the last one, but still has an associated trough affect northern NSW and southern QLD.

8 DAY RAIN OUTLOOK (Friday to Friday)

The rain projections show the low on grazes the far southwest... and most of it's action is focused in VIC, southern NSW and northeast TAS this time. This is due next week - see the day by day maps for timing and movement of the rain, with each day in detail.

Then the end of next week and next weekend may bring a new inland low system affecting the interior. The models have very different projections for which areas this one is likely to affect - it could be northern WA and NT, or it could spread east and drench QLD.

NEXT WEEK RAIN COMPARED TO AVERAGE (Monday to Sunday)

This map shows the activity in the southeast, then the potential for that interior system to affect a large part - this model has the NT and QLD in the zone.

This is a map of rain vs average conditions for this time of year, and that interior/QLD rain is very early in the season.

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 NIGHTS COMPARED TO AVERAGE

The pattern of colder than average for this time of year continues at night for a large part of the country - except in TAS and the tropics.

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 at this time of year during the day continues for a large part of the country too - except in TAS and the 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.

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.

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 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. High pressure is more likely to be the dominant feature in 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.

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, and the latest forecast shows the MJO will continue to not 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.