Friday 5th Aug, 12:09 AM

5th August Seasonal and Rain Update - another inland rain event brewing for next week

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 Southern Ocean (meaning multiple low centres that are connected), driving fronts and troughs over southern Australia. They began in the west but have now moved eastwards with high pressure to follow - and this will continue to all progress eastwards. The strongest front extends right up into QLD with a link to tropical moisture from the Indian Ocean, producing widespread rain.

If you have been following along over the past few weeks you will see the strong front and think 'negative SAM', and the connection to the Indian Ocean and think 'negative IOD' - and you will see that you are correct in the charts to follow :)

The satellite shows that we are on the tail end of the moisture coming from the Indian Ocean, and that the air in the south is turning speckled, indicating it is changing from warmer than average in NW winds, to cold air coming up from Antarctica. The lines on the weather map are literally like roads for air, and they show you where the air is travelling from (moving clockwise around a low, anticlockwise around a high).

There is another cold front approaching from the southwest, arriving in SW WA on Sunday, and the low well out over the Indian Ocean (the blob in the top left corner on the satellite) shows that there may be further feeds of tropical moisture to come.

The 8 day rain outlook has out of season rain in southern QLD/northern NSW - that clears by later tomorrow and it's back to dry conditions. The southeast is generally over the next two days as well, followed by a dry spell under the high. The west ramps up again over the weekend with the next front - and there is another area of rain spreading across the inland into the southeast late in the period. This is important, and you will see why it is occurring in the following maps.

See here for the day by day maps to make the rain projections clearer.

Comparing the projected rain to average shows strong signs of another rain event sweeping down from the northwest. Southwest WA and western TAS are drier than usual with this pattern.

Another 30 to 60 % chance of not just above average rain, but unusually high rainfall, across the interior into inland parts of the eastern states - the second week in a row.

Colder than average nights return to much of the south. Set your cold temperature alerts to stay ahead of the frost risk. This is likely when the high moves through early next week (after the cold surge comes up from the south this weekend).

The days map shows colder days are likely next week across much of the country, except in the far north and TAS.

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 is is gaining yellow - but the remaining blue is deep. It is offset by warm water off NW AUS and Indonesia. See the Indian Ocean outlook graph for more.

The Pacific Ocean shows we are out of La Nina (green area), properly sitting in neutral, and nowhere near El Nino (brown area) - but heading back towards La Nina (green). You may hear different things from the USA as they use a weaker threshold, putting that observation right next to La Nina.

The Consensus of the models shows that we may return to be closer to Australia's La Nina (green) threshold during spring, and remain there for summer. If it crosses the threshold that would be our third La Nina in a row. Even if we don't cross the threshold, the ocean continues to show La Nina-like characteristics (ie the blue out there and warm orange off QLD), and no model likes El Nino. This all acts to send moisture from the Pacific Ocean towards Australia.

PACIFIC OCEAN (EL NINO/LA NINA)

The Indian Ocean has been in the green zone for two months now, so the BoM can declare we are in a negative Indian Ocean Dipole (IOD). The observation line has risen in the past few weeks (the yellow taking over the box a bit more) but it is still well below the threshold. All models keep us in a negative IOD until summer. This acts to send tropical moisture from the Indian Ocean towards Australia. The recent inland rain, and the potential for more next week 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, and our brief return to negative around the 11th July and now a huge dip into negative (which was not forecast by the model if you look at last week's update).

Snow resorts generally need a negative SAM to get good snowfalls (which occurred at the beginning of winter, again around the 11th of July, but not in the past few days). You also need cold enough air, and that usually isn't possible when the lows driving the fronts are back to the west of the alps. The cold air arrives when they push eastwards.

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. This is likely to do a bit of a drive by of Australia over the next week (move through the green zone) - why we may have another big inland rain system.

MADDEN JULIAN OSCILLATION (MJO)

MONTHLY OUTLOOKS

The rain outlook for August.

The rain outlook for September.

I'll 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.