Friday 22nd Jul, 3:04 AM

22nd July Seasonal and Rain Update - train of cold fronts for the south

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 Tuesday and 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 large high sitting over the Tasman Sea, driving the rest of the nation's weather.

The energy from a cold front in the southeast a few days ago moved up to QLD and let a low develop off the coast. The high helps to push tropical moisture in from the Pacific Ocean, and they meet up to produce rain south of the low, and strong winds near the low. But... look how far off the coast this low is sitting.. the main action (wind and rain) is right near that low... offshore. So, we're missing the full brunt of this one, and the low is set to keep moving away from the coast.

A cold front brought rain to southwest WA but it should slide as it crosses the southeast on the weekend, with the rain dissipating as it slides.

The next cold front (just above the title box on the current map) pushes up over southwest WA on Saturday with further rain, cold air and wind. That'll arrive in SA later Sunday, and VIC/TAS on Monday.

After that, cold fronts continue to cross the south (very typical in winter) while the north looks settled - see SAM further down for more on why we are entering this pattern.

The 8 day rain outlook has that frontal rain activity across the south, creeping up into eastern NSW - and the remains of the current low affecting southeast QLD and northeast NSW. Those areas are only wet on days 1 and 2, and dry for the rest of the outlook.

Comparing the projected rain to the average in the following chart, you can see a lot of drier than average is depicted. This is from Monday 25th to Sunday 31st.

The activity in the southwest is typical to low for this time of year, but the cold front's aren't strong enough to bring the southeast the amounts of rain it would normally expect.

This seasonal model is picking up something coming to eastern QLD that isn't on the day by day models at the moment. One to watch.

The nights map shows quite a change from recent weeks - much less colder than average, and a lot more warmer than average.

As always in autumn, winter and spring, frost is a risk in between cold fronts, so make sure to check your location, and set an alert if you're a premium member.

The days map has only one part of the country colder than average - eastern QLD/northeast NSW, associated with the wetter than average rain map above.

The cold front pattern described on the current map has the fronts weakening as they cross, so they don't have huge amounts of cold air in behind them.. as shown on the days map. Residents of the south should be aware though that cold fronts increase the wind which makes it feel colder!

THE DRIVERS OF THIS PATTERN

Looking at the two boxes of interest in the Pacific and Indian Oceans, the Pacific continues to be lightly shaded cool blue with a few patches of warmer yellow in the east. See the Pacific Ocean outlook graph for more on this.

The Indian box has an area of cool blue in the north and west that is quite deep in colour (cold), however, it is offset by a warm yellow area in the southeast that takes up around half of the available space. See the Indian Ocean outlook graph for more.

The Pacific Ocean shows we are well out of La Nina (green area), properly sitting in neutral, and nowhere near El Nino (brown area). 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 models all have a similar outlook - known as a negative Indian Ocean Dipole (green area). The Indian Ocean box (from the sea temp map) had the blue 'winning' over the yellow. We crossed the threshold into this negative IOD in late May and should remain there until the end of the year. This acts to send a lot of moisture from the Indian Ocean towards Australia.

INDIAN OCEAN (IOD)

So:

- the Pacific Ocean pushes a little moisture towards Australia this winter and spring (less moisture than we had over summer and autumn, but not taking the moisture away)

- the Indian Ocean pushes a lot of moisture towards Australia this winter and spring.

- this is helped by warm oranges around much of Australia to help drive that moisture in... and the very warm Tasman Sea encourages low pressure to form just off the coast (one of the ingredients for flooding rain).

- next summer we may have a lot of moisture pushed towards Australia from the Pacific Ocean, if we go back into La Nina.

But... 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).

So, 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 cold 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, a potentially wishy washy pattern.

There was a strong negative outbreak at the beginning of winter, and our brief return around the 11th July (you may notice these are the only two times this winter we've had good alpine snowfalls...). The signal is now back in positive, encouraging the lows in QLD/NSW, but may turn negative for a time, encouraging a cold front pattern in the next week.

SOUTHERN ANNULAR MODE (SAM)

The MJO is a pulse of tropical energy, and whenever that comes near Australia it acts to bring the lows and moisture together. Then the forecast shows it rapidly moving away and remaining in the brown for a while (not encouraging rain in Australia's north).

MADDEN JULIAN OSCILLATION (MJO)

MONTHLY OUTLOOKS

The rain outlook for August.

Chances of seeing unusually high rainfall (ie in the wettest 20%).

Chances of seeing unusually low rainfall (ie in the driest 20%).

I'll update these each week going forward 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 and more information about them under our Rain Outlook and Seasonal Outlook pages within Jane's Update.