Monday 18th Jul, 11:43 PM

19th July Seasonal and Rain Update - quiet period in southeast, active QLD/NSW/WA

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 twice weekly (as that is when most indicators update).

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THE NEXT WEEK

The current weather maps shows a large high coming into the southeast. That will be like an anchor over the next week, gradually moving to sit just east of Tasmania (leading to frosty nights and sunny days, slowly thawing out as it moves eastwards). While the southeast is quiet, the northeast and southwest should be very active. The northeast is impacted by a complex low from Wednesday to Sunday. The southwest has a few lows, the first just off to the west today.

You can see those elements in action here on the next 8 days rain outlook. Rain in eastern parts of QLD/NSW; rain in the southwest. Only a little bit makes it through to the southeast, and not until early next week.

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 Saturday to Sunday, and most of the QLD rain falls before Saturday, so it's back to dry winter and usual from the weekend. The NSW coast still has a few days of that rain system, then back to generally dry. The lows in the southwest are typical for this time of year.

The nights map shows a large part of central and eastern Australia in a period of colder than average nights. This map is from Saturday, so it doesn't include the frosty period in the southeast before then, but parts of QLD and NSW could have a frosty period after the low moves away - make sure to check your location, and set an alert if you're a premium member.

The days map shows the effects of the cloud lingering over eastern QLD and NSW after the low, while the rest looks warmer than average. With the high to the east of Tasmania that brings northerly winds to the southeast and lets it 'warm' up (ie Melbourne could reach 18C, possibly twice).

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. See the Pacific Ocean outlook graph for more on this.

The Indian box's area of blue continues to deepen in colour in the north and west of the box, however, the warm yellow in the southeast has grown. 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). The Consensus of the models shows that we may return to be closer to the 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). 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, but if they peak in Perth they slide over Melbourne, and vice versa.

- 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

The chart shows the end of the strong negative outbreak at the beginning of winter, and our brief return last weekend (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 and in WA.

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. We had big rain in QLD and NSW at the end of June, and the next QLD/NSW rain will be helped by the signal being in the green again. Then the forecast shows it rapidly moving away and remaining in the brown for a while (not encouraging rain).

MADDEN JULIAN OSCILLATION (MJO)

Finally, we have the outlook for August (same as Friday's as it only updates once a week).

The next update is due on Tuesday morning. 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.