I like to do these 2 week outlooks a few times a week to give some sense of where things are heading, great for planning purposes.
Rainfall Outlook % chance of exceeding the median rainfall for the coming 2 weeks.
Not much change in the coming week. Drier signals continue for the next 7-10 days with the persistence forecast. Westerly winds dragging the dry air from the inland across the nation to the east. There are some signs we may see a cloud band next weekend that could bring some light rainfall to inland areas but not enough to override the below average rainfall signals. The south wet, with a lot of frontal weather, elsewhere, seasonal rainfall expectations.
Temperature Outlook - Next 2 weeks
Temperatures are the big shift with the warmer signal creeping south with the strong northwest winds ahead of the cold fronts dragging the warmer air southeast and south. The southwest will remain cooler than normal with the coldest air impacting that part of the nation in line with where the long wave is expected to be positioned during this time.
This week the pattern is very much set in stone, we know there is more frontal weather developing in association with the long wave trough and the negative SAM. We know the impact of that will be more of the same, frequent frontal weather for the coming 7 days at least, with some moderation likely as the SAM trends neutral and then possibly positive into the second week of August.
SAM Forecast - Coming 2 weeks. The negative dive this week, will recover by next weekend, with generally neutral conditions developing towards the end of the first week of August. This is where we are seeing some signals of the pattern shifting. This will be the catalyst for shifting the pattern over the region during the coming fortnight.
That is where I am anticipating the pattern flip for the nation where we see the high pressure take control of the southern states, easterly winds develop over the east bringing back coastal showers and moisture and heat building over the north, which eventually spills south.
But the clues also lay over the Indian Ocean, and looking upstream in the westerly wind belt, you can see the shift in conditions, with the westerly wind belt starting to retreat further south over the course of the next 10-12 days.
18Z GFS - Surface Pressure Values over the coming 2 weeks.
Note the wavy flow that is sitting further north during the period, bringing up cold front after cold front after cold front through the next 7 days at least. But then as the SAM trends more neutral, the westerly winds starts to move further south and the high pressure belt begins to dig in from the north and this allows the jet stream to also seep further south. Under such guidance is would be reasonable to surmise that the weather would become warmer, less wet for the south and the chances of a pattern flip increase where we see easterly winds develop over eastern Australia and the moisture to build further off the northwest of Australia, and also that moisture begins to seep further south as well.
Thursday 29th of July 2021
Note the weather is dominated by a large long wave over the southern half of the nation with the very strong ridge over the western and central Indian Ocean helping to propel the westerly winds into the southern states. This looks to be a good solution this week weather expected to remain unchanged for the coming week.
Monday 9th of August 2021
By the time we reach the second week of August, the pattern has changed, and modelling is suggesting that the ridging over southern Australia will shunt the westerly wind belt to the south and allow the moisture and lower air pressure to build over the Indian Ocean. That moisture with the upper level winds would then be propelled to the southeast into a trough or front that approaches from the west during the second week of August. So that could be where we see rainfall signals increase over the interior. This period the weather will be much warmer than this week coming, with above average temperatures to replace the cold and blustery weather of the past few weeks and the week coming.
Moisture looks to reset over the course of the next week offshore the northwest of the nation while the frontal weather continues to ram through the southern states. I have drawn this in recent days as well for people to see the movement of this weather. You can see it playing out in the national wraps that I do twice daily which helps to supplement the weather information for your part of the world.
I will use the Indian Ocean as the foundation for this two week forecast as it is the dominant feature of this period. This is where the inland areas will get the rainfall into mid August onwards as what has been pointed out in my Climate updates which you can review here
18Z GFS Precipitable Water Values for the coming 2 weeks
The moisture does build slowly over the coming 2 weeks. You can see those tropical waves moving north of the nation, the southern branch of that moisture being drawn southwest and south through the Indian Ocean as the moisture rotates around the upper high over the NT. As the upper high breaks down, the steering currents then break down and the moisture sits in the region and continue to build in the region south of Indonesia. It is not until the jet stream realigns itself over the nation in response to the next wave approaches, that the moisture is then drawn south and southeast into WA and the NT. This is something that I will be watching during the next week or so and I think we will have something more significant showing up in short term forecasting in the next week as the frontal barrage continues.
18Z GFS Precipitable Water Values Saturday 7th of August 2021
Deep moisture building in the classic position/region for the negative IOD, this moisture is looking to be drawn south ahead of frontal passages which are further south in this period than the coming week, but these four zones I have pointed out, means there is an large shift in conditions over the basin into the second week of August, meaning that the moisture will be surging towards Australia. That could see elevated rainfall chances for the nation as we get through the second half of August and then peaking as pointed out in my climate analysis during September to November as the seasons shift.
Monday 9th of August 2021
And you can see two days later the impact of that surging moisture with the major long wave/front moving closer to Australia, scooping up and drawing the moisture out of the northwest to the southeast, but as it moves closer to Australia, it will help to pull that stagnant area of heavy convection over the northern Indian Ocean southeast towards Australia for mid month. Again it is a way out but this is what is underpinning the forecasts into the 2-4 week range here on this website and of course there will be some refining in this as we get closer. But it does suggest to me the pattern will flip at the end of the first week of August.
Back to the mainland, how does that look in terms of temperatures and rainfall?
12Z Euro Ensemble Rainfall - Next 15 days.
There will be little change in the rainfall this week which has been well covered. Dry for the north and east and wetter over the southern states with the gusty winds. But then note in the modelling below, the green shading is generally increasing for the period beyond August 6th. So it hasn't quite captured the impact of moisture that comes in mid month, but I will update Tuesday. But for those wanting more generous follow up it looks like the period August 10-20. So again the short term modelling won't quite capture this well but keep watching the trends. A better look on this period will be available from about this time next week.
12Z GFS Ensemble Rainfall - Next 15 days.
Very similar to the Euro in that the moisture impacts come in after this period, but note that there is still signals for the pattern flipping at the surface level with the westerly wind belt retreating and warmer settled weather taking hold of the nation at the beginning of the August.
12Z CMC Ensemble Rainfall - Next 15 days.
This model is interesting as it brings in the impact of the moisture a little earlier, with about 80% of the 30 member data set suggesting a larger cloud band next weekend and again for the first week of August working eastwards with light falls for many areas.
Temperatures are also an indication of the pattern flip. There will be a warming trend beyond the freezing weather this weekend. The warmer air is likely to being drawn southeast ahead of frontal weather this week, with periods of above average temperatures before the front arrives, so it will be a spring preview at times this week. The following week looks much warmer with the high pressure starting to beat down the frontal weather, and then dragging in the warmer air over the southern and eastern inland with the temperatures along the southern and eastern coastal areas likely to be above average for the 2nd week of this outlook. The west with the long wave parked over the southwest of the nation could keep that area colder than normal for many areas.
12Z GFS Temperature Anomalies- Next 15 days
This has been suggesting the inland will remain warmer than normal, with the warmer air being drawn south and southeast with upper level winds bringing in the milder days ahead of the frontal rainfall and westerly wind changes. The coldest air will impact the southwest of the nation through this period, and this could extend inland with the cloud increasing from the west.
12Z Euro Temperature Anomalies- Next 15 days
Much warmer weather expected for the second week of this period, particularly once we lose the westerly wind regime with this bitterly cold air. We will see the warm air drawn in ahead of frontal weather from this point on before the colder rainy weather arrives. That will bring that spring feel with temperatures bouncing around. However I will point out that overall, it will likely finish up warmer than normal over this period, with the coldest air not as severe as this last event we have experienced.
12Z CMC Temperature Anomalies- Next 15 days
Similar to the models above, note the colder signal through over inland WA and that is in association with the cloud band developing from the Indian Ocean.
More weather details to come this afternoon with the national wrap and a look at the short term forecasting for the state by state fly around.