MEDIUM TERM FORECAST UPDATE

The twists and turns continue through the medium term, making it tricky to read the play for you all into the 2 week range, however over the past day, which I touched on yesterday, we have seen a shift to rainfall developing over the east coast with a developing trough or low pressure system sitting offshore.


That continues in the modelling today.


Rainfall for the next 4 days

It is easier to break down what is falling when and where. So this is coming during the next 4 days but carries some level of caution as we try and how far north the frontal weather will eventually be and how deep the moisture is surging into the systems from the northwest. The latest GFS has brought back the coupling of the elements so a little more rainfall is now expected for parts of southern Australia. Rainfall largely unchanged for the southwest of WA.

UKMET - Next 6 days until Monday morning, starting to bring in the stronger system early next week.

Again the shorter term, the next 5 days or so you can track that rainfall mainly over southern areas of the nation, and then stacking up on and west of the divide through southern NSW. Depending on two factors, how much moisture gets drawn into the frontal weather and how far north the systems are, this will dictate how much rainfall develops.

ICON/German - Next 5 days until Sunday afternoon.

Light and patchy rainfall for the inland but more concentrated rainfall for coastal areas of southern WA through southeast SA into VIC and heavy falls over western parts of Tasmania. Most of the inland as covered off during the past 24hrs suggesting that the weather will remain dry for the next 5 days.

Lets try and make sense of the medium term now.


SYNOPSIS.


A strong cold front is expected to pass through early next week, what form that takes during Monday and through Tuesday is becoming a little clearer. It looks like the front will move through steadily over the southeast unlikely to cut off into a low pressure system over the region, but that may occur offshore NSW or southern QLD.


There is a large chunk of tropical air that will be drawn south through the Coral Sea through the Pacific into the Tasman and this could interact with the colder southerly surge moving up the NSW coast. This may lead to showers increasing finally for those along the east coast a little worried it is getting too dry (though many areas still sitting well ahead of the YTD totals)


The moisture that is hanging back through parts of QLD and northern NSW due to onshore winds over the coming days may also be lifted by a front passing throughout the region early next week into showers and storms. That is a lower chance if you are living through QLD and NSW but something that needs to be watched.


Also through the early part of next week we could see showers develop for the northern areas of the NT, the models expressing a spike in humidity more than rainfall, the seasonal shift early this year with higher heat levels being experienced at the moment.


Out west we are seeing a surge of moisture coming in again through the west of the nation from this time next week ahead of another frontal passage that will again trigger showers for some parts of WA. That may track rain through southern Australia from later next week.


As you can see the rainfall opportunities continue into the medium term to round out the month. The GFS for example showing two cut off low pressure systems bringing that rainfall over the southern and eastern inland.


The north of the nation however may get drier air after days of the signals suggesting the humidity would increase, so that remains unclear. The more moisture there is up north, the better it is for those looking for rainfall.


For now the confidence in the forecast remains low, there has been a shift in bias for rainfall over the eastern inland of the nation next week and possibly persisting into the end of the month. But we will see.


Lets look at the data


18Z GFS Surface Pressure Pattern for the coming 16 days

The weather in the medium term is interesting, we do see rainfall opportunities to continue until the end of the month, there is no signal of dominating blocking high pressure stopping rainfall bearing systems nationwide. So that is a good thing. The issue at the moment is the dominance of the subtropical ridge through the southern parts of the nation and the phasing of cold fronts, which at this stage, timing it well with the dry air finding the weakness in the ridging over the inland and surging north. The opposite of this was being modelled over the weekend where moisture was surging south over the nation finding the weakness in the ridge over the southern parts of the nation and then being lifted into areas of widespread rainfall for inland areas. Now that we are seeing inland areas being dominated by dry air for the coming week at least, it is safer and considered to keep rainfall totals very low. The medium term offers a new front passing through the SWLD next week with a further band of showers. That system then turns into a low pressure system over the Bight and sweeps through SA with limited moisture but the model says moisture over the east would then be lifted by this system and produce more rain in the east. Another system moves in from the west during early September.

18z GFS Precipitable Water Anomalies for the coming 16 days

The battle between the dry air surging north through the nation and the moisture trying to overrun that dry air being driven south continues to play havoc with the forecast coverage of rainfall, cloud cover and the overall impact of weather systems that come in from the west during the next fortnight. The weather is expected to remain warmer than average over most inland areas which means that higher than average moisture values could be observed, more than what is being shown here, which could increase rainfall chances through northern and eastern parts of the nation. The weather over the west and northwest of the nation could be dry, hot and sunny under persistent dry air, that air surging offshore over the eastern Indian Ocean (not unusual) and suppressing the region that would provide us the biggest chance of rainfall during spring. But this will start to break down over the coming 4 weeks as we continue to move towards early build up conditions. So at this time, the data is supporting rainfall for eastern areas of the nation more than the northwest and west during the coming 2 weeks. But this is low confidence.

18z GFS Rainfall for the next 16 days.

Rainfall is coastal as discussed for the next 5 days over the southern parts of the nation, perhaps a few showers for the QLD coast, more extensive over FNQ before that eases this weekend. Then a stronger system approaching next week could bring another burst of rainfall to the southeast states, with the heavier falls likely to bypass SA, and peak over central and eastern VIC and through southern inland NSW west of the divide. That colder surge of air will help to promote showery periods along the east coast with the chance of moderate falls for areas that have largely missed out in recent months which is normal for winter. The pattern then in the medium term keeps the rainfall bias over the south but introduces the idea of easterly winds developing once again to end the month and to kick off September. Any system rolling out of the west into moisture from the easterly winds would see rainfall break out over inland areas. Must also keep watch over the moisture surge possible from the NT and northern QLD through the remainder of the eastern inland ahead of frontal weather coming out of the southern parts of the nation too. The western inland largely dry for now with very hot weather.

12z CMC Rainfall for the next 10 days.

The model is unchanged from overnight, with southern coastal areas of the nation expected to see the bulk of rainfall during the next week. Note the moisture over the northeast of the nation wit showers increasing over the east coast. Also CMC does like to promote the idea of showers over northern areas of the nation with a pulse of moisture giving way to showers next week. This is probably a little too widespread, and if it verifies, it will see moisture increase over the eastern inland waiting for the next system to move out of the west.

12z KMA Rainfall for the coming 12 days

Some indications that we could see a deeper low pressure system developing just offshore the east coast next week if this model verifies. The shower activity more widespread, the moisture deeper over the east, seeing the showers and storms ride up the NSW ranges into southern QLD. The showers over the west also quite widespread in the early part of this period, which takes in the next 3 days and then a larger scale system rolling through later next week. Rain also breaks out over SA with moisture surging south through the state being lifted by that larger system coming out of the west at the end of the month. The north not as humid as CMC or GFS but still has showers developing for the tropics.

12z ACCESS Rainfall for the next 10 days.

Much more conservative compared to above, but does have more moisture being lifted over inland areas of southern and eastern Australia with the system coming in from the west early next week and then rolling up the NSW coast. Deeper moisture over the eastern inland of QLD could be lifted into isolated showers and thunderstorms, with that also being replicated over the northern tropics Dry over the northwest of the nation with above average temperatures, this will play a part into providing more dynamic weather in September. This model does not support the idea of a secondary feature rolling out of the west of the nation later in the outlook hence why there is not the coverage of rainfall as per the KMA above.

12z CMC Ensemble Rainfall for the next 15 days.

The nation is coloured in fairly extensively in the broader data sets on this modelling. CMC has been running fairly wet in recent weeks but hasn't quite verified so lets see how this pans out in the coming week or so, but I am suggesting confidence is low. It did perform well with the winter westerly belt of June and July. Now the tropics are transitioning clearly at the moment, models don't handle this period well. Rainfall over the east coast is in association with high pressure moving into the Tasman Sea and this easterly wind regime helping to bring moisture and shower activity back to the eastern inland. The cold fronts are still expected to play a part in bringing rainfall through southern coastal areas for southwest WA, southern and southeast SA, VIC and southern NSW too.

12z Euro Ensemble Rainfall for the next 15 days.

This model is much more conservative and I think closest to the post in terms of where rainfall coverage. Drier air over the northern inland of the nation which is seasonal will see temperatures rise dramatically and that will play a part in rainfall increasing through the nation during September. The eastern inland is expected to see rainfall chances return thanks to easterly winds developing as high pressure move into areas near NZ. The cold fronts will continue to move through southern parts of the nation but they are not as potent as other models, hence why rainfall coverage is less extensive and intense. It is important to contrast and compare the data, this is the driest look across the board and it is safest to lean with this based on current observations.

Rainfall for the coming 2 weeks.

Rainfall looks to be bias along the eastern seaboard on current data, with that moisture creeping west through QLD and into NSW. Showers and the odd rumble of thunder passing over southern Australia with frontal weather next week with another set of systems expected to follow for the end of the month. There are a range of solutions outlined above, so at this time I have drawn the middle ground of all the data. The driest parts of the nation look to be the central and northwestern interior, as it is should be over the desert country. This area will warm up dramatically if this verifies leading to a dynamic atmosphere as we track into September.

I will more on the medium term data in the evening updates after 5pm.

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