LETS LOOK TO THE MEDIUM TERM

The forecast has been incredibly tricky for the medium term, mixed signals, inconsistent modelling, not unexpected, all hallmarks of the seasonal shift into spring.


August through October/November can be notoriously tricky when it comes to forecasting anything beyond 5 days and we are seeing that come to pass.


Lets have a look at the latest data sets and try and curate a forecast for the nation.


Rainfall for the next 5 days.

The modelling supports now an increase in rainfall for the system coming through on Sunday through Monday over VIC and NSW in particular, some of this now creeping into southern QLD now with the guidance suggesting moisture hangs back over SA and that is drawn into the front that passes in Sunday through Monday through the southeast states. The bulk of the rainfall looks to peak further east SA this time around. The west will see the wettest weather today through Saturday morning. Showers at times along the FNQ coast.

18Z German Model - Rainfall for the next 5 days.

Can see that moisture that hangs back over SA through the weekend gets lifted by the stronger front passing through the southeast inland west of the divide during Sunday night through Monday. That looks to be the wettest system for the southeast and east during the coming week at this time. Some of those falls could be moderate to heavy in scattered pockets through VIC and along the GDR in NSW. The rainfall peaking east of SA seems to be a more plausible outcome at this time, though light rainfall expected for the region.

12Z UKMet Model - Rainfall for the next 5 days.

Much less resolution in the model, shows more moisture of use over QLD through this period, we will track that trend in the coming 2 days. Showers more extensive over the southern parts of SA and less rainfall than what is being advertised over the eastern inland of NSW and northeast VIC. But the consensus is for more rainfall developing with the secondary system bringing the colder airmass from the south later Sunday into Monday. The west will see the bulk of the rainfall during today through Saturday.

18Z GFS - Rainfall for the next 5 days.

To show you the more robust global models in the coming days extending that rainfall over southern QLD and showers increasing for the coast, but numbers are coming up over the eastern inland. Lets see how that trend holds. Also decent falls for southeast SA and southwest VIC in the onshore flow as well as western TAS.

Lets take it out further so now that you can see how much rainfall has already fallen in the modelling in the coming days. It sometimes gets confusing to see what falls when.


SYNOPSIS


A drier period is expected to unfold from mid next week through most of the nation as a large high pressure system passes through. Now that cold air may be trapped for a number of days, leading to below average temperatures but also increasing the risk of frost, possibly severe, through eastern SA, VIC, NSW and southern QLD (dependent on wind flow). That would be risk to the more vulnerable crops that are getting up there now with new growth, promoted by this warmer spell of weather.


The weather then will largely stay dry through the end of next week for the inland with seasonal weather returning.


Along the east coast, there is some model conjecture with some of the global modelling showing an east coast low forming on the front next week over VIC and NSW (hence the rainfall increasing above), the development of that has shifted from northern NSW through to offshore southern NSW today. The moisture that is drawn south from the tropics appears to occur mainly offshore for now.


Showers are likely to develop for the east coast of NSW and perhaps parts of QLD, the spread and intensity of that shower activity determined by how close the low pressure sits to the coastal areas. Winds cold southerly shifting southeasterly mid week before easing as high pressure moves in. The low pressure system may waft out offshore for quite some time next week.


Out west the modelling shows a front passing through in about a week with a showery burst passing through after a warmer spell once again. The amount of moisture that gets involved in this feature is uncertain still with a higher amplitude pattern developing on the charts meaning that we see a more wavy flow appear. That lends itself to injecting a lot of dry air from the south behind the cold fronts whipping through the nation but moisture being drawn in ahead of the fronts is also occurring. That is spring weather!


With the higher amplitude pattern of stronger high pressure and stronger cold fronts moving through, larger temperature clashes are likely to unfold with moisture passing through this battle zone, that will lead to widespread rain and storms developing. Again the GFS shows this higher amplitude pattern developing in the medium term today.


GFS 18Z - Surface Pressure Pattern for the coming 16 days

The pressure pattern is unchanged for the coming 5 days however I will point out that the GFS in this latest data set is suggesting a low pressure system might form on the front over NSW during Monday which would increase rainfall throughout the state and extend that into QLD. Will keep an eye on that. The weather is largely drying out over the west for next week with a warmer trend and warm and humid over the tropics with showers to start the week, but dry air will see the dry season conditions return with hot day time temperatures. The next front approaches the southwest of the nation during late next week with a decent chance of moderate rainfall with that feature. That passes eastwards. But ahead of that a weak upper low may bring showers to the southeast states later next week. There are two frontal passages expected to impact the southern parts of the nation to end the month and another to kick off September with some chance of inland rainfall similar to what is developing Sunday, to also occur through this period. The east coast will see an uptick in showery weather in easterly winds over the north.

GFS 18Z - Precipitable Water Anomalies for the coming 16 days

The moisture in the short term is now coming into agreement across the board with moisture hanging back over inland SA and western NSW then another impulse coming through QLD into NSW ahead of the stronger system due in Sunday into Monday, that is why rainfall numbers are coming up. I will have more on that this evening, but into the medium term, and you can see the colder dry air that clears out the majority of the inland with a southerly wind passing over the eastern states. The west also turning dry with a high pressure ridge from the weekend through much of next week with drier air. But then we see the emergence of moisture over the west with a northwest flow aloft dragging in another impulse of moisture from the Indian Ocean. Like I said yesterday we need to see more activity from the Indian Ocean to verify rainfall forecasts across all global climate models, and it is a little quiet so will be watching. The moisture also increases over northern parts of the nation via those trade winds which will at times, interrupt the dry season over the NT and QLD. This will become more frequent in the coming weeks as we begin to transition. This run of the GFS is drier in the medium term as opposed to the model data earlier today.

GFS 18Z - Rainfall for the coming 16 days

I outlined above at the top that a good portion of rainfall comes in the next 5 days. So beyond that period you can track the low pressure system that moves slowly away from the NSW coast, with showers easing mid next week as winds ease. The slower the system moves away the longer the drier spell for the southern and eastern states with high pressure unable to move over and into the Tasman, opening the door for frontal weather coming in from WA later next week. So a few issues to iron out in the coming days on those parameters as models are not sold on how the low performs offshore and indeed where it forms. Into the last week of August and two fronts will likely impact the south of the nation and moisture may be drawn in on the fronts over inland areas, which could extend rainfall back through the southern and eastern inland, just like this feature passing through over the weekend into next week. There are more rainfall opportunities and one of these systems will bring all the ingredients together to develop a major rainfall event for parts of the nation.

Lets look at the other data sets from the global models surrounding rainfall and what they are saying, seeing as this is the most requested data sets from you out on the land.


CMC 12Z - Rainfall for the coming 10 days

The CMC is a little drier than GFS for the east with the system passing through tomorrow and indeed over Sunday and into early next week for NSW, but reasonable totals for VIC and southern SA. The system passes through quicker and doesn't quite collect that moisture over the inland, instead bringing that all together offshore the NSW coast with a low forming further offshore. Otherwise another front will bring a further burst of rainfall for southwest WA during this time next week.

KMA 12Z - Rainfall for the coming 12 days

Mixed odds with the KMA data this morning, showing moisture being lifted into areas of rainfall through the east with the system passing through later Sunday into next week. Showers over the southern parts of the nation quite moderate with the system Sunday into Monday with follow up falls coming in as early as next week. The model does develop a low closer to the NSW coast next week but moves it away quicker so again another solution on the table that we have to factor in. Tropics also a little more active early and later in the period than what other models have suggested.

ACCESS 12Z - Rainfall for the coming 10 days

Widespread rainfall being indicated by this model for the system that passes through the southeast Sunday through Monday with a low likely to form on the front as that passes. Note the rainfall has been increasing in recent runs over northern and eastern inland areas of NSW in particular. Reasonable rainfall over VIC and SA, with a secondary system passing through this time next week. While ACCESS develops a low offshore NSW it moves it off quicker, opening the door for that rainfall from WA this time next week passing over SA and then into the eastern states. Tropics quite active for late August with moisture pooling ahead of a southeasterly surge mid next week.

Rainfall for the next 16 days

So this includes the rainfall that was drawn in the top chart. Two rainfall events to watch in the medium term. The most likely chance of rainfall will be over in WA that spreads through SA and into VIC. Another system likely to form over the west spreading through inland areas in early September as well with moisture being drawn in from the north and west of the nation. I would like to see the Indian Ocean wake up a little more and see that dry and sinking air overrun as the area that was forecast to be wet, is currently forecast to have NO rainfall so that does concern me that some of the guidance has been tainted for the outlooks. That said will keep watching and adjust the monthly forecasts next week if this signal continues despite the IOD in negative phase.

Temperatures are pretty much looking bang on what was anticipated, with warmer than normal weather, intersected with cold outbreaks, but the cold outbreaks are now shorter duration in line with the seasons shifting. The cooler than average temperature bias is reserved for that SWLD region which will experience a number of cooler days over the next week before the weather warms. The north is looking hot with and without the humidity. Temperatures come down after a warm weekend through the east, but they will spike back above average next week ahead of another wave of low pressure arriving from WA and SA. Even though there are warmer days, the frost risk is still high at times during the next week and there will be grazier issues related to the cold blast next Monday and Tuesday. The risk is not as defined beyond this period for the remainder of the month.

I will have another look at the medium term tomorrow and maybe a sneaky small update tonight after 9pm.


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