MEDIUM TERM FORECAST UPDATE

There are more interesting signals from the models today, the weather very hard to pin down for the mid August period through to the end of the month, BUT, there will likely be widespread rainfall over parts of Australia. WHO gets what is yet to be determined.


This is all part of the beginnings/rumblings of a seasonal shift, very common to see this in August. August and September are traditionally where we see the model accuracy at its lowest, this persists into October before accuracy increases again over summer and autumn.


So expect the medium term model data to be quite erratic and volatile through the next 8-12 weeks.


Rainfall forecast next 14 days

A dry week for the most part across the inland is expected to persist through to mid next week with the air stable with the ridge moving through further south than previous six weeks. The pattern flip is underway this week and will continue through next week, so not much wet weather is expected over inland areas. The wet weather as outlined in the AM forecast packages is coastal for the coming week. Then there are rumblings, as you can see below, of inland rainfall developing. Now the higher chance of that inland rainfall developing is over NW WA, but there is also a risk of rainfall developing over the eastern inland of QLD and NSW however the risk is lower at this time. The tropics also becoming soupy with the chance of morning showers about coastal areas and a few showers over inland areas possible during the afternoon and evening later next week.

Temperature anomalies next 14 days

No real change in the guidance with a near certainty following the dry surge this week over the northern parts of the nation, the temperatures will largely be above average for the northern tropics, that warmer air then spreading south and east through QLD and the NT. After the colder weather this week over southern and eastern areas, the weather slowly warms up in a northwest flow that develops from Sunday and persists next week as frontal weather collapses through the Bight. That will assist in dragging the warmer weather from northwest to the southeast. With the frontal weather continuing to impact the southwest of the nation, after the warmth of the coming days, cooler than normal weather will develop from the weekend and throughout next week.

Rainfall anomalies next 14 days

Rainfall anomalies reflect that pattern flip with a slight bias for above average falls along the QLD coast as what is normal for this time of year and that spread of moisture coming through the northwest of the nation and coming southeast through the NT, SA and the southeast inland. The rainfall slightly above average over the SWLD but again this is against another wet month seasonally for the southwest of the nation. Seasonal expectations are expected elsewhere.

PATTERN FLIP.

I posted this the other day, but a pattern flip is where the pattern literally flips. In this case, high pressure that was over the northern half, relocates to the southern half. The moisture that was over the southern half of the nation, relocates to the northern half of the nation.


Where we were on Sunday 1st of August

Westerly wind belt starting to move south and relax with a trough moving through SA. Ridging building over the mainland.

The pattern flip in about 7 days on Thursday 12th of August, high pressure is now over the southern states, the westerly wind belt is well south now with minimal impact away from SWLD of WA. And moisture starting to build up north.




THE DATA SETS


18z GFS - Surface Pressure Pattern next 16 days

GFS continues to show the pattern drying out and the pattern flipping through the course of this week. The high pressure has replaced the frontal barrage of the past 6 weeks for southern Australia, the impact already clearly seen on the maps today. The weather will remain fairly stable over most of the inland of the nation as dry air continues to hold firm for the coming week. Coastal areas of NSW and QLD will see showers redevelop in the easterly winds that return next week which for some communities will be the first rainfall in many weeks. The high pressure in the medium term looks to strengthen over the southeast of the nation then move into the Tasman. This will allow for moisture to build through the northern tropics via northeast winds, over the eastern inland via easterly winds and over northwest Australia, with frontal weather allowed to move closer to the WA region once again, but acting as a pulling mechanism to drag in the jet stream which at this stage, be carrying moisture from the Indian Ocean into the western inland. That is what is a higher chance of bringing the inland rainfall mid month onwards.

18z GFS - Precipitable Water Anomalies next 16 days

As per the pressure pattern, you can see the moisture replenishing across the nation during the coming 2 weeks, the dry air eventually is overrun by the moisture that builds via onshore winds and the lack of dry cold surges moving over the nation. This is critical to seeing the full impact of not only the onshore winds but the full impact of the negative IOD impacts. The IOD is still strengthening and this is yet to be fully realised. We will see that over the coming months but hints of what is to come in the next fortnight.

18z GFS - Rainfall next 16 days

Rainfall over the coming 2 weeks indicates that moisture building and being drawn south and then southeast, with lifting mechanisms such as frontal weather and troughs coming through WA the higher chance of seeing rainfall develop. The moisture over the eastern inland and northern inland of the nation also needs watching with some models showing showers over the eastern inland creeping further west from mid month, if this does occur, then that moisture will also be lifted by frontal weather or troughs running in from SA and VIC to produce more widespread rainfall for the eastern states. For the Ag areas of SA and VIC, it is a seasonal outlook, but this could change dramatically in coming days with that moisture starting to be lifted by a stronger system later next week, hence why you see the heavier falls moving east. The Euro as you can see below has also got rainfall moving southeast.

12z Euro M - Precipitable Water Values next 15 days

Similar to the GFS, the Euro shows that drier weather being overrun by the moisture building up over the course of the next week, so a benign week to come through the nation's interior. The pattern shift can be seen with the moisture coming in via the easterly winds over QLD and NSW, moisture pooling over the northern parts of the nation via northeast winds and moisture increasing from the northwest via the jet stream. So there are multiple ports of moisture to work with, we just need the lifting mechanism in place to see rainfall become widespread. There is a greater than 50% chance of rainfall becoming widespread over the course of mid month onwards.

12z Euro M - Rainfall next 15 days

The rainfall from the ensemble data set (50 members) is in support of that process of moisture being lifted from mid month onwards over inland areas occurring, then that rainfall moving south and east. There are some members that have absolute whopping rainfall events, but there are some that continue the lighter rainfall events coming. The nation needs to warm more to hold more moisture and then to produce heavier rainfall events through interior and southern inland parts. So I think the below is a fair representation of what the medium term offers. The end of the month is looking active. I will have more on that coming up later in the weekend.

12z Euro C - Rainfall next 15 days

The control member is one example showing that moisture streaming from northwest to southeast and you can see the higher resolution planting a months worth of rainfall for many areas from mid month onwards, particularly from around 9-10 days from now. That is a reasonable expectation given the current short term forecasting. The wet weather then progressing over the eastern inland after this period. But note the IOD impacts finally starting to appear over Indonesia and northwest of Broome. That is the area to watch. That impulse and in feed of moisture is more reminiscent of what you would see during October and November, especially from that direction.

12z CMC M - Rainfall for next 15 days

This model has been picking the mid month transition from dry to wet over inland areas for weeks now and seems to be on the ball, like Euro suggesting widespread falls developing from mid month. The weather expected to get very wet if this is to verify for the last 10 days of August for many areas, with many areas seeing above average rainfall on this particular data set.

12z KMA - Rainfall for next 12 days

The shorter term KMA model showing that wave of moisture slowly being lifted in the next 10 days and then starting to lift eastwards across the nation for the second half of the month. This is one example of the deterministic data expressing that moisture is increasing and it will likely bring rainfall back to the inland areas out west and north before spreading east. The lower chance of inland rainfall for the eastern parts of QLD and NSW still exists, but it is low.

12z CMC M - Temperature Anomalies for next 15 days

The CMC is showing a much cooler spread of temperatures over the coming 10 days for the nation, however this is also suggesting that the rainfall will increase as early as mid to late next week. That may be a little too quick, but will watch trends. Overall, the temperature anomalies expressed below are reasonable for the weather that develops in September during in which the IOD will be building further. So I don't think this is correct for this period, but a hint of what is to come for spring.

12z Euro M - Temperature Anomalies for next 15 days

This is probably closer to the pin, noting that the warm signal has been lost over inland WA with the cloudy skies taking over snuffing the heat out a bit. QLD and NSW as well as VIC and eastern SA looks milder for the coming week ahead, once we lose the cooler southwest to westerly flow. The tropics are looking warm as well with humidity building next week.

SAM Forecast - Next 2 weeks.

Finally the SAM is expected to remain neutral though the next fortnight, however there is a bias for it to turn positive during mid to late month, and if that occurs then the east will see an increased chance of rainfall and moisture will increase dramatically for eastern inland areas of the nation and humidity will rise exponentially over the northern tropics. It will also allow for the moisture to pool over northern and western parts of the nation with a lack of drier southerly surges. BUT this climate driver is quite tricky to pin down well ahead of time.

SAM Forecast - Next 2 weeks.

I will have a look at the medium term again on Sunday, let the models settle a little more over the coming days. Climate Drivers Update comes on Friday with a look at how that La Nina talk is stacking up against the data sets.
















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