MEDIUM TERM FORECAST UPDATE - THE HEAT IS ON AND FOLLOW RAINFALL TOO.

Lets look at the event next week spreading from the west through to the east and a look beyond that system as of course many tongues have been wagging about major rainfall into mid September....


Rainfall expectations for the coming 2 weeks

Temperature Anomalies for the coming 2 weeks

RAIN EVENTS TO WATCH - August 30 through September 12 2021.


  1. QLD - There is a weak trough expected to be fed by onshore winds and deeper moisture leading to showers increasing from the weekend into early next week. Models struggling to place how far inland the trough will be of the coast, which will ultimately determine the spread of rainfall away from the coast, but there will be some. This could be the first modest thunderstorm outbreak for the subtropics.

  2. WA - A strong cold front is expected to rise through the region next week bringing a band of rain and much colder shift, 10-20mm possible for the SWLD, could be more if the system slows down. Then cooler with showers for the remainder of the outlook.

  3. SA through VIC and NSW - That strong cold front approaches next week and a low is likely to form along the front. The question is, does the low come east, or does it dive south? We have seen a different look each run from the models. So that will be the key to forecasting rainfall later next week into the weekend.

  4. SA and NSW - A secondary follow up rain band is likely to form along the stalled trough into the second week of September over inland parts with the bias throughout SA and NSW at this time. It may extend into southern QLD as well.

  5. SA, NSW and VIC - A third shot of moisture coming through the jet stream could link up with another front during the period September 10-13th with the stalled boundary on the charts remaining there from last night. This is a signal which we will review again on Monday.

TEMPERATURES


The temperatures are expected to deviate from the cooler than normal weather over the southeast to becoming much warmer than normal from next week and clearly the hottest weather since March is possible for many areas of SA, through northern VIC into western NSW and QLD. Many locations will rocket above 30C.


The west by contrast following the frontal passage next Wednesday, will be likely cooler than normal for a long period into the second week of September. The signal continues to strengthen for a large temperature battle to play out for a number of days over central and eastern Australia which will become the focus of instability, cloud bands developing and areas of rainfall.


The longer this battle plays out, the better chance that temperatures will eventually moderate for the second half of September.


LETS LOOK AT THE DATA UNDERPINNING THE FORECASTS


GFS 18Z - Surface Pressure Pattern and Rainfall for the coming 2 weeks.

You can see the battle zone developing between airmasses, with less frontal weather appearing on the charts and more low pressure systems forming in response. That increases the chances of larger rainfall events developing throughout the nation which are more productive (higher temperatures is able to hold more moisture = more rainfall develops). The bias of rainfall chances shifts from the west to the east during the coming week and then stays there. I have outlined above the different rainfall event chances throughout the period and you can see them firing below on the GFS.

GFS 18Z - Surface Pressure Pattern and Rainfall for the coming 2 weeks.

The moisture that races through from the northeast and east of the nation during the coming week is critical to how the rainfall opportunity develops next week for central, southern and then the eastern inland of the nation. How far west that moisture is propelled into the front is critical. If the front is slower (which models are seeming to agree on), then a larger rainfall event will form over the nation. The moisture is deepest over central and eastern Australia, with some of the rainfall charts further below picking up on this and spreading larger rainfall totals through the outback and then spreading it on east. Note the moisture then continues to surge in while the event is underway, that seeds the possible events that follow the lead system later next week for the second week of September so again more on that next week. For now there is no doubt the drier air will likely be overrun by the moisture throughout next week, which brings the humidity levels up for the tropics, and that then infiltrating the well above average temperatures throughout the east and central parts of the nation, giving us a preview of the spring ahead, warm, humid more rainfall and scattered storms about.

GFS 18Z - Rainfall deterministic output 2 weeks.

The confidence in the rainfall in the medium term is low confidence, but I share this to show the movement of the moisture across the nation and where the low pressure meets it to lift rainfall across the inland. You can see the major system moving through next week, fed by what happens over northeast QLD with that weak trough and storm outbreak. Then the system stalls out next weekend and it appears in this data set, 2 more rounds of rainfall are possible along the boundary from the southern NT, through SA and into NSW and QLD. This will move around more and more as we go through the coming days, but the SIGNAL remains for a shift towards more spring like rainfall, less frontal weather, more warmer days into the outlook period, which means more productive rainfall events when they pass through. If you are living out over the west, you may be sitting in colder periods with showers from time to time over the southwest, but up north the humidity will begin to increase so build up weather may be knocking on the door. That humidity over the north may also play out well for QLD and northern NSW into the back half of September.

OTHER RAINFALL DATA FROM MODELS - RAINFALL IS THE MOST SOUGHT AFTER DATA SO HERE IT IS WITH MY ANALYSIS.


KMA Rainfall Next 12 days

Again note the spread through the inland of SA back through the NT and then spreading through eastern Australia, with the front and trough running into the deeper moisture sweeping in from the east and northeast. This is a reasonable outcome. Clearly the better rainfall over the southeast inland, though the heavy falls you see over southeast NSW is with rainfall today, west of the divide through the southeast is for later next week onwards. Also note the moisture building up over the northern tropics and heavier rainfall for FNQ. Less rainfall being picked for southwest parts of the nation with the bias shifting east.

Access Rainfall Next 10 days

Again note the sharp increase of rainfall over parts of southern NT and throughout SA with the moisture surging into the front a little further east of where the KMA has it so once again, timing is everything with the front and where that interaction takes place. The rainfall then spreads east into VIC and NSW after this period, so the model suggesting a slower progression of the front. Moisture increases over northern Australia and some thunderstorms break out over NSW ahead of the major rainfall later next week.

CMC Rainfall Next 10 days

The CMC moves the system through quicker, so it captures the moisture further east, and you can see it exploding along the tristate border region back through the NT and into southeast SA and western VIC. The system being further east in phase with the moisture, the rainfall will bypass the EP and western parts of SA under such guidance. Otherwise fairly good agreement between all remaining models with the west much drier, the north looking humid and becoming unsettled and the storm outbreak over the east of QLD early next week looking productive closer to the coast.

LOOKING AT THE LARGER DATA SETS


Euro C Rainfall Next 15 days

Note the control member of the Euro is wetter for QLD with the storm outbreak and trough early this week, so will have to watch trends in the data in the coming days. Clearly the more widespread the rainfall there, the more moisture eventually spills through QLD into Central Australia as the high redevelops over NSW. The rainfall event not as widespread next week as indicated by the deterministic data, but I would be suggesting larger rainfall is possible for SA, VIC and NSW under such guidance above. Note the moisture refires over central Australia after the first rain band moves through, so that idea is gathering pace for further follow up rainfall along the trough during the second week of September.

GFS M Rainfall Next 16 days

Similar shape in the rainfall spread coming up over the next 2 weeks with the coverage broader, but the heavier rainfall is possible over eastern QLD, with onshore winds, a rainfall event running through the south of the nation, clearly bringing the heaviest falls to southern inland parts and the southeast inland of the nation, but rainfall may extend deeper inland with lighter falls for now, but watch this space, with moisture hanging back and troughs in place, further rainfall events may emerge beyond later next week for the second week of September, with the bias over the eastern side of the nation.

CMC M Rainfall Next 15 days

Similar spread to other global ensemble data with the spread broad, heaviest falls over QLD and through the southeast inland and southern inland of the nation, but note the rainfall extends through most of the inland so that suggests to me, more potential for follow up rainfall is on the cards for the second week of September.

Euro M Rainfall Next 15 days

The Euro is much the same as above, but has a higher resolution and is picking that drier pocket over the NW of Australia likely to continue - this will be factored into the September update next week if the trend in modelling continues.

TEMPERATURES

There is a sharp rise in temperatures next week and this is expected to persist for the east throughout the coming 2 weeks. So despite the cooler and cloudy weather about the south and east today, with below average temperatures, this will shift markedly after a changeable weekend.


Temperatures this time next week could be up to 20C different in some locations so make preparations for the warmer weather and the prospect of those areas that have seen less rainfall in recent times, to dry out rapidly.


Out west it is a different story, but at least consistent to forecast, a run of below average temperatures set to continue from mid next week when the front passes through and then a period of onshore westerly winds with weaker fronts rolling with high pressure out west, continuing the cooler trend.


The north of the nation, hot and sultry.


GFS M Temperature Anomalies August 31-September 11 2021

Euro M Temperature Anomalies August 31-September 10 2021

Euro C Temperature Anomalies August 31-September 10 2021

I will have another 2 week forecast update coming through on Sunday afternoon.

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