NATIONAL WEATHER PICTURE WITH A LOOK INTO THE MEDIUM TERM - MONDAY 25TH OF JULY 2021

Headlines


  • Frontal weather is on by the east but more windy and wet weather is expected throughout the week.

  • Wet weather focus shifts to the southwest of the nation with a strong cold front and decent moisture supply combining to bring a wet day.

  • Hot and dry over the north with very high fire dangers over the NT.

  • Lets look at the potential for that inland rainfall later this week.

National Satellite Picture - Monday 25th of July 2021

Can see the more wavy flow over the southern parts of the nation as the westerly wind belt becomes negative this week. The long wave that forms over the Australian region will peak over WA this week. The systems will race across the southern states and then weaken somewhat. The long wave will likely then begin to weaken as it approaches the east, but windy showery weather is still expected for much of the south this week. Note the inactive area of upper level moisture drifting through QLD today as high cloud. We just need the trigger to lift that into rainfall, that is still a few weeks away from happening. And the large wave of moisture moving west north of the NT will park itself northwest of the nation later this week, that will be part of the weather to watch next week as it starts to filter into the nation via the jetstream

Rainfall for the next 10 days - Monday 25th of July 2021

Rainfall for the coming 10 days is largely unchanged with the westerly flow dominating the period, but the rainfall totals will start to come down for some parts of the coast line as the winds veer more northwesterly instead of the persistent westerly barrage. The wettest part of the nation for the coming 7-10 days will be the southwest of WA and western TAS. Moderate falls for the southwest of VIC and southeast SA also looks of higher confidence as well as moderate falls for the VIC and NSW Alpine areas where the snow will continue.

Rainfall for the next 16 days - Monday 25th of July 2021

I am hedging my bets for that westerly wind belt to continue to impact TAS and western parts of WA through the coming fortnight, but I do feel that moisture will be introduced into the mid lattitude westerly belt bringing up rainfall chances for inland areas into mid month. Nothing overly heavy for the coming 2 weeks but the rainfall should creep back inland from the second week of August. The rainfall into the second half of August looks much more pronounced for inland areas and as we move into September through December, this is where we will see a lot more colour over the inland on these charts.

Medium Term Rainfall Outlook


This is to supplement the update yesterday for the coming 2 weeks. The main drive that is in control aside from the negative IOD is the SAM. I have just had a look at the latest members and updates and there is now a shift towards a second negative surge into the first week of August, which is probably why the medium term modelling is keeping the frontal weather in place for yet another week next week. I am NOT totally convinced of this but again the forecasts will be adjusted to factor this in if the trend for the negative trend continues. Bottom line, the weather may remain unchanged this week and all of next week if that verifies, but pinning down the SAM is very challenging.


18Z GFS Rainfall for the next 15 days - Monday 25th of July 2021

I am using the ensemble for this update to show the impact of the model divergence. The GFS updated this morning for the coming 15 days shows the impact of the SAM data being fed back into the updated modelling. As you can see the drier signal persists on the GFS because it sees the frontal weather not totally leaving over the coming 2 weeks. The model has limitations in the medium term and it doesn't perform as well as the Euro, however it did pick the long duration westerly wind event we have experienced in the past 4 weeks. So you have to factor that into the forecasts into the medium term, perhaps there is some merit to the westerly wind belt remaining within reach to keep the forecast largely unchanged over the coming 2 weeks. I am not leaning towards this solution, however you cannot rule it out given the new data this morning.

12Z Euro Rainfall for the next 10 days - Monday 25th of July 2021

The Euro is leaning to the SAM returning to neutral and staying there from early next week, still has frontal weather but the impact on rainfall is reduced over southern areas from early next week and starts to increase over northern areas of the nation, that in part due to the moisture increasing in the northwest and north of the nation as the jet stream begins to shift from it's current lower latitude location and return to the northern areas, helping to filter moisture through the northern half of the nation. This lends itself to moisture being better utilised ahead of troughs and fronts, as it dives more north to the south, rather than west to east. The current configuration of the moisture running west to east which brings good rainfall to southern areas is good for the southern states. This configuration however does not bring the widespread long duration rainfall through the eastern inland that we see in the later August and into Spring. So that is something to watch over the next 10 days if that can verify. The Euro shows some decent falls developing in the medium to long term from mid August and that has been a very strong signal for the past 2 months.

12Z CMC Rainfall for the next 10 days - Monday 25th of July 2021

CMC is clearly the most aggressive with the moisture moving south and east through the nation ahead of the weakening westerly wind belt, but the moisture becomes stuck through the inland areas with the jet stream beginning to become more wavy. This then lifts the rainfall chances through most of the nation as we head into mid month. Now while CMC performs not so great in the medium term, this is an example of the SAM tending neutral and turning positive.


So you can see the three phases playing out in the medium term across the global modelling, which also expresses uncertainty in the medium term, so all forecasts will carry low confidence beyond 7 days due to this.


One element that the models do agree on is the warmer than average temperatures being drawn throughout most of the country. We are feeling that shift today with the winds adopting a more northwesterly flow, and all of a sudden we have lost the severe wind chill within 24hrs and many areas in the mid to high teens, 2 days ago we could not even escape single figures.


The one part of the nation expecting below average temperatures will be the southwest of the nation, thanks in part to this week and into the weekend with the day time maximums beginning to move into the mid teens.


So expect more variability in temperatures moving forward.


12Z Euro Temperature Anomalies for Friday July 30th through August 9th 2021. - Monday 25th of July 2021

Elevated temperatures over the north and east will drift further south during this period and that is in line with the westerly wind belt starting to shift south and it's orientation is changing through the coming weeks, this will see the temperatures become more variable over this period and a taste of early spring in the air (nature has been signaling that of late).

18Z GFS Temperature Anomalies for Friday July 30th through August 9th 2021. - Monday 25th of July 2021

The GFS model has relaxed the warmer than average signal in its recent update and that is to signal the potential for that SAM tending negative again through the period meaning another period of colder weather is possible under that scenario if it verifies. We will have to keep watch.

2 Week Temperature Outlook

No change from this over the coming days, we can already see the synoptic pattern supporting this outlook this week and indeed on the charts for next week.

And the SAM Forecast for the coming 2 weeks. Note the members turn positive briefly but then surge back negative. If this continues to be a trend, this will again reinforce the westerly wind belt and colder and showery weather once again for those areas who have been wet these past 3-4 weeks.

More weather details to come.



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