MEDIUM TERM MOISTURE WATCH

The moisture is still wafting around in the medium term, heralding a likely shift to wet conditions to finish the month of August, likely heading into a wet spring.


Moisture Watch Dashboard - August 12-19th 2021.

This a quick overview of the areas to watch. This will be updated again on Sunday.


This will be the focus for those on the land who are looking for follow up rainfall and or even just some rainfall, especially for those in QLD.


Lets take a look at the latest information


The modelling is volatile in the medium term. Unlike the westerly wind domination that was easily picked by the longer term modelling back in June for the month of July, this period weather is very trick to bed down ahead of time and this will create a challenge when it comes to forecasting who gets exactly what from mid month.


But there is a lot of moisture wafting about, so it is only a matter of time in the medium term that the rainfall will be back over inland parts of Australia.


The models will continue to struggle in the medium term trying to pin down the rainfall events. This will become a little clearer as we get to Sunday but more so from this time next week. For the short term, if you are living over inland areas in southern and eastern Australia, a chance to dry out for the most part is likely. Southwest WA will see the bulk of the rainfall over the coming week and the eastern seaboard.


Below is a great example of what we are dealing with from run to run.


PREVIOUS DATA FROM THIS MORNING

12Z GFS Precipitable Water Anomalies for the next 16 days

The PW values are increasing through the outlook, that carries high confidence, but it is the positioning of the lower pressure systems to lift the moisture that remains very hard to pin down from this far out. Note that the accuracy will improve during the next 5-7 days for this period.

12Z GFS Rainfall for the next 16 days

Moisture deepens over the western inland with a trough forming over the region lifting the moisture into areas of rainfall, that rainfall becoming more widespread as we track a cold front through the region. The rest of the nation becomes more humid and warmer as we go through the coming 2 weeks which will set the stage for more rain developing beyond this period. This was the rainfall guide this morning from GFS for the medium term. It has the rainfall over in the west of the nation with the moisture deepening near a trough. That would bring heavy inland rainfall to WA spreading into SA mid month under this solution

LATEST DATA

18Z GFS Precipitable Water Anomalies for the next 16 days

The latest data from the same model shows the moisture building up over the north of the nation overriding the drier airmass. That idea looks fairly good now. But now this model which had the moisture deepening over the west, deepens it over the east and lifts it via a trough with areas of rain developing for QLD. This keeps WA dry. So this expresses the uncertainty over the medium term still.

18Z GFS Rainfall for the next 16 days

This is the latest data on the rainfall which updated mid morning, it has taken away the rainfall for the west and now increasing rainfall for the eastern inland of QLD and NSW. This shows the lack of confidence in forecasting with any great accuracy about who gets the rainfall but the signal is the same, moisture increasing from mid month.

A better way to look at this is the larger data sets which I did a few days ago.


12Z GFS Ensemble (35 members) PW Anomalies for Thursday 19th of August

I have chosen the data as to where the models agree the moisture overrides the dry air and this seems to be the 2 weeks from today. Now in saying that, this does not mean it will be dry for the entire period before this point, but the more widespread rainfall is more likely to develop from August 16 onwards for parts of the nation and then spread through larger parts of the nation from that point on. The difference in the model impacts the output of rainfall coming in the medium term. The GFS is the driest out of all modes for the coming period. But still these values are impressive for August traditionally a dry time of year for the nation.

18Z GFS Ensemble (35 members) Rainfall for the next 16 days.

Again you can see the spread of rainfall through the inland which is a good sign of the pattern starting to break over the dry areas of inland Australia. This will not be as intense as I thought it would be back in late July, but the pattern flip is still there. This is the driest of all the global modelling products today.

12Z CMC Ensemble (30 members) PW Anomalies for Thursday 19th of August

The wettest out of the medium term global models. Note the shape of the moisture plume over the nation, this suggesting that a larger in feed from the northwest and north is likely, with an easterly flow also expected to develop, modelling supporting high pressure strengthening over the southeast states, pumping in a long fetch easterly wind and moisture therefore building over the tropics and the eastern inland too during this period.

12Z CMC Ensemble (30 members) Rainfall for the next 15 days.

The CMC still very bullish on inland rainfall with heavy falls for the APY Lands through to central and southern SA. Rainfall also widespread most everywhere else with an active tropics too. This continues to be the wettest of all the global modelling.

12Z Euro Ensemble (50 members) PW Anomalies for Thursday 19th of August

Similar shape in the moisture plume across the nation, once, again suggesting the majority of the moisture will be coming out of the Indian Ocean and spreading from northwest to southeast. The weather largely becoming more humid over the north of the nation and with the frontal weather moving in from the west, the fronts could pick up the moisture and bring more rainfall for WA. Though the more widespread falls are anticipated for Central Australia into the southern and eastern inland under such guidance at this time.

12Z Euro Ensemble (50 members) Rainfall for the next 15 days.

The Euro sits in between all the models, and I think is likely to be closest to the post over the coming 2 weeks. It has been consistent on the pattern flip coming through over the next 5 days with the high pressure replacing the westerly wind belt and also beating down the cold fronts that move through WA, further south of the nation as the high strengthens over in the east. That high likely to bring moisture back to the east and we can see better falls for somewhat thirsty parts of the east coast. The Euro has more moisture for the eastern inland during this period.

During periods where the pattern is flipping from one phase to another, the climate drivers are in transition to a new phase as well, the shorter term modelling is always going to struggle.


The takeaway?


  • Dry for much of the inland for the coming 7-10 days, warming up and becoming humid over the northern and eastern inland.

  • Moisture may surge through the Indian Ocean but then be driven south of the nation as a high pressure ridge slowly moves over the southeast states.

  • Easterly winds return through the outlook period with the weather turning showery for the east coast, especially north of Newcastle to Cape York.

  • A taste of an early build up over the Top End of the NT and Cape York, with that humidity spreading west through the Kimberly.

  • The Indian Ocean likely to provide the better opportunity for rainfall in the medium term for SA, VIC and western NSW.

  • The easterly winds providing a better opportunity for inland rainfall for QLD and northern and eastern NSW. We may see that impact with a weakening front mid next week.

Tomorrow I will have a look at the Climate Drivers with more details on their influence through August and another look at the La Nina chances as we get more data overnight from the global climate agencies.


This forecast will be updated again on Sunday.




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