MODELS AND RAINFALL - THE JOYS OF FORECASTING NARROW RAIN BANDS.

The short term rainfall is very tricky to forecast through southern WA and into SA with a narrow band of rainfall expected to develop as a low deepens on a trough Friday into Saturday. You will notice on some of the model data tonight that the band is super thin - only 20-30km wide on some data sets, meaning a location under the core of the rain band could see 50mm while down the road will see 5mm. This is a likely outcome for SA and parts of western VIC. So be aware of that - your forecast confidence in this region is low in terms of totals and high in terms of rainfall developing.


Now for the east and southeast your rainfall totals are getting much clearer with the models converging in the idea of heavy rainfall of up to 1 month's value over southern parts of NSW, maybe expanding to central areas, but more likely south. Heavy rainfall over central and eastern VIC as well. The ACT could record enough rain to get very close to equalling or breaking the record rainfall for November.


More rainfall is possible next week with a secondary system but that feature looks to be moving further north and favouring the southern NT, QLD and NSW as well as the ACT.


The tropics are becoming more active with strong storms and heavy falls redeveloping. Watch that tropical mischief offshore WA next week, as that could become the first named storm of the season. Another tropical low within a broad open tropical wave is expected to approach the QLD coast towards the end of the run.


Finally, the east coast needs to watch the forecast trends in the next week as we finish the month and kick off December with a positive SAM developing which may increase rainfall totals via easterly winds. The above average SSTs have already provided falls over 100mm from showers in northern QLD. A sign of what is to come.


The risk of further flooding as mentioned earlier is now expected over southeast inland areas of the nation with this lead system.

Lets take a look at the data sets


SHORT TERM

The short term is clearly dominated by the low coming through southern WA through SA then into VIC and NSW over the course of Friday to Sunday with moderate to heavy falls about. The weather is expected to clear later Sunday with only residual showers over the eastern seaboard early next week. Another low may redevelop over the interior parts from this time next week, though modelling is VERY POOR on this feature with divergence continuing.


UKMET - Rainfall Next 7 Days - LOW CHANCE

The rainfall spread is moving back to the major model guidance but still carries a low confidence. Watch the moisture plume offshore WA, that tropical mischief needs watching in the coming week as it feeds moisture southeast.

MEDIUM TERM

The potential follow up system has a higher chance of bringing rainfall into NSW and QLD as well as interior parts from mid next week and then continuing into the end of the month. The models are doing strange things quite frankly. Looking at the climate drivers, such as the SAM, its more likely to go positive than not, but some of the models suggest a large scale high will adopt a more northern location in the Bight drying out the nation. I do not buy that, with the positive SAM, I expect that high to be further south allowing easterly winds to return.


ICON - Rainfall Next 8 Days - MODERATE CHANCE

The spread of rainfall has come back into line with the major global models, and like the UKMET watch the tropical moisture off WA next week to see if that feeds moisture across the country. The rainfall over southern and eastern areas would bring up to 1 months worth of rainfall.

CMC - Rainfall Next 10 Days - MODERATE CHANCE

Rainfall over the east is heavier than the above and the rain band thinner over the south. Note the moisture over the tropical north spreading south towards Australia increasing rainfall and the moisture profile. The regular rainfall chances over the north increasing which will increase rainfall chances for the south and east.

EURO - Rainfall Next 10 Days - HIGH CHANCE

Again note that thin rain band through the south of the nation connected to the tropical mischief offshore WA, keep watch of that feature. The rain band will frustrate a few in SA and may bypass the southeast districts and southwest VIC. The rainfall has increased over the southeast of NSW and southern inland of NSW with this feature. The east coast could see a storm outbreak next week with the secondary trough. The tropics turning active.

ACCESS - Rainfall Next 10 Days - MODERATE CHANCE

This is an interesting run, with the model suggesting the same as the others with widespread rainfall passing over southern and southeast Australia. Then the secondary system develops and deepens into an inland low over NSW tapping into deep moisture and bringing heavy rainfall. Now while it is out on it's own in terms of this scenario, other models have shown this at one time or other so keep watch on trends. This would be a major severe flood event for inland NSW and QLD maybe VIC if it eventuated.

KMA - Rainfall Next 12 Days - LOW CHANCE

Similar to last night as is more aligned with GFS in having a large scale come in during later this month and sits further north bringing only one rainfall event through the remainder of the month. I do not buy that solution. However it probably renders the northern and western parts of the nation well and the storm potential over QLD - but it is still a low chance.

GFS - Rainfall Next 16 Days - LOW CHANCE

Too dry - but again as all models have, keep an eye on the tropical moisture offshore WA which may be the first named system of the year.

ENSEMBLE DATA

The broader data sets are consistent in their approach, the further east and north you are the wetter you will be, that has been the theme that has played out so far this Spring and will likely end the season. The models are suggesting some heavier rainfall may develop to kick off December, but CMC sees this where the others do not.


GFS - Rainfall Next 16 Days (median of 35 members) - MODERATE CHANCE

Still too dry over the interior and north but has the east about right.

CMC - Rainfall Next 15 Days (median of 30 members) - HIGH CHANCE

I am still siding with this solution however it has dried a little in recent runs for the interior but rainfall numbers are coming up over the east and north.

EURO - Rainfall Next 15 Days (median of 51 members) - MODERATE CHANCE

Too dry across the interior. But has the east right and the north is probably a tad too dry. The west is right on all three models.

LONG TERM

Wet....


CFS - Rainfall Next 6 Weeks - MODERATE CHANCE

Maxing out the rainfall potential over QLD. That could be replicated anywhere over the east coast, the numbers over the inland represent more warmer and convective rainfall potential. Note the moisture coming south from the tropics.

More coming up tomorrow from 7am EDT.


Note - I have to attend a funeral tomorrow afternoon so there will be a modified update during the evening but I hope to produce this update again tomorrow night still.