MODELS AND ALL THINGS RAINFALL - THE EMERGENCE OF A MONSOON TROUGH PLAYING HAVOC WITH FORECASTS

And that is typical and happens every year at this time, I have been through this many times so buckle up and expect the forecasts to remain low confidence over the coming medium term. There is no skill in forecasting where or IF tropical lows/cyclones form, if the monsoon trough forms close enough to Australia to have any impact, but the trends are signaling that all of those features will be in play during the Christmas and New Year Period.


The system that carries the highest confidence relating to rainfall is the trough passing through the eastern inland over the weekend producing a band of rain and thunderstorms, some of that could be moderate to heavy in places over the NSW ranges and into the ACT.


Otherwise the weather turns more humid and unstable as we track through the middle of next week over northern and eastern Australia, but how quickly that comes south and how much of that rainfall comes south all remains to be seen.


Lets take a look


SHORT TERM

As mentioned the system that offers the most widespread rainfall across the nation is once again the trough passing through the eastern inland from Saturday into Sunday. There may be a few moderate to heavy falls about along the trough with thunderstorms training and running up the western face of the GDR. The other area of rainfall that is of significance will be the trough deepening over northern Australia leading to increasing showers and thunderstorms. Most elsewhere dry and stable.


UKMET - Rainfall Next 7 Days - HIGH CHANCE

Still remaining very much on track for the rain to return over the eastern inland of NSW and VIC during the weekend and then extending north into QLD early next week. The tropics turning active while the west and far south remains dry.

MEDIUM TERM

The tropical weather is expected to increase further through the Christmas period with heavy rainfall developing for coastal areas of northern Australia, a more narrow focus will come when the data becomes clearer on where the developing trough and low pressure ends up. The rest of the nation may remain drier than first indicated but will watch closely. For now the confidence is low and will remain low for a number of days.


ICON - Rainfall Next 8 Days - LOW CHANCE.

Still too dry but it is picking up on that moisture creep at the end of the run into troughs over the interior, born out of the heatwave conditions. Dry elsewhere.

CMC - Rainfall Next 10 Days - MODERATE CHANCE

Seeing the front and trough through the southeast this weekend with a few moderate to heavy falls, the tropical weather increases over the northern tropical regions and then the easterly winds return with a trough hanging on the coast with the chance of moderate falls. Patchy rainfall but mostly dry in remaining areas.

EURO - Rainfall Next 10 Days - MODERATE CHANCE

Similar to the CMC but amplifies the moisture content over the tropics and brings it further southwards at the end of the run into an inland trough. The east coast trough looks to be gathering pace later next week on the top of a high, moisture feeds into that trough in easterly winds and rainfall is expected to increase.

ACCESS - Rainfall Next 10 Days - MODERATE CHANCE

Has fallen back into line with the other models but is following the Euro in the moisture creeping further south more readily. The moisture over the east later next week into the weekend as we move through Christmas could lead to heavier rainfall and below average temperatures for eastern NSW and QLD. Rainfall over the southeast states is associated with the trough passing through this weekend.

KMA - Rainfall Next 10 Days

Also falling into line with the Euro and ACCESS now with more widespread rainfall over northern Australia and building the rainfall over eastern areas as winds turn easterly. The west and south relatively dry for the majority of this week.

GFS - Rainfall Next 16 Days - MODERATE CHANCE

From the driest model yesterday to the wettest over the north, as I mentioned keep watching this increase as we go along through the outlook period, and the spread of rainfall will come through the southern and eastern states as well as the model recognises the climate drivers support troughs deepening over the eastern states and high pressure falls south.


ENSEMBLE DATA

These data sets have been getting much wetter as we have travelled through the week, the GFS which was super dry has now flipped signaling the rainfall is set to increase. And looking at the charts, we may have to start factor the northwest flow over northern Australia which will increase rainfall throughout much of the nation.


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

Now coming into line with the other two models but it is the driest out of the three. This will adjust in the days ahead.

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

Still leaning with this solution and the Euro as we move forward.

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

As per the CMC this is supportive of a monsoonal burst coming through the north at the end of the year.

This is the single run of the above data and also shows that favorable move towards a monsoon up north, that flow dictating the rainfall for the remainder of the nation.

LONG TERM

The wet signal is very aggressive now for the coming 6 weeks, and a lot of this will likely come down during January.


GFS - Rainfall Next 6 Weeks - MODERATE CHANCE

The GFS tonight finally falling into line with the other climate models and seeing that monsoonal flow up north and rainfall from there will spill south and east, with SA perhaps seeing little impact under persistent ridging.

CFS - Rainfall Next 6 Weeks - MODERATE CHANCE

I am very concerned for the east of the nation with the guidance bringing in the monsoon, this coupled with the persistent easterly flow, positive SAM and La NIna, this is very reasonable. It does mean rainfall for the remainder of the south and west is limited however, under subsidence and drier air.

More coming up tomorrow including your 6 week outlook update and a lot more!!