MEDIUM TERM - WATCHING THE NORTHERN TROPICS AND MOISTURE FLOATING ABOUT, MORE FOLLOW UP RAIN?

There is a lot of information on next wave of rain passing through from WA through SA into the southeast over the weekend, with a pretty decent band of storms and rain likely for VIC, NSW and the ACT Friday into Saturday. But what happens after this event?


The models are struggling with a system that emerges in the Bight next week, with a strong thermal gradient likely to be attached to the feature, a sharp cold air field has been forecast to move north and cut off into a low pressure system in recent days, today the models keep the system in tact and the flow more zonal, meaning less rainfall now looks likely for large areas of the nation.


But how much faith do we put in that forecast? And what elements need watching through this period?


MOISTURE

The moisture load for the country is key for inland rainfall to keep going and this region is one to watch as we move through the cooler months. The Indian Ocean is running somewhat warmer than normal at the moment and that is allowing for late season tropical waves to be more robust, leading to higher than normal moisture content being drawn into the jet stream and this leading to elevated rainfall chances. This is a prognostic for later next week and there is plenty to work with well to the west of the country.

Another area to watch is the moisture content over northern Australia with the lingering wet season thanks to the waning La Nina. This could lead to significant moisture being drawn southwards to the jet stream, which in turn is drawn south into cold fronts approaching the west. Northwest cloud bands can certainly form in response to this leading to widespread rainfall over southern and eastern Australia.

UPPER AIR PLAYS A PART.

Not all the weather makers appear at the surface and I refer to the upper charts in great detail daily as that shows you the flow pattern at about 18000ft above our heads. This is where we can diagnose where systems are going and what influence they will have. Now in terms of the sequence next week, the placement of an upper low is crucial. Below is the GFS today vs yesterday and the difference from run to run, can drive you spare if you are looking for a reliable forecast in terms of rainfall and temperature. When we have cut off systems, we need the feature to be modelled in real time to get better guidance on what one location can expect from such an event. This has a severe weather look to it.

This was the same model about 24hrs ago and you can see the difference for mid to late next week from run to run means the confidence is low. Is there potential for further widespread rainfall and unsettled weather next week? Yes. But who gets what is unknown and poorly modelled right now.

RAINFALL

The Euro Model shows that moisture over the northern tropics being drawn south and southeast through the country, but it is not well supported by other modelling, but certainly if we have moisture lingering over the north and northwest, this will be drawn to the south and southeast leading to elevated rainfall chances spreading across the country. If that was to verify below, we will see above average rainfall for large areas of northern Australia, then marching southwards.

But the latest GFS says hold your horses, once we get the rainfall event out of the eastern inland this weekend, we may be dry for a while. But as you can see above, that is not necessarily the case.

More to come in your state based forecasts this evening.