Weekly Update: 21st March 2025

Big rainfalls moved through parts of southeastern Australia and this week we focus on why some parts received huge rain and others hardly anything.

In order to make it rain you need two things to work together: moisture and instability.

The moisture side of the equation has a huge tick mark, the instability is the variable part.

As long as high pressure is 'in the way' then low pressure can't move into the spots that most need the rain - especially these areas in significant drought in the south:

The latest rain system delivered big falls further east of these drought zones, but once again missed these key drought-affected areas:

What will the next few months look like?

Moisture

Australia is surrounded by warmer than average water, so there is more moisture available to us than 'normal'. This has been the case for the past five years, either in La Nina, or El Nino (as the drying trend was offset by this alternative source of moisture), and in a negative or positive IOD (again as the drying trend was offset by this alternative source of moisture).

- 2025 is showing weak signs of heading into El Nino (or close to) later in the year (later Spring/Summer), with neutral conditions before that.

- 2025 is showing stronger signs of heading into a negative IOD in Winter, with neutral conditions before that.

So that's the Indian Ocean sending moisture our way, and the Pacific suppressing it - but the waters around Australia offset that suppression.

Moisture gets a big tick.

Instability

This is the hardest thing to forecast.

We don't have confidence in where a weather system will move a week out, let alone the next few months or year.

One climate driver to look at is the Southern Annular Mode (see daily updates here - scroll to the SAM map).

If that is positive (as it was for much of the past year) then it encourages high pressure to sit across southern Australia. It doesn't tell us where it will sit, but it is one factor in stopping big rainfalls from reaching those drought affected areas - just like we saw with the most recent weather system.

For my full analysis settle in for my weekly update video: