NORTHERN parts of the Southern Grampians experienced good rains last week, prompting community discussion about the arrival of the autumn break.

Anzac Day appears to be the point at which many farmers measure whether the break is early or late, but the definition of what constitutes an autumn break is varied and subject to much professional discussion.

A paper written by CSIRO meteorologist, Dr Michael Pook et al, for the Royal Meteorological Society in 2008, addressed the issue with the following text in the introduction.

The first significant rainfall event of the winter growing season, the so-called autumn break, is a keenly anticipated occurrence in the southern Australian agricultural calendar.

It sets the time when successful sowing of grain crops can proceed, and initiates pasture growth which can then be ‘autumn saved’ for grazing during the dormant winter period.

Failure to have an autumn break can have severe financial repercussions, particularly when available soil moisture is low following a dry summer.

The definition of the autumn break is problematic since farmers and graziers can have differing opinions on the effectiveness of autumn rainfall depending on the predominant enterprise of the farm, soil types, regional climatology and farming methods.

Hence, the definition of the autumn break will vary to some extent from farmer to farmer and district to district.

The paper noted four categories of synoptic systems – cutoff lows, cold fronts of all types, tropical troughs in the easterlies and all others – that play a role in bringing the rain.

Cutoff low-pressure systems are defined as slow-moving lows in the upper part of the troposphere (the innermost layer of Earth’s atmosphere up to 10km high) that have become completely detached from the main westerly airstream current.

It is these systems that play the main role in bringing autumn breaks - the paper calling them “the dominant mechanism responsible”.

As for the actual amount of rain falling being considered the break, the Bureau of Meteorology (BOM) have settled on this definition for the Glenelg/Hopkins area - 15mm over three days after April 1 - and a spokesperson said “the month-to-date suggest the far south-west region of Victoria has not reached this criteria”, even though “areas just to the north have in the last few days”.

BOM have forecast average rainfall conditions continuing into autumn.

“The outlook for May across the region is for no strong swing in the odds towards wetter or drier than average conditions,” the spokesperson said.

“The dry conditions in recent months have been caused by persistent blocking highs in the Australian Bight and South Tasman Sea, which have diverted cold fronts and moist tropical air away from the region.

“A persistent positive phase of the Southern Annular Mode (SAM) in the Southern Hemisphere, and the La Niña event in the tropical Pacific, have contributed to this.”

The SAM refers to the (non-seasonal) north-south movement of the strong westerly winds that blow almost continuously in the mid- to high-latitudes of the southern hemisphere.

La Niña usually means above-average winter–spring rainfall for Australia, particularly across the east and north.

It occurs when equatorial trade winds become stronger, changing ocean surface currents and drawing cooler deep water up from below.

The six wettest winter–spring periods on record for eastern Australia occurred during La Niña years.

Rainfall scenario maps can be accessed at bit.ly/3JYPR0w.

Rain is forecast for the south-west this week beginning today and a BOM Regional Weather and Climate Guide for the Glenelg Hopkins area can be found at bit.ly/37uG9Gc.