The Fast Break – Victoria

Seasonal climate risk information for Victoria

Volume 18, Issue 5, 24 May 2023

May has been drier than average to date in most areas except for the southwest coastal fringe, east Melbourne to West Gippsland, and far east Gippsland.

April rain allowed some early sown crop and pasture to germinate but low rainfall, wind and frosts have dried the surface so much that further rains are now needed to support germination in the northwest and northern country. Southwest and east-northeast regions are ideal. Soil moisture probes only rose significantly in those areas with close to average rainfall.

The Pacific Ocean sea surface temperatures are in a weak El Niño condition in the east but a neutral condition in the central region, which is believed to affect Victoria more. The western Pacific still remains very warm at the surface and at depth as a good moisture source but ways of getting it down from there have been hard to find.

The Pacific atmosphere has shown limited to no interest in coupling with the ocean. Cloud, wind and pressure patterns have been normal but in the last 10 days the pressure has increased at Darwin and there is a weak burst of westerly wind off the Solomon Islands. Both these will be worth watching if they continue, as it will indicate a chance for the atmosphere to couple.

Classic El Niño effects tend to only occur if the system is coupled. If the current burst of the Madden Julian Oscillation (providing the reversed trade winds) fails to kick of central Pacific warming or western cooling it will be another 40 or so days before it gets another chance. All models surveyed predict a central Pacific El Niño to form and hang around for at least six months.

The Indian Ocean is poised to be a positive Indian Ocean Dipole but currently only at depth. The ocean surface is currently warmer or normal and stronger easterly winds off Sumatra have been short lived to get the colder water to up-well. Cloud and pressure patterns have been more in keeping with the MJO rather than the underlying ocean conditions. Keep an eye on the big high-pressure systems as a really big one could provide enough easterly wind in the Timor Sea to flip it over cool. Nearly all models predict a +IOD to form and hang around till November.

While El Niño and +IOD historically give greater chances of drier and warmer conditions, the model predictions are predicated on these climate drivers actually forming and coupling. It they don’t, the rainfall outcomes are more uncertain.

The Southern Annular Mode (SAM) has been moderate to strongly positive over May and appears to be dragging weather systems south particularly north of the divide. It is predicted to behave itself at normality in coming weeks.

Unusually with a +SAM, the Sub Tropical Ridge of high pressure has been at a favourable latitude for allowing fronts up, but probably only benefitting southern areas. The actual pressure has been slowly increasing, which is probably the key factor to why rain has been hard to find in the north.

My assessment of 11 climate models for Victoria shows a likely drier rainfall and likely warmer temperatures for the next three months.

Soil moisture

Compared to 30 days earlier than 22 May 2023

Probes from Agriculture Victoria, Dookie Land Management Group, Gecko Clan, Perennial Pasture Systems, and Riverine Plains Inc. networks.

The Australian Water Outlook (AWO) predicts Victoria has slightly dried out in the north but is unchanged in the south. The northwest and northern country are ranked drier and below average, the alps and far southwest are ranked above average. Probes show little change for the month, most with single figure decreases. The Jancourt perennial pasture rose 24 percentage points from 39 to 63 % and the Brim crop decreased by eight percentage points dropping from 88 to 80 %.

Model distribution summary for the next 3 months

Predicted rainfall – June to August 2023

Predictions for June to August 2023, the outlook from 11 global model forecasts is for likely drier across Victoria.

Model distribution summary for Victoria

Predicted temperature – June to August 2023

Predictions for June to August 2023, the outlook from 10 global model forecasts is for likely warmer across Victoria.

Model distribution summary for the next four-to-six months

Predicted rainfall – September to November 2023

Predictions for September to November 2023, the outlook from eight global model forecasts is for likely drier over Victoria.

Predicted temperature – September to November 2023

Predictions for September to November 2023, the outlook from eight global model forecasts is for likely warmer across Victoria

Model consensus forecast for the next 6 months

Current outlook (24 May 2023)

Previous outlook (27 April 2023)

Sea surface temperature (SST) anomalies

The Equatorial Pacific Ocean has warmed further during May. The oceanic values for NINO3 and NINO3.4 are +1.02oC and +0.55oC respectively (as of 24 May). NINO3 is above the El Niño threshold of 0.8 oC but NINO 3.4 is lagging behind and remains neutral.

Waters to our northeast remain very warm as a legacy of the La Niña. The Indian Ocean is neutral and the (DMI) is -0.15oC, which is normal for this time of the year. It will be important to watch the cooling in the Timor Sea to our northwest for further propagation eastwards and northwards.

Sea surface temperatures are the key to the world’s rainfall. For more information on how they are measured, maps created and how to read them, check out our eLearn.

Equatorial Pacific sub-sea temperature anomalies

The Pacific Ocean Equatorial sub-surface temperatures have warmed further with an eastern Pacific warm upwelling, but no central Pacific warming to depth yet.

Southern Oscillation Index (SOI)

30-day moving SOI

The SOI has declined into negativity during May, currently at -7.9 (as of 20 May 2023). Pressure patterns around the equator are close to an El Niño value of -8.0.

© Copyright Commonwealth of Australia 2023. Bureau of Meteorology

Dipole Mode Index (DMI)

The DMI is the difference between the temperature of the west and east boxes of ocean monitored for the IOD. The DMI has stayed at normal values for May. A +IOD value would be above + 0.4 oC.

© Copyright Commonwealth of Australia 2023. Bureau of Meteorology.

Pacific Ocean surface wind anomalies

Wind direction and strength anomalies for the last 30 days

Operational data: Surface winds (m/s) 30-day anomaly for: Saturday 22 April 2023 to Sunday 21 May 2023.

The Pacific Ocean easterly trade winds are at normal values. The beginnings of a potential pulse of westerly wind have recently appeared north of PNG and the Solomon Islands. El Niño behaviour would be indicated by continued westerly wind off the Solomon Islands in the western Pacific.

The eastern Indian Ocean shows normal winds, although a brief but unsustained week of easterly wind occurred mid-month. Positive IOD behaviour would be consistent easterly wind off Sumatra in the eastern Indian Ocean.

These two regions are the key indicators for either El Niño’s and/or +IOD development this season.

(NCEP Operational climatology data: 1985 -1996, smoothed with five-day running mean) Source: NOAA

World cloudiness anomalies

OLR Anomalies: Average of 20 April 2023 to 20 May 2023

Cloud over the international dateline junction with the equator is now normal. This was the last indicator to retreat from La Niña values for the first time in eight months. Greater cloud exists over PNG associated with the warm water in that region and a pulse of the Madden Julian Oscillation. There has been less cloud over Victoria.

Source: Bureau of Meterology

Southern Annular Mode (SAM)

AAO: Observed and GFS forecasts

Observations 22 January 2022 to 21 May 2023

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) went strongly positive in May. SAM’s behaviour in autumn can be erratic and has a poor correlation with rainfall but due to the closeness of winter it is tempting to correlate the drier conditions with weather systems being pulled south by this climate driver. The Bureau of Meteorology and NOAA both predict the SAM to return to neutral in a week and remain there for the next week.

Source: NOAA

Air pressure

Last 30 days air pressure

Operational data: Sea level pressure (mb) 30-day mean for: Saturday 22 April 2023 to Sunday 21 May 2023.

In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) remained in a normal to slightly higher latitudinal position for autumn centred near the top of the Bight. The pressure which had been building from the west is now positioned over the whole of southern Australia on average.

The latitude of the pressure ridge is far enough north to be allowing fronts through, particular south of the divide. The extent of the high pressure has not been enough to drastically overturn the Timor sea cooler.

Source: NOAA

Air pressure anomalies

Last 30 days air pressure anomaly

Operational data: Sea level pressure (mb) 30-day anomaly for: Saturday 22 April 2023 to Sunday 21 May 2023.

The STR gained pressure over May meaning high pressure systems were more dominant and slower moving. This has a fair bit to do with the drier weather over May. Pressure was also increasing to our tropical northeast and into Darwin, with slightly lower pressure at Tahiti. This is why the SOI is positive.

(NCEP Operational climatology data: 1985 – 1996, smoothed with five-day running mean). Source: NOAA

Climate definitions

Read a list of climate acronyms and explanations.

Modelled climate and ocean predictions for Victoria from May 2023 run models

Twelve climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria.

View a colour-coded version of this table: Modelled climate and ocean predictions for Victoria from May 2023 run models (WORD - 67.5 KB)

Our new e-learn explains this table.

Four coupled global circulation model forecasts

* Abbreviations:

JJA = July, August, September

ASO = August, September, October

SON = September, October, November

Four coupled global circulation model forecasts

* Abbreviations:

JJA = July, August, September

ASO = August, September, October

SON = September, October, November

Three ensembles and a statistical model forecast

* Abbreviations:

JJA = July, August, September

ASO = August, September, October

SON = September, October, November

Image references

Original images used in this document are sourced from the Bureau of Meteorology under a Creative Commons 3.0 licence and from the NOAA who have a Public Domain policy. Annotations highlighting areas of interest have been added by Dale Grey.