The Fast Break – Victoria

Seasonal climate risk information for Victoria

Volume 19 | Issue 9 | 24 September 2024

Another drier month in the north has seen crops and pastures with access to soil moisture rapidly using it up. In the south, frequent low rainfall events are yet to lead to runoff in the southwest.

The Pacific Ocean is still neutral, having only made minor progress towards a La Niña in the last 30 days. Ocean temperatures at depth have cooled and this is starting to outbreak a small amount at the surface. Early in the month a burst of easterly wind probably helped this to occur, but it hasn’t been sustained and the trade winds have returned to normal in the central equatorial Pacific. A phase of positive La Niña like Southern Oscillation Index (SOI) values has also not been sustained. Cloud has picked up north of Australia along with some lower pressure, but this could be associated with a passing phase of the Madden Julian Oscillation (MJO). Nearly all models predict a cooling of the central Pacific to form a ‘modoki’ style La Niña for spring and summer but there is still much work needed by the trade winds to get this to occur.

The Indian Ocean has warmed up across the basin and the area of cooler water (drier pattern) off Indonesia has faded away. Pressure and cloud have returned to more normal values from positive Indian Ocean Dipole (drier) like patterns. Despite this, northwest cloud band activity has been supressed and moisture in the Timor Sea has been moving away from Australia. Most models predict the Indian Ocean to stay warmer but in a neutral phase for the rest of spring.

The Southern Annular Mode (SAM) has spent time in both positive and now negative phases, but SAM’s reliability as a climate driver in spring is poor. A negative position could mean more frontal activity for southern areas but also less easterly moisture flow from the tropics.

The MJO has recently passed across northern Australia with some strength, but no passing weather systems were able to get a connection into that moisture.

Pressure has been lower south of the Great Divide with many frontal systems passing by. This is due to the position of high-pressure systems being much further north of the average Adelaide position in spring. Higher pressure to the northwest of Victoria has been blocking moisture transport from the northwest.

My assessment of 12 climate models for Victoria is for increased chances of wetter, although our confidence in the wetter forecasts is low given the poor form of models in recent months. Temperatures are likely to be warmer for the next three months.

Soil moisture

Compared to 30 days earlier than 20 August 2024

The BoM Australian Water Outlook decile (AWO) deciles for perennial pasture rank most of the northwest, northern country, Wimmera, northeast, and eastern southwest at decile one. The far coastal southwest, central and east Gippsland are ranked average or wetter. The moisture probe network shows most northern crop sites significantly drier as well as two sites in central Gippsland. The median change of probes that dried in the past month was minus 30 percentage points. The Coonooer Bridge wheat decreased 39 percentage points from 88 to 49% and the Jancourt dairy pasture increased 54% from 46 to 100%

Soil moisture probe data can be accessed in real time at Agriculture Victoria’s soil moisture monitoring website.

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

Model distribution summary for the next 3 months

Predicted rainfall: October to December 2024

Predictions for October to December 2024, the outlook from 12 global model forecasts is for likely wetter across Victoria.

Model distribution summary for Victoria

Predicted temperature: October to December 2024

Predictions for October to December 2024, the outlook from 11 global model forecasts is for likely warmer across Victoria.

Model distribution summary for the next 4 to 6 months

Predicted rainfall: January to March 2025

Predictions for January to March 2025, the outlook from nine global model forecasts is for likely wetter across Victoria.

Predicted temperature – January to March 2025

Predictions for January to March 2025, the outlook from 9 global model forecasts is for likely warmer across Victoria.

Model consensus forecast for the next 6 months

Current outlook (23 September)

Previous outlook (20 August)

Sea surface temperature (SST) anomalies

The equatorial Pacific Ocean surface cooled over September particularly in the central region. The oceanic values for NINO3 and NINO3.4 were -0.15ºC and -0.21ºC respectively (15 September). Both these values remain normal. A La Niña value would be cooler than -0.8ºC. Ocean temperatures are warmer than normal in the Coral Sea to our northeast as a better moisture source.

The Indian Ocean has rewarmed across the basin and the cooling around Sumatra that existed last month has disappeared. The Dipole Mode Index is currently -0.07ºC (15 September), a neutral value. The Timor Sea is warmer to our northwest as a better moisture source.

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

National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch Daily 5 km SST Anomalies (Version 3.1) 21 September 2024.

Equatorial Pacific subsea temperature anomalies

Analysis done 19 September 2024

The Pacific Ocean Equatorial sub-surface temperature increased in size and lowered in temperature during September. This looks like a stronger pre-La Niña pattern. Cooler water has started to upwell in the central Pacific.

Southern Oscillation Index (SOI)

30-day moving SOI

The SOI climbed strongly into La Niña like positive values in September but has been unable to sustain it, recently dropping below the +7 threshold. The SOI is currently at +4.0 (as of 21 September 2024). The strongly positive SOI was driven mainly by higher pressure at Tahiti rather than lower pressure at Darwin. Recently the pressure has lowered around Darwin but this could be due to the passing phase of the MJO.

© Copyright Commonwealth of Australia 2024. Bureau of Meteorology. Climatology period 1933–92.

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 IOD is behaving normally (currently -0.07^oC at 15 September) with temperatures warm across the whole Ocean. A few models are predicting a late forming negative IOD (wetter), but most just keep the ocean warmer with no differential in temperature between east and west.

© Copyright Commonwealth of Australia 2024. 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 24 August to Sunday 22 September 2024.

A burst of stronger easterly wind in the first weeks of September occurred across the Equatorial Pacific. This has helped the Pacific Ocean surface to cool further. This burst of wind was possibly kicked off by the MJO being positioned west of Australia and drawing wind from the east into it. Currently the winds are normal, indicating no consistent strengthening of the easterlies.
Off Indonesia, the winds are currently normal but had previously been a bit stronger southeasterly with moisture in that zone moving away from Australia.

(NCEP Operational climatology data: 1985–96, smoothed with 5-day running mean.) Source: NOAA.

World cloudiness anomalies

OLR anomalies: Average of 22 August to 21 September 2024

Cloud at the junction of the dateline is slightly less but shows no La Niña like pattern. Greater cloud existed in the north from the warmer Coral Sea but this could also be an effect of a passing MJO cloud wave phase. A lack of northwest cloud band activity into Western Australia is indicative of that being a problem for many months in Victoria as well.

Source: Bureau of Meteorology.

Southern Annular Mode (SAM)

AAO: Observed and GFS forecasts

26 May 2024 to 22 September 2024

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has been weakly positive in the first half of September and is currently weakly negative. SAM has historically led to variable results in spring for Victoria compared to greater dependability of influence in winter or summer. A negative SAM could push fronts up closer to Victoria which could be useful for some coastal areas but could also lead to less easterly flow from the tropics. In the next 14 days both BoM and NOAA predict the SAM to go moderately strong negative before returning to neutral values.

Source: NOAA.

Madden Julian Oscillation (MJO)

MJO Phase Diagram for 13 August to 21 September 2024

For September the MJO cloud band weakly formed west of Indonesia then made a stronger rush across northern Australia in the last fortnight.  Unfortunately, no passing front or low was in a position to get a connection to it. The BoM ACCESS model predicts the MJO to spend the next 30 days traversing the Pacific and Indian Ocean, arriving off Indonesia with no strength.

Source: Bureau of Meterology.

Air pressure

Last 30 days air pressure

Sea level pressure (mb) 30-day mean for: Saturday 24 August to Sunday 22 September 2024.

In the past 30 days the Sub Tropical Ridge of High Pressure (STR) has been further north of a normal spring position of Adelaide. This shifting of patterns north has meant a lot of westerly winds flow, and plenty of frontal activity south of the Divide. The pressure ridge has been doing a good job of blocking connections to tropical moisture.

Source: NOAA.

Air pressure anomalies

Last 30 days air pressure anomaly

Sea level pressure (mb) 30-day anomaly for Saturday 24 August to Sunday 22 September 2024.

The Sub Tropical Ridge was lower in pressure south of the Great Divide, but higher in pressure north of Victoria. This was indicative of many fronts passing in the south and an inability to connect to the tropics in northern Victoria.  Some lower pressure north of Australia is a bit more La Niña like but the pressure in Tahiti has shifted from higher to normal. This is why the SOI is neutral.

(NCEP Operational climatology data: 1985–96, smoothed with 5-day running mean.) Source: NOAA.

Climate definitions

Read a list of climate acronyms and explanations.

Modelled climate and ocean predictions for Victoria from September 2024 run models

Twelve climate models show their predictions for the next 6 months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria.

View a colour-coded version of this table (WORD - 77.2 KB)

Our e-learning module How to read the Fast Break table explains this table.

Four coupled global circulation model forecasts

*OND = October, November, December DJF = December, January, February JFM = January, February, March

Four coupled global circulation model forecasts

*OND = October, November, December DJF = December, January, February JFM = January, February, March

Three ensembles and a statistical model forecast

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 which has a public domain policy. Annotations highlighting areas of interest have been added by Dale Grey.