The Fast Break – Victoria

Seasonal climate risk information for Victoria

Volume 18 | Issue 7 | 26 July 2021

July rainfall in Victoria has been average to wetter so far, due to many fronts passing over the state. As is typical of such conditions, Gippsland has been drier as a result of moisture coming from the north west.

Soil moisture has pleasingly started to increase in the north west quarter, but elsewhere it’s wet, wet, wet and a long winter of waterlogging looks likely. Mallee probes have finally started to detect moisture at 30-40 centimetres and are keen for more as the days start to lengthen.

The Indian Ocean surface made a more convincing move into negative IOD territory by warming in the eastern box, such that the Bureau of Meteorology (BoM) officially declared it an event. The western box is closer to normal. The undersea temperatures are much warmer off Sumatra, but the trade winds backed off to be only moderately like -IOD. The pressure and cloud patterns are also only weakly like -IOD demonstrating that this event is not yet a fully-fledged bird. Models predict a -IOD to hang around until November. The -IOD has historically increased the chances of wetter seasonal rainfall.

In the Pacific Ocean, the surface cooled a little, showing some blue wiggles signalling possible movement towards the La Niña side of the scales but as yet it still sits in the bounds of neutral. The undersea also cooled a little. The SOI went strongly positive and the trade winds were stronger in a sign of La Niña like behaviour, but the cloud patterns at the dateline were more normal. A few models are sniffing the possibility of a La Niña this year.

The SAM was less of an influence this month, with some mixed positive and negative activity.

The major influence on Victorian rainfall was the second month in a row of a more northerly sub-tropical ridge, resulting in lower pressure over the state. Both conditions usually leading to average or wetter conditions in winter.

My assessment of 12 climate models for Victoria shows likely wetter/neutral rainfall forecasts with likely neutral to warmer temperatures, for the next three months.

Soil moisture

Compared to 30 days earlier than 26 July 2021.

The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture for pasture, predicts soil moisture is wetter than average over the Alps, central Victoria, central Gippsland, south west coast and the Mallee. The Agriculture Victoria Perennial Pasture Systems soil probe network shows most probes bar the Wimmera-Mallee are now saturated, many remaining so for the last few months. A number of Wimmera and Mallee probes have significantly “wet up” in the last month. Lawloit lucerne increased by 30 percentage points from five to 35 per cent and Elmore crop increased 23 per cent to reach 100 per cent.

Model distribution summary for the next three months

Predicted rainfall – August to October 2021

Predictions for August to October 2021, the outlook from 12 global model forecasts is for likely wetter/neutral rainfall across Victoria for the next three months.

Predicted temperature – August to October 2021

Predictions for August to October 2021, the outlook from 12 global model forecasts is for likely neutral/warmer temperatures across Victorian for the next three months.

Model distribution summary for the next four to six months

Predicted rainfall – November 2021 to January 2022

Predictions for November 2021 to January 2022 the outlook from nine global model forecasts is for Victoria to experience likely wetter/neutral rainfall conditions.

Temperature – November 2021 to January 2022

Predictions for November 2021 to January 2022, the consensus from nine global model forecasts, is for Victoria to experience likely neutral/warmer conditions.

Model consensus forecast for the next six months

Current outlook (26 July  2021)

Previous outlook (28 June 2021)

Sea surface temperature (SST) anomalies

NOAA Coral Reef Watch Daily 5 km SST Anomalies (Version 3.1) 24 July 2021

The Equatorial Pacific Ocean remains at a normal temperature with NINO3 and NINO3.4 cooling slightly during July. The NINO3 and NINO3.4 values are -0.09^oC and -0.01^oC respectively (as of 26 July). Most models are predicting the surface to be slightly cool in coming months with a few predicting progression to a La Niña in spring. The water off Indonesia warmed significantly in July and the Dipole Mode Index (DMI) for the Indian Ocean Dipole has been below the -0.4^oC threshold for more than eight weeks (currently at -0.88^oC as of 26 July). With water off Africa being closer to normal, this -IOD event seems to be driven by Sumatran warming. Water also remains warmer to depth off Sumatra. Most models predict the -IOD to hang around for the rest of this season. Water is warmer than normal everywhere to the north of Australia 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 NEW eLearn.

Equatorial Pacific sub-sea temperature anomalies

Analysis done 22 July 2021.

The Pacific Ocean Equatorial sub-surface temperatures have slightly cooled at depth in the last month.  The eastern Pacific sub-sea will be worth watching for further La Niña like cooling in coming months.

Southern Oscillation Index

30-day moving SOI

© Copyright Commonwealth of Australia 2021. Bureau of Meteorology

The SOI took a meteoric climb into positive territory over July, currently at +15.6 (at 26 July 2021). Pressure patterns around the equator are behaving like La Niña.

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 26 June to Sunday 25 July 2021.

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

Trade winds strengthened dramatically over July, in La Niña like fashion, to be stronger easterly over the western half of the Pacific. This will be pushing warmer water to the north of Australia into the Coral Sea and is likely to cause further sub-sea and surface cooling.  In the Indian Ocean, westerly trade wind strength backed off to be only moderately moving towards Sumatra. In a proper coupled -IOD we would expect greater westerly wind strength.

World cloudiness anomalies

OLR Anomalies: Average of 24 June 2021 to 24 July 2021

Source: BoM

Cloud at the junction of the dateline with the Equator is close to normal, where we would expect to see a lot less cloud in La Niña like conditions. Cloud increase over Indonesia is in keeping with -IOD but the location would normally be over the eastern IOD box. Some north west cloud band activity was streaming from the central Indian into WA, rather than over Broome into Victoria. This would be more -IOD like.

Southern Annular Mode (SAM)

AAO: Observed and GFS forecasts

Observations 28 March 2021 to 25 July 2021

Source: NOAA

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) started July in moderate positivity before going weakly negative. Negative values of the SAM over winter would normally indicate greater chances of wetter for South West Victoria and West Gippsland. The NOAA (above) model predicts a dip into negativity before returning to normal and the BoM forecast predicts a rise into weak to moderate positivity in the coming 14 days.

Air pressure

Last 30 days air pressure

Operational data: Sea level pressure (mb) 30-day mean for: Saturday 26 June to Sunday 25 July 2021.

Source: NOAA

In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has remained at a more northern position to normal which would usually be the Great Australian Bight. This has allowed numerous frontal systems to sweep across the state. If this was to be a permanent feature this winter, it statistically increases the chances of wetter rainfall.

Air pressure anomalies

Last 30 days air pressure anomaly

Operational data: Sea level pressure (mb) 30-day anomaly for: Saturday 26 June to Sunday 25 July 2021.

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

The sub-tropical ridge of high pressure was much lower than normal over south east Australia due to the passage of lows and fronts allowed for by the pressure ridge moving north. Pressure finally normalised to the north of Australia (a good sign) and at Darwin where Tahiti was much higher. This is why the SOI is positive. Some weak lower pressure off Sumatra is roughly in keeping with -IOD, but like the cloud patterns, the pressure patterns have not convincingly locked into a classic pattern of lower in the east box and higher in the west box.

Climate definitions

Read a list of climate acronyms and explanations.

Modelled climate and ocean predictions for Victoria from July 2021 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 July 2021 run models (WORD - 58.2 KB)

If this table makes no sense, visit our new e-learn which explains it.

Four coupled global circulation model forecasts

* ASO = August, September, October NDJ = November, December, January

Four coupled global circulation model forecasts

* ASO = August, September, October NDJ = November, December, January

Three ensembles and a statistical model forecast

* ASO = August, September, October NDJ = November, December, January

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.