Victoria River 12-month Rainfall Forecast

10 5 0
Nov 18
6
Dec 18
6
Jan 19
7
Feb 19
5
Mar 19
5
Apr 19
7
May 19
7
Jun 19
1
Jul 19
1
Aug 19
9
Sep 19
1
Oct 19
8
Rainfall deciles
10 Well above normal
8-9 Above normal
4-7 Near normal
2-3 Below normal
1 Well below normal

Issue Notes

ENSO status: Neutral (El Nino Alert)
Indian Ocean Dipole (IOD): Positive

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral values, but are now borderline El Nino thresholds.

The Nino3.4 index lingered between 0.7 and 0.8 during October, just shy of the El Nino threshold. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 3.0 in October.

Current consensus suggest El Nino threshold will be met this month. The risk of an El Nino developing during summer is now over 80%, according to the International Research Institute for Climate and Society. The Australian Bureau of Meteorology has continued to maintain its outlook at an El Nino Alert. In the long run, seven out of eight international seasonal models maintain El Nino threshold through to March.

The most significant climate feature over the past month is the significant warming of the Sea Surface temperatures (SSTs) off the NSW and eastern Victorian coasts. This warmer than average area of the Tasman Sea has injected significant moisture into the atmosphere east of the Great Dividing range leading to significant thunderstorm and rainfall activity from southeastern Qld. Cooler SSTs off the northwest have experience some warming over the past week, heralding the beginning of the end for the current positive Indian Ocean Dipole (IOD) phase.

With an El Nino on the cards, climate forecasts are favouring average-to-below average rainfall for southeastern Australia through the summer period. East of the Great Dividing Range, however, odd are neutral due to the elevated SSTs off the coasts.

Despite some decent rainfall across NSW, the 10-month average rainfall, continue to show serious-to-severe rainfall deficiencies across NSW, Victoria, parts of southern Qld and the eastern half of South Australia.

In the longer term, the risk of an El Nino during summer favours a drier than average outlook for the eastern half of the country during the during then end of summer and early autumn. If a marginal El Nino does eventuate, rainfall values could return to normal in early autumn. Higher humidity levels towards the end of summer/early autumn could bring some good rainfall east of the Great Dividing Range and perhaps enhanced thunderstorm activity, leading to flash floods.

The risk of an El Nino brings the risk of:
- A significant reduction of rainfall during the eastern states' wet season,
- A later onset of the North Australian Monsoon,
- A reduction of tropical cyclone activity across northern Australia during the summer (leading to a reduction in rainfall for western Qld).

Issued Nov 9

Forecast Explanation

Notes on the concept of deciles

If all the data in a record are ranked from lowest to highest they can then be divided into 100 equal blocks. These blocks are known as percentiles. The values that fall into the lowest 10% range (from 0 to 10%) are said to be in the first decile, those in the group 10+% to 20% are in the second decile, and so on. Those in the group 90+% to the maximum value recorded are in the 10th decile. The 50% value is a special one known as the 'median'. It is noteworthy since there is the same number of records above and below its value.

Deciles have been found to be very useful for analysing rainfall in particular as its distribution is not the normal bell-shape distribution but is skewed towards many low values with only a few high values. The deciles can be described in qualitative terms. A table is provided in the accompanying results.

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Some good falls of rain for parts of NSW

11:11 AEDT A low pressure trough has chucked a fair amount of rainfall over southern parts of New South Wales, with more to come.

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