Northern Tablelands 12-month Rainfall Forecast

10 5 0
Jul 17
Aug 17
Sep 17
Oct 17
Nov 17
Dec 17
Jan 18
Feb 18
Mar 18
Apr 18
May 18
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.
Indian Ocean Dipole (IOD): Neutral.

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral ENSO boundaries.

The Nino3.4 value remained within neutral at 0.5 in May, though continuing to show a warming tendency, 0.2 degrees compared to the previous month. The Southern Oscillation Index (SOI) recorded a value of 0.5 during the same month, also within neutral.

Current consensus continues to suggest El Nino side of neutral during the early winter, but only 4 out of 8 international models are indicating an El Nino threshold by October. More significantly, Australia??s model (POAMA), is now indicating a flip to cooler than average conditions across the Pacific (La Nina side of neutral) later this year.

For the coming three months, climate forecasts continue to favour drier than average conditions for the southern half of Australia during winter and early spring.

During this period, southern Australia sees most of its rainfall through rain-bearing cold fronts traversing the south. However, a stronger than average high pressure ridge over Australia is looking to keep these at bay. Models also continue to favour average to above average rainfall for the tropics during the early dry season.

Despite the overall drier than average conditions for the south, east of the divide, east coast lows can always bring extreme rainfall events with significant rainfall in short periods of time.

In the longer term, a possible El Nino scenario, or at least El Nino side of neutral, will increase the risk of drier than average conditions for the late winter and spring. Moreover, climate models are also indicating the risk of a positive Indian Ocean Dipole (IOD) towards the end of winter and through spring. These outlooks are also being downplayed in the latest model runs. During positive IOD events, southeastern Australia in particular tend to see an increased risk of drier than average conditions for southern Australia.

Issued Jun 13

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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Dry fronts equating to frosty nights

15:03 AEST This winter so far has been one of dry fronts for southeastern Australia, bringing little rain and priming the air for frosty nights to follow.

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