Hunter 12-month Rainfall Forecast

10 5 0
Apr 19
May 19
Jun 19
Jul 19
Aug 19
Sep 19
Oct 19
Nov 19
Dec 19
Jan 20
Feb 20
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 Watch)

Sea surface temperatures (SST's) have dropped below El Nino threshold across the equatorial Pacific Ocean during the past few weeks, prompting the Bureau of Meteorology to decrease their level of advice to a Watch.

The Nino3.4 index lingered between 0.5 and 0.3 during January, neutral but remaining within the El Nino side of neutral. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -0.6 in January. Moreover, sub-surface equatorial water temperatures have cooled down significantly in the east but remain warmer than average in the west. There are early indications an oceanic Kelvin Wave could transport some of the western pacific heat back to the east in coming months.

Current consensus suggest the marginal El Nino thresholds (related to SST's) will continue through the early autumn, with only two out of eight international seasonal models maintaining El Nino thresholds through to at least April.

Around Australia, SST's off Western Australia and the Northern Territory have experienced significant cooling in recent months with temperatures one to two degrees cooler than average. Further east, the Coral Sea and the Gulf of Carpentaria continue to see average-to-below average SST's. The most significant feature of the SST's is the oceanic heatwave which is bringing extreme temperatures to the Tasman Sea as well as surrounding Tasmania.

Climate forecasts favour below average rainfall for northern and western Australia through the end of the summer and early autumn. Below average rainfall is also favoured for the remainder of summer over southern South Australia. On the contrary, average to above average rainfall is likely for the remainder of summer and spring along the eastern states, particularly in New South Wales, Victoria and Tasmania. SST's are very much warmer than average over the Tasman Sea increasing the humidity in the region. This higher humidity levels can bring some good rainfall, especially east of the Great Dividing Range in short but intense periods with enhanced thunderstorm activity, leading to flash floods.

The 10-month average rainfall shows serious-to-severe rainfall deficiencies in northern and western parts of NSW, Victoria‚??s Gippsland, most of southern Qld and the eastern half of South Australia.

Issued Feb 11

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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